Marine microorganisms

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| caption1 = Microbial mats are the earliest form of life on Earth for which there is good fossil evidence. The image shows a cyanobacterial-algal mat.

| image2 = Stromatolites in Sharkbay.jpg

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| caption2 = Stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.

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File:Relative sizes of microscopic entities 2.jpg

{{see also|Evolution of cells}}

Microorganisms make up about 70% of the marine biomass. A microorganism, or microbe, is a microscopic organism too small to be recognised adequately with the naked eye. In practice, that includes organisms smaller than about 0.1 mm.Glöckner, F.O., Gasol, J.M., McDonough, N., Calewaert, J.B. et al. (2012) [http://archives.esf.org/fileadmin/Public_documents/Publications/MarineBoard_PP17_microcean.pdf Marine microbial diversity and its role in ecosystem functioning and environmental change]. European Science Foundation, Position Paper 17. {{isbn|978-2-918428-71-8}}{{rp|13}}

Such organisms can be single-celled or multicellular. Microorganisms are diverse and include all bacteria and archaea, most protists including algae, protozoa and fungal-like protists, as well as certain microscopic animals such as rotifers. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses as microorganisms, but others consider these as non-living.

Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. Some microorganisms are pathogenic, causing disease and even death in plants and animals. As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements.

{{clade

|label1=Marine microorganisms

|1={{clade

|1 = Viruses 45 px

|label2=  Prokaryotes  

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|1 = Bacteria 45 px

|2 = Archaea     25 px

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|label3=Eukaryotes

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|1 = Protists 55 px

|2 = Microfungi   25 px

|4 = Microanimals   45 px

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{{Quote box

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|quote = While recent technological developments and scientific discoveries have been substantial, we still lack a major understanding at all levels of the basic ecological questions in relation to the microorganisms in our seas and oceans. These fundamental questions are:

1. What is out there? Which microorganisms are present in our seas and oceans and in what numbers

do they occur?

2. What are they doing? What functions do each of these microorganisms perform in the marine environment and how do they contribute to the global cycles of energy and matter?

3. What are the factors that determine the presence or absence of a microorganism and how do they influence biodiversity and function and vice versa?

|source = – European Science Foundation, 2012{{rp|14}}

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File:Relative scale.svgs (bacteria and archaea) and viruses relative to those of other organisms and biomolecules]]

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File:Marine critters.jpg

Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems.{{cite journal |last=Suttle |first=C.A. |title=Viruses in the Sea |journal=Nature |year=2005 |volume=437 |issue=9 |pages=356–361| doi=10.1038/nature04160 |pmid=16163346 |bibcode=2005Natur.437..356S |s2cid=4370363}} Most marine viruses are bacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.Shors p. 5.{{fcn|date=February 2024}} They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.Shors p. 593.{{fcn|date=February 2024}} Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.{{cite journal |vauthors=Suttle CA |title=Marine viruses—major players in the global ecosystem |journal=Nature Reviews Microbiology |volume=5 |issue=10 |pages=801–12 |year=2007 |pmid=17853907 |doi=10.1038/nrmicro1750 |s2cid=4658457}}

File:Ocean mist and spray 2.jpg, and can travel the globe before falling back to earth.]]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[https://www.smithsonianmag.com/science-nature/living-bacteria-are-riding-earths-air-currents-180957734/ Living Bacteria Are Riding Earth's Air Currents] Smithsonian Magazine, 11 January 2016. Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.{{cite news |last=Robbins |first=Jim |title=Trillions Upon Trillions of Viruses Fall From the Sky Each Day |url=https://www.nytimes.com/2018/04/13/science/virosphere-evolution.html |date=13 April 2018 |work=The New York Times |access-date=14 April 2018}}{{cite journal |last1=Reche |first1=Isabel |last2=D'Orta |first2=Gaetano |last3=Mladenov |first3=Natalie |last4=Winget |first4= Danielle M |last5=Suttle |first5= Curtis A |title=Deposition rates of viruses and bacteria above the atmospheric boundary layer |journal=ISME Journal |volume=12 |issue=4 |pages=1154–1162 |date=29 January 2018 |doi=10.1038/s41396-017-0042-4 |pmid=29379178 |pmc=5864199|bibcode=2018ISMEJ..12.1154R }}

Microscopic organisms live throughout the biosphere. The mass of prokaryote microorganisms — which includes bacteria and archaea, but not the nucleated eukaryote microorganisms — may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons).{{cite web |author=Staff |title=The Biosphere |url=http://www.agci.org/classroom/biosphere/index.php |date=2014 |work=Aspen Global Change Institute |access-date=10 November 2014 |archive-date=2 September 2010 |archive-url=https://web.archive.org/web/20100902111048/http://www.agci.org/classroom/biosphere/index.php |url-status=dead }} Single-celled barophilic marine microbes have been found at a depth of {{convert|10900|m|ft|abbr=on}} in the Mariana Trench, the deepest spot in the Earth's oceans.{{cite web |last=Choi |first=Charles Q. |title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date=17 March 2013 |publisher=LiveScience |access-date=17 March 2013}}{{cite journal |last1=Glud |first1=Ronnie |last2=Wenzhöfer |first2=Frank |last3=Middelboe |first3=Mathias |last4=Oguri |first4=Kazumasa |last5=Turnewitsch |first5=Robert |last6=Canfield |first6=Donald E. |last7=Kitazato |first7=Hiroshi |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth |doi=10.1038/ngeo1773 |date=17 March 2013 |journal=Nature Geoscience |volume=6 |issue=4 |pages=284–288 |bibcode = 2013NatGe...6..284G}} Microorganisms live inside rocks {{convert|580|m|ft|abbr=on}} below the sea floor under {{convert|2590|m|ft|abbr=on}} of ocean off the coast of the northwestern United States,{{cite web |last=Oskin |first=Becky |title=Intraterrestrials: Life Thrives in Ocean Floor |url=http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=14 March 2013 |publisher=LiveScience |access-date=17 March 2013}} as well as {{convert|2400|m|ft mi|abbr=on}} beneath the seabed off Japan.{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |title=Microbes discovered by deepest marine drill analysed |url=https://www.bbc.com/news/science-environment-30489814 |date=15 December 2014 |work=BBC News |access-date=15 December 2014}} The greatest known temperature at which microbial life can exist is {{convert|122|°C|°F|abbr=on}} (Methanopyrus kandleri).{{cite journal |display-authors=4 |vauthors=Takai K, Nakamura K, Toki T, Tsunogai U, Miyazaki M, Miyazaki J, Hirayama H, Nakagawa S, Nunoura T, Horikoshi K |title=Cell proliferation at 122°C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation |journal=Proceedings of the National Academy of Sciences of the United States of America |date=2008 |volume=105 |issue=31| pages=10949–54 |doi=10.1073/pnas.0712334105 |pmid=18664583 |pmc=2490668 |bibcode=2008PNAS..10510949T |doi-access=free}} In 2014, scientists confirmed the existence of microorganisms living {{convert|800|m|ft|abbr=on}} below the ice of Antarctica.{{cite journal |last=Fox |first=Douglas |title=Lakes under the ice: Antarctica's secret garden |date=20 August 2014 |journal=Nature |volume=512 |issue=7514 |pages=244–246 |doi=10.1038/512244a |bibcode=2014Natur.512..244F |pmid=25143097 |doi-access=free}}{{cite web |title=Life Confirmed Under Antarctic Ice; Is Space Next? |url=https://www.forbes.com/sites/ericmack/2014/08/20/life-confirmed-under-antarctic-ice-is-space-next/ |last=Mack |first=Eric |date=20 August 2014 |work=Forbes |access-date=21 August 2014}} According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are." Marine microorganisms serve as "the foundation of all marine food webs, recycling major elements and producing and consuming about half the organic matter generated on Earth each year".{{cite journal |last1=Armbrust |first1=E.V. |last2=Palumbi |first2=S.R. |year=2015 |title=Uncovering hidden worlds of ocean biodiversity |journal=Science |volume=348 |issue=6237| pages=865–867 |doi=10.1126/science.aaa7378 |bibcode=2015Sci...348..865A |pmid=25999494 |s2cid=36480105}}{{cite journal |last1=Azam |first1=F. |last2=Malfatti |first2=F. |year=2007 |title=Microbial structuring of marine ecosystems |journal=Nature Reviews Microbiology |volume=5 |issue=10| pages=782–791 |doi=10.1038/nrmicro1747 |pmid=17853906 |s2cid=10055219}}

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File:Marine virus-host interactions.jpg

{{main|Marine viruses}}

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.{{cite journal

|vauthors=Koonin EV, Senkevich TG, Dolja VV

|title=The ancient Virus World and evolution of cells

|journal=Biology Direct

|volume=1

|page=29

|year=2006

|pmid=16984643

|pmc=1594570

|doi=10.1186/1745-6150-1-29

|doi-access=free

}}

When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles, also known as virions, consist of two or three parts: (i) the genetic material (genome) made from either DNA or RNA, long molecules that carry genetic information; (ii) a protein coat called the capsid, which surrounds and protects the genetic material; and in some cases (iii) an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of these virus particles range from simple helical and icosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an optical microscope. The average virion is about one one-hundredth the size of the average bacterium.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity. Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection. However, they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life"{{cite journal |vauthors=Rybicki EP |year=1990 |title=The classification of organisms at the edge of life, or problems with virus systematics |journal=South African Journal of Science |volume=86 |pages=182–186}} and as replicators.{{cite journal |title=Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question |journal=Studies in History and Philosophy of Biological and Biomedical Sciences |date=October 2016 |volume=59 |vauthors=Koonin EV, Starokadomskyy P |pages=125–134 |doi=10.1016/j.shpsc.2016.02.016 |pmid=26965225 |pmc=5406846}}

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| header = Bacteriophages (phages)

| header_align = center

| footer = Phage injecting its genome into bacteria

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| image1 = Phage.jpg

| alt1 =

| caption1 = Multiple phages attached to a bacterial cell wall at 200,000x magnification

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| image2 = Tailed phage.png

| alt2 =

| caption2 = Diagram of a typical tailed phage

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Viruses are found wherever there is life and have probably existed since living cells first evolved.{{cite journal

|vauthors=Iyer LM, Balaji S, Koonin EV, Aravind L

|title=Evolutionary genomics of nucleo-cytoplasmic large DNA viruses

|journal=Virus Research

|volume=117

|issue=1

|pages=156–84

|year=2006

|pmid=16494962

|doi=10.1016/j.virusres.2006.01.009|url=https://zenodo.org/record/1259447

}} The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose.{{cite journal |vauthors=Sanjuán R, Nebot MR, Chirico N, Mansky LM, Belshaw R |title=Viral mutation rates |journal=Journal of Virology |volume=84 |issue=19 |pages=9733–48 |date=October 2010 |pmid=20660197 |doi=10.1128/JVI.00694-10 |pmc=2937809}}

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.{{cite book |editor1=Mahy, W.J. |editor2=Van Regenmortel, M.H.V. |title=Desk Encyclopedia of General Virology |publisher=Academic Press |location=Oxford |year=2009 |pages=28 |isbn=978-0-12-375146-1}}

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators and as "organisms at the edge of life".

File:Caudovirales.svgs of different families of tailed phages}}]]

File:Cyanophages.pngs, viruses that infect cyanobacteria (scale bars indicate 100 nm)}}]]

Bacteriophages, often just called phages, are viruses that parasite bacteria and archaea. Marine phages parasite marine bacteria and archaea, such as cyanobacteria.{{cite journal |vauthors=Mann NH |title=The Third Age of Phage |journal=PLOS Biology |volume=3 |issue=5 |pages=753–755 |date=2005-05-17 |doi=10.1371/journal.pbio.0030182 |pmid=15884981 |pmc=1110918 |doi-access=free}} They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,{{cite journal |vauthors=Wommack KE, Colwell RR|title=Virioplankton: viruses in aquatic ecosystems|journal=Microbiology and Molecular Biology Reviews |volume=64 |issue=1 |pages=69–114 |year=2000 |pmid=10704475 |pmc=98987 |doi=10.1128/MMBR.64.1.69-114.2000}}{{cite journal |vauthors=Suttle CA |title=Viruses in the sea|journal=Nature |volume=437 |pages=356–361 |year=2005 |issue=7057 |pmid=16163346 |doi=10.1038/nature04160 |bibcode=2005Natur.437..356S |s2cid=4370363}} although estimates of viral abundance in seawater can vary over a wide range.{{cite journal |vauthors=Bergh O, Børsheim KY, Bratbak G, Heldal M|title=High abundance of viruses found in aquatic environments |journal=Nature|volume=340|issue=6233|pages=467–8 |year=1989 |doi=10.1038/340467a0 |bibcode=1989Natur.340..467B |pmid=2755508 |s2cid=4271861}}{{cite journal |vauthors=Wigington CH, Sonderegger D, Brussaard CP, Buchan A, Finke JF, Fuhrman JA, Lennon JT, Middelboe M, Suttle CA, Stock C, Wilson WH, Wommack KE, Wilhelm SW, Weitz JS |title=Re-examination of the relationship between marine virus and microbial cell abundances |journal=Nature Microbiology |volume=1|pages=15024|year=2016 |issue=3 |pmid=27572161 |doi=10.1038/nmicrobiol.2015.24 |s2cid=52829633 |url=http://www.vliz.be/imisdocs/publications/23/301523.pdf}}

For a long time, tailed phages of the order Caudovirales seemed to dominate marine ecosystems in number and diversity of organisms.

However, as a result of more recent research, non-tailed viruses appear to be dominant in multiple depths and oceanic regions, followed by the Caudovirales families of myoviruses, podoviruses, and siphoviruses.{{cite journal |vauthors=Brum JR, Schenck RO, Sullivan MB |title=Global morphological analysis of marine viruses shows minimal regional variation and dominance of non-tailed viruses |journal=The ISME Journal |volume=7 |issue=9 |pages=1738–51 |date=September 2013 |pmid=23635867 |pmc=3749506 |doi=10.1038/ismej.2013.67 |bibcode=2013ISMEJ...7.1738B}}

Phages belonging to the families:

Corticoviridae,{{cite journal |title=Putative prophages related to lytic tailless marine dsDNA phage PM2 are widespread in the genomes of aquatic bacteria |journal=BMC Genomics |year=2007 |volume=8 |pages=236 |doi=10.1186/1471-2164-8-236 |pmid=17634101 |vauthors=Krupovic M, Bamford DH |pmc=1950889 |doi-access=free}}

Inoviridae,{{cite journal|title=High frequency of a novel filamentous phage, VCY φ, within an environmental Vibrio cholerae population|journal=Applied and Environmental Microbiology |year=2012|volume=78|issue=1|pages=28–33 |doi=10.1128/AEM.06297-11 |pmid=22020507 |vauthors=Xue H, Xu Y, Boucher Y, Polz MF |pmc=3255608|bibcode=2012ApEnM..78...28X }}

Microviridae,{{cite journal|title=Evolution and diversity of the Microviridae viral family through a collection of 81 new complete genomes assembled from virome reads |journal=PLOS ONE |year=2012 |volume=7 |issue=7 |pages=e40418 |doi=10.1371/journal.pone.0040418 |pmid=22808158 |vauthors=Roux S, Krupovic M, Poulet A, Debroas D, Enault F |pmc=3394797 |bibcode=2012PLoSO...740418R|doi-access=free}}

and Autolykiviridae{{cite journal |vauthors=Kauffman KM, Hussain FA, Yang J, Arevalo P, Brown JM, Chang WK, VanInsberghe D, Elsherbini J, Sharma RS, Cutler MB, Kelly L, Polz MF |year=2018| title=A major lineage of non-tailed dsDNA viruses as unrecognized killers of marine bacteria |journal=Nature |volume=554 |issue=7690| pages=118–122 |doi=10.1038/nature25474 |pmid=29364876 |bibcode=2018Natur.554..118K |s2cid=4462007}}{{cite web |website=Sci News |url=http://www.sci-news.com/biology/autolykiviridae-05664.html |title=Scientists Find New Type of Virus in World's Oceans: Autolykiviridae |date=25 January 2018}}{{cite web |last=Dockrill |first=Peter |url=https://www.sciencealert.com/entirely-unknown-family-of-infectious-viruses-discovered-in-the-ocean-parasites-tail-less-autolykiviridae |title=Never-Before-Seen Viruses With Weird DNA Were Just Discovered in The Ocean |website=Science^alert |date=25 January 2018}}{{cite journal |vauthors=Schoch CL |display-authors=etal |journal=NCBI Taxonomy: A Comprehensive Update on Curation, Resources and Tools |type=Database (Oxford) |date=2020 |at=baaa062 |pmid=32761142 |pmc=7408187 |url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=2184034 |title=Autolykiviridae|volume=2020 |doi=10.1093/database/baaa062 }}

are also known to infect diverse marine bacteria.

There are also archaean viruses which replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.{{cite journal|vauthors=Lawrence CM, Menon S, Eilers BJ|title=Structural and functional studies of archaeal viruses |journal=Journal of Biological Chemistry |volume=284 |issue=19 |pages=12599–603 |year=2009 |pmid=19158076 |doi=10.1074/jbc.R800078200 |pmc=2675988 |doi-access=free}}{{cite journal |vauthors=Prangishvili D, Forterre P, Garrett RA |title=Viruses of the Archaea: a unifying view |journal=Nature Reviews Microbiology |volume=4 |issue=11 |pages=837–48 |year=2006 |pmid=17041631 |doi=10.1038/nrmicro1527 |s2cid=9915859}} These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.{{cite journal |vauthors=Prangishvili D, Garrett RA |title=Exceptionally diverse morphotypes and genomes of crenarchaeal hyperthermophilic viruses |journal=Biochemical Society Transactions |volume=32 |issue=Pt 2 |pages=204–8 |year=2004 |pmid=15046572 |doi=10.1042/BST0320204 |s2cid=20018642 |url=https://curis.ku.dk/ws/files/51497971/0320204.pdf}}

Microorganisms make up about 70% of the marine biomass. It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms, which often kill other marine life.{{cite web

|url=https://www.cdc.gov/hab/redtide/

|title=Harmful Algal Blooms: Red Tide: Home {{!}} CDC HSB

|publisher=www.cdc.gov

|access-date=2014-12-19

}}

The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.{{cite journal|vauthors=Canchaya C, Fournous G, Chibani-Chennoufi S, Dillmann ML, Brüssow H|title=Phage as agents of lateral gene transfer|journal=Current Opinion in Microbiology |volume=6 |issue=4 |pages=417–24 |year=2003 |pmid=12941415 |doi=10.1016/S1369-5274(03)00086-9}} It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth.{{cite journal |vauthors=Forterre P, Philippe H |title=The last universal common ancestor (LUCA), simple or complex? |journal=The Biological Bulletin |volume=196 |issue=3 |pages=373–5; discussion 375–7 |year=1999 |pmid=11536914 |doi=10.2307/1542973 |jstor=1542973}} Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.

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File:Electron microscopic image of a mimivirus - journal.ppat.1000087.g007 crop.png]]

File:Tupanvirus.jpeg, named after Tupã, the Guarani supreme god of creation]]

Viruses normally range in length from about 20 to 300 nanometers. This can be contrasted with the length of bacteria, which starts at about 400 nanometers. There are also giant viruses, often called giruses, typically about 1000 nanometers (one micron) in length.

All giant viruses belongto phylum Nucleocytoviricota (NCLDV), together with poxviruses.

The largest known of these is Tupanvirus. This genus of giant virus was discovered in 2018 in the deep ocean as well as a soda lake, and can reach up to 2.3 microns in total length.{{cite journal|title=Tailed giant Tupanvirus possesses the most complete translational apparatus of the known virosphere| first1=Jônatas| last1=Abrahão| first2=Lorena| last2=Silva| first3=Ludmila Santos| last3=Silva| first4=Jacques Yaacoub Bou| last4=Khalil| first5=Rodrigo| last5=Rodrigues| first6=Thalita| last6=Arantes| first7=Felipe| last7=Assis| first8=Paulo| last8=Boratto| first9=Miguel| last9=Andrade| first10=Erna Geessien| last10=Kroon| first11=Bergmann| last11=Ribeiro| first12=Ivan| last12=Bergier| first13=Herve| last13=Seligmann| first14=Eric| last14=Ghigo| first15=Philippe| last15=Colson| first16=Anthony| last16=Levasseur| first17=Guido| last17=Kroemer| first18=Didier| last18=Raoult| first19=Bernard La| last19=Scola| display-authors=4| date=27 February 2018| journal=Nature Communications| volume=9| issue=1| pages=749| doi=10.1038/s41467-018-03168-1| pmid=29487281| pmc=5829246| bibcode=2018NatCo...9..749A}}

The discovery and subsequent characterization of giant viruses has triggered some debate concerning their evolutionary origins.{{Cite journal |last=Reardon |first=Sara |date=2017-04-06 |title=Patchwork giant viruses spark debate over the tree of life |journal=Nature |language=en |doi=10.1038/nature.2017.21798 |s2cid=89815981 |issn=1476-4687}} The two main hypotheses for their origin are that either they evolved from small viruses, picking up DNA from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction.{{cite news |first=Rae Ellen |last=Bichell |title=In Giant Virus Genes, Hints About Their Mysterious Origin |url=https://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin |work=All Things Considered}} What sort of complicated organism giant viruses might have diverged from is also a topic of debate. One proposal is that the origin point actually represents a fourth domain of life,{{cite journal |journal=American Scientist |title=Giant Viruses |first=James L. |last=Van Etten |name-list-style=vanc |date=July–August 2011 |volume=99 |issue=4 |pages=304–311 |doi=10.1511/2011.91.304 |url=http://www.americanscientist.org/issues/feature/2011/4/giant-viruses}}{{cite journal |vauthors=Legendre M, Arslan D, Abergel C, Claverie JM |title=Genomics of Megavirus and the elusive fourth domain of Life |journal=Communicative & Integrative Biology |volume=5 |issue=1 |pages=102–6 |date=January 2012 |pmid=22482024 |pmc=3291303 |doi=10.4161/cib.18624}} but this has been largely discounted.{{cite journal |vauthors=Schulz F, Yutin N, Ivanova NN, Ortega DR, Lee TK, Vierheilig J, Daims H, Horn M, Wagner M, Jensen GJ, Kyrpides NC, Koonin EV, Woyke T |title=Giant viruses with an expanded complement of translation system components |journal=Science |volume=356 |issue=6333 |pages=82–85 |date=April 2017 |pmid=28386012 |doi=10.1126/science.aal4657 |bibcode=2017Sci...356...82S |s2cid=206655792 |url=https://escholarship.org/content/qt0kf9t6gn/qt0kf9t6gn.pdf?t=oruwia| doi-access=free}}{{cite journal |vauthors=Bäckström D, Yutin N, Jørgensen SL, Dharamshi J, Homa F, Zaremba-Niedwiedzka K, Spang A, Wolf YI, Koonin EV, Ettema TJ |title=Virus Genomes from Deep Sea Sediments Expand the Ocean Megavirome and Support Independent Origins of Viral Gigantism |journal=mBio |volume=10 |issue=2 |pages=e02497-02418 |date=March 2019 |doi=10.1128/mBio.02497-18 |pmid=30837339 |pmc=6401483}}

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{{main|Marine prokaryotes}}

File:Pelagibacter.jpg, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.]]

File:Vibrio vulnificus 01.png, a virulent bacterium found in estuaries and along coastal areas]]

File:Synechococcus elongatus PCC 7942 electron micrograph showing carboxysomes.jpeg. Carboxysomes appear as polyhedral dark structures.]]

{{see also|Bacterioplankton|Bacterial motility}}

Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,{{cite journal |vauthors=Fredrickson JK, Zachara JM, Balkwill DL, Kennedy D, Li SM, Kostandarithes HM, Daly MJ, Romine MF, Brockman FJ | title = Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the Hanford site, Washington state | journal = Applied and Environmental Microbiology | volume = 70 | issue = 7 | pages = 4230–41 | year = 2004 | pmid = 15240306 | pmc = 444790 | doi = 10.1128/AEM.70.7.4230-4241.2004| bibcode = 2004ApEnM..70.4230F }} and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.{{cite journal |vauthors=Woese CR, Kandler O, Wheelis ML | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 87 | issue = 12 | pages = 4576–9 | year = 1990 | pmid = 2112744 | pmc = 54159 | doi = 10.1073/pnas.87.12.4576 | bibcode = 1990PNAS...87.4576W| doi-access = free }}

The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.{{cite journal | vauthors = Schopf JW | title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 91 | issue = 15 | pages = 6735–42 | year = 1994 | pmid = 8041691 | pmc = 44277 | doi = 10.1073/pnas.91.15.6735 | bibcode = 1994PNAS...91.6735S| doi-access = free }}{{cite journal |vauthors=DeLong EF, Pace NR | title = Environmental diversity of bacteria and archaea | journal = Systematic Biology | volume = 50 | issue = 4 | pages = 470–8 | year = 2001 | pmid = 12116647 | doi = 10.1080/106351501750435040 | citeseerx = 10.1.1.321.8828}} Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.{{cite journal |vauthors=Brown JR, Doolittle WF | title = Archaea and the prokaryote-to-eukaryote transition | journal = Microbiology and Molecular Biology Reviews | volume = 61 | issue = 4 | pages = 456–502 | year = 1997 | pmid = 9409149 | pmc = 232621 | doi = 10.1128/mmbr.61.4.456-502.1997}}

Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.{{cite journal |last1=Dyall |first1=Sabrina D. |last2=Brown |first2=Mark T. |last3=Johnson |first3=Patricia J. |author-link3=Patricia J. Johnson |date=9 April 2004 |title=Ancient Invasions: From Endosymbionts to Organelles |journal=Science |volume=304 |issue=5668 |pages=253–257 |bibcode=2004Sci...304..253D |doi=10.1126/science.1094884 |issn=0036-8075 |pmid=15073369|s2cid=19424594 }}{{cite journal |vauthors=Poole AM, Penny D | title = Evaluating hypotheses for the origin of eukaryotes | journal = BioEssays | volume = 29 | issue = 1 | pages = 74–84 | year = 2007 | pmid = 17187354 | doi = 10.1002/bies.20516| s2cid = 36026766 }} This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.{{cite journal |vauthors=Lang BF, Gray MW, Burger G | title = Mitochondrial genome evolution and the origin of eukaryotes | journal = Annual Review of Genetics | volume = 33 | pages = 351–97 | year = 1999 | pmid = 10690412 | doi = 10.1146/annurev.genet.33.1.351}}{{cite journal | vauthors = McFadden GI | title = Endosymbiosis and evolution of the plant cell | journal = Current Opinion in Plant Biology | volume = 2 | issue = 6 | pages = 513–9 | year = 1999 | pmid = 10607659 | doi = 10.1016/S1369-5266(99)00025-4| bibcode = 1999COPB....2..513M }} This is known as secondary endosymbiosis.

File:Sulphide bacteria crop2.jpg|The marine Thiomargarita namibiensis, largest known bacterium

File:Potomac river eutro.jpg|Cyanobacteria blooms can contain lethal cyanotoxins

File:Glaucocystis sp.jpg|The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.{{cite journal|journal = American Journal of Botany|year = 2004|volume = 91|pages = 1481–1493|title = Diversity and evolutionary history of plastids and their hosts|author = Patrick J. Keeling|doi = 10.3732/ajb.91.10.1481|issue = 10|pmid = 21652304|s2cid = 17522125}}

File:Marinomonas arctica.jpg| The bacterium Marinomonas arctica grows inside Arctic sea ice at subzero temperatures

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Pelagibacter ubique and its relatives may be the most abundant organisms in the ocean, and it has been claimed that they are possibly the most abundant bacteria in the world. They make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water. The total abundance of P. ubique and relatives is estimated to be about 2 × 10²⁸ microbes.{{cite web |title=Candidatus Pelagibacter Ubique |publisher=European Bioinformatics Institute |date=2011 |work=Bacteria Genomes |url=http://www.ebi.ac.uk/2can/genomes/bacteria/Candidatus_Pelagibacter_ubique.html |access-date=8 Jan 2012 |archive-url=https://web.archive.org/web/20081201014010/http://www.ebi.ac.uk/2can/genomes/bacteria/Candidatus_Pelagibacter_ubique.html |archive-date=1 December 2008}} However, it was reported in Nature in February 2013 that the bacteriophage HTVC010P, which attacks P. ubique, has been discovered and "it probably really is the commonest organism on the planet".{{cite news |url=https://www.economist.com/news/science-and-technology/21571843-newly-discovered-virus-may-be-most-abundant-organism-planet-flea |title=Flea market: A newly discovered virus may be the most abundant organism on the planet |date=16 February 2013 |newspaper=The Economist |access-date=16 February 2013}}{{Cite journal |vauthors=Zhao Y, Temperton B, Thrash JC, Schwalbach MS, Vergin KL, Landry ZC, Ellisman M, Deerinck T, Sullivan MB, Giovannoni SJ |doi=10.1038/nature11921 |title=Abundant SAR11 viruses in the ocean |journal=Nature |volume=494 |issue=7437 |pages=357–360 |year=2013 |pmid=23407494| bibcode=2013Natur.494..357Z |s2cid=4348619}}

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains {{convert|0.75|mm|μm|abbr=on}}.{{Citation|work=Max Planck Institute for Marine Microbiology |date=8 April 1999 |title=The largest Bacterium: Scientist discovers new bacterial life form off the African coast |url=http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |url-status=dead |archive-url=https://web.archive.org/web/20100120043846/http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |archive-date=20 January 2010}}{{Citation |title=List of Prokaryotic names with Standing in Nomenclature – Genus Thiomargarita |url=https://lpsn.dsmz.de/genus/thiomargarita}}

File:Morning-Glory Hotspring.jpgs living in harsh environments, such as the yellow archaea pictured here in a hot spring, but they have since been found in a much broader range of habitats.{{cite journal |vauthors=Bang C, Schmitz RA |title=Archaea associated with human surfaces: not to be underestimated |journal=FEMS Microbiology Reviews |volume= 39|issue= 5|pages= 631–48|date=2015 |pmid=25907112 |doi=10.1093/femsre/fuv010|doi-access=free }}]]

{{see also|Marine prokaryotes}}

The archaea (Greek for ancient[http://www.etymonline.com/index.php?l=a&p=41 Archaea] Online Etymology Dictionary. Retrieved 17 August 2016.) constitute a domain and kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells.

Archaea were initially classified as bacteria, but this classification is outdated.{{cite journal |vauthors=Pace NR |title=Time for a change |journal=Nature |volume=441 |issue=7091 |page=289 |date=May 2006 |pmid=16710401 |doi=10.1038/441289a |bibcode=2006Natur.441..289P |s2cid=4431143 |doi-access=free}} Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukaryota. The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their nucleic acids in samples from their environment.

Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi.{{cite journal |vauthors=Stoeckenius W |title=Walsby's square bacterium: fine structure of an orthogonal procaryote |journal=Journal of Bacteriology |volume=148 |issue=1 |pages=352–60 |date=1 October 1981|pmid=7287626 |pmc=216199 |doi=10.1128/JB.148.1.352-360.1981}} Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, such as archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species forms spores.

Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle. Thermoproteota (also known as eocytes or Crenarchaeota) are a phylum of archaea thought to be very abundant in marine environments and one of the main contributors to the fixation of carbon.{{cite book |veditors=Madigan M, Martinko J |title=Brock Biology of Microorganisms |edition=11th |publisher=Prentice Hall |year=2005 |isbn=978-0-13-144329-7}}

File:RT8-4.jpg| Eocytes may be the most abundant of marine archaea

File:Halobacteria with scale.jpg|Halobacteria, found in water nearly saturated with salt, are now recognised as archaea.

File:Haloquadratum walsbyi00.jpg|Flat, square-shaped cells of the archaea Haloquadratum walsbyi

File:Methanosarcina barkeri fusaro.gif|Methanosarcina barkeri, a marine archaea that produces methane

File:Thermophile bacteria2.jpg|Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C

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All living organisms can be grouped as either prokaryotes or eukaryotes. Life originated as single-celled prokaryotes and later evolved into the more complex eukaryotes. In contrast to prokaryotic cells, eukaryotic cells are highly organised. Prokaryotes are the bacteria and archaea, while eukaryotes are the other life forms — protists, plants, fungi and animals. Protists are usually single-celled, while plants, fungi and animals are usually multi-celled.

It seems very plausible that the root of the eukaryotes lie within archaea; the closest relatives nowadays known may be the Heimdallarchaeota phylum of the proposed Asgard superphylum. This theory is a modern version of a scenario originally proposed in 1984 as Eocyte hypothesis, when Thermoproteota were the closest known archaeal relatives of eukaryotes then.

A possible transitional form of microorganism between a prokaryote and a eukaryote was discovered in 2012 by Japanese scientists. Parakaryon myojinensis is a unique microorganism larger than a typical prokaryote, but with nuclear material enclosed in a membrane as in a eukaryote, and the presence of endosymbionts. This is seen to be the first plausible evolutionary form of microorganism, showing a stage of development from the prokaryote to the eukaryote.{{cite web |title=Deep sea microorganisms and the origin of the eukaryotic cell |url=http://protistology.jp/journal/jjp47/JJP47YAMAGUCHI.pdf |access-date=24 October 2017}}{{cite journal|last1=Yamaguchi |display-authors=et al|first1=Masashi |title=Prokaryote or eukaryote? A unique microorganism from the deep sea |issue=6 |journal=Journal of Electron Microscopy |volume=61 |pages=423–431 |doi=10.1093/jmicro/dfs062 |pmid=23024290 |date=1 December 2012}}

{{main|Marine protists}}

Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated as single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor).

Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungal-like,{{cite journal |last1=Whittaker |first1=R.H. |last2=Margulis |first2=L. |year=1978 |title=Protist classification and the kingdoms of organisms |journal=Biosystems |volume=10 |issue=1–2| pages=3–18 |doi=10.1016/0303-2647(78)90023-0 |pmid=418827 |bibcode=1978BiSys..10....3W}} or a mixture of these.{{cite journal |vauthors=Faure E, Not F, Benoiston AS, Labadie K, Bittner L, Ayata SD |year=2019 |title=Mixotrophic protists display contrasted biogeographies in the global ocean |journal=ISME Journal |volume=13 |issue=4| pages=1072–1083 |doi=10.1038/s41396-018-0340-5 |pmid=30643201 |pmc=6461780 |bibcode=2019ISMEJ..13.1072F}}

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Protists are highly diverse organisms currently organised into 18 phyla, but are not easy to classify.{{Cite journal|author=Cavalier-Smith T |title=Kingdom protozoa and its 18 phyla |journal=Microbiological Reviews |volume=57 |issue=4 |pages=953–94 |date=December 1993 |pmid=8302218 |pmc=372943 |doi=10.1128/mmbr.57.4.953-994.1993}}{{Cite journal|author=Corliss JO |title=Should there be a separate code of nomenclature for the protists? |journal=BioSystems |volume=28 |issue=1–3 |pages=1–14 |year=1992 |pmid=1292654 |doi=10.1016/0303-2647(92)90003-H |bibcode=1992BiSys..28....1C }} Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.{{Cite journal|vauthors=Slapeta J, Moreira D, López-García P |title=The extent of protist diversity: insights from molecular ecology of freshwater eukaryotes |journal=Proceedings of the Royal Society B: Biological Sciences |volume=272 |issue=1576 |pages=2073–81 |year=2005 |pmid=16191619 |doi=10.1098/rspb.2005.3195 |pmc=1559898}}{{Cite journal|vauthors=Moreira D, López-García P |title=The molecular ecology of microbial eukaryotes unveils a hidden world |journal=Trends in Microbiology |volume=10 |issue=1 |pages=31–8 |year=2002 |doi=10.1016/S0966-842X(01)02257-0 |url=http://download.bioon.com.cn/view/upload/month_0803/20080326_daa08a6fdb5d38e3a0d8VBrocN3WtOdR.attach.pdf |pmid=11755083}} There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist biomass.{{cite journal |vauthors=Leles SG, Mitra A, Flynn KJ, Stoecker DK, Hansen PJ, Calbet A, McManus GB, Sanders RW, Caron DA, Not F, Hallegraeff GM |year=2017 |title=Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance |journal=Proceedings of the Royal Society B: Biological Sciences |volume=284 |issue=1860| page=20170664 |doi=10.1098/rspb.2017.0664 |pmid=28768886 |pmc=5563798}} Since protists are eukaryotes they possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Protists are asexual but can reproduce rapidly through mitosis or by fragmentation.

File:Diatoms through the microscope.jpg|Diatoms are a major algae group generating about 20% of world oxygen production.{{cite web |author=Andrew Alverson |url=https://www.livescience.com/46250-teasing-apart-the-diatom-genome.html |title=The Air You're Breathing? A Diatom Made That |date=October 14, 2022 |website=LiveScience}}

File:Diatom algae Amphora sp.jpg|Diatoms have glass like cell walls made of silica and called frustules.{{cite web |title=More on Diatoms|website=University of California Museum of Paleontology |url=http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html |access-date=11 October 2019|archive-url=https://web.archive.org/web/20121004130024/http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html|archive-date=4 October 2012|url-status=dead}}

File:Triceratium morlandii var. morlandii.jpg|Fossil diatom frustule from 32 to 40 mya

File:Podocyrtis papalis Ehrenberg - Radiolarian (30448963206).jpg|Radiolarian

File:Gephyrocapsa oceanica color (lightened).jpg|Single-celled alga, Gephyrocapsa oceanica

File:CSIRO ScienceImage 7609 SEM dinoflagellate.jpg|Two dinoflagellates

File:Euglenoid movement.jpg|Euglenoid

File:Zooxanthellae.jpg|Zooxanthellae is a photosynthetic algae that lives inside hosts like coral

File:Paramecium bursaria.jpg|A single-celled ciliate with green zoochlorellae living inside endosymbiotically

File:The ciliate Frontonia sp.jpg|This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.

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File:Frontonia ingesting a diatom.ogg| Ciliate ingesting a diatom

File:Amoeba engulfing diatom.ogv| Amoeba engulfing a diatom

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In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.{{Cite journal|author=Devreotes P |title=Dictyostelium discoideum: a model system for cell-cell interactions in development |journal=Science |volume=245 |issue=4922 |pages=1054–8 |year=1989 |pmid=2672337 | doi=10.1126/science.2672337|bibcode=1989Sci...245.1054D}} Other marine protist are neither single-celled nor microscopic, such as seaweed.

{{anchor|Macroscopic protists}}

File:Chaos carolinensis Wilson 1900.jpg|The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm

File:Gromia in situ closeup.png|Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm.{{cite journal |author1=Mikhail V. Matz |author2=Tamara M. Frank |author3=N. Justin Marshall |author4=Edith A. Widder |author5=Sonke Johnsen |title=Giant Deep-Sea Protist Produces Bilaterian-like Traces |journal=Current Biology |volume=18 |issue=23 |pages=1849–1854 |url=http://www.biology.duke.edu/johnsenlab/pdfs/pubs/sea%20grapes%202008.pdf |publisher=Elsevier Ltd |date=2008-12-09 |archive-url=https://web.archive.org/web/20081216211211/http://www.biology.duke.edu/johnsenlab/pdfs/pubs/sea%20grapes%202008.pdf |archive-date=2008-12-16 |doi=10.1016/j.cub.2008.10.028 |doi-access=free |pmid=19026540 |bibcode=2008CBio...18.1849M |s2cid=8819675}}

File:Spiculosiphon oceana AB.png|Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.

File:Xenophyophore.jpg|The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm across.{{Cite journal |last1=Gooday |first1=A. J. |last2=Aranda da Silva |first2=A. |last3=Pawlowski |first3=J. |date=2011-12-01 |title=Xenophyophores (Rhizaria, Foraminifera) from the Nazaré Canyon (Portuguese margin, NE Atlantic) |journal=Deep-Sea Research Part II: Topical Studies in Oceanography |series=The Geology, Geochemistry, and Biology of Submarine Canyons West of Portugal |volume=58 |issue=23–24 |pages=2401–2419 |doi=10.1016/j.dsr2.2011.04.005 |bibcode=2011DSRII..58.2401G}}

File:Giant Kelp.jpg|Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m

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Protists have been described as a taxonomic grab bag of misfits where anything that doesn't fit into one of the main biological kingdoms can be placed.Neil A C, Reece J B, Simon E J (2004) [https://books.google.com/books?id=lRhFAQAAIAAJ&q=protists+%22taxonomic+grab+bag%22 Essential biology with physiology] Pearson/Benjamin Cummings, Page 291. {{ISBN|9780805375039}} Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.{{Cite journal | doi = 10.1007/s10539-012-9354-y| title = The other eukaryotes in light of evolutionary protistology| journal = Biology & Philosophy| volume = 28| issue = 2| pages = 299–330| year = 2012| vauthors = O'Malley MA, Simpson AG, Roger AJ| s2cid = 85406712}}{{cite journal | vauthors = Adl SM, Simpson AG, Farmer MA, Andersen RA, Anderson OR, Barta JR, Bowser SS, Brugerolle G, Fensome RA, Fredericq S, James TY, Karpov S, Kugrens P, Krug J, Lane CE, Lewis LA, Lodge J, Lynn DH, Mann DG, McCourt RM, Mendoza L, Moestrup O, Mozley-Standridge SE, Nerad TA, Shearer CA, Smirnov AV, Spiegel FW, Taylor MF | title = The new higher level classification of eukaryotes with emphasis on the taxonomy of protists | journal = The Journal of Eukaryotic Microbiology | volume = 52 | issue = 5 | pages = 399–451 | year = 2005 | pmid = 16248873 | doi = 10.1111/j.1550-7408.2005.00053.x| s2cid = 8060916 | doi-access = free | url = http://doc.rero.ch/record/14409/files/PAL_E1847.pdf }} This more constrained definition excludes many brown, multicellular red and green algae, and slime molds.{{Cite book | url = https://books.google.com/books?id=9IWaqAOGyt4C | title = Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth | last1 = Margulis | first1 = Lynn | last2 = Chapman | first2 = Michael J. | name-list-style = vanc | date = 2009-03-19 | publisher = Academic Press | isbn = 9780080920146}}

{{main|Protist locomotion}}

Another way of categorising protists is according to their mode of locomotion. Many unicellular protists, particularly protozoans, are motile and can generate movement using flagella, cilia or pseudopods. Cells which use flagella for movement are usually referred to as flagellates, cells which use cilia are usually referred to as ciliates, and cells which use pseudopods are usually referred to as amoeba or amoeboids. Other protists are not motile, and consequently have no movement mechanism.

Flagellates include bacteria as well as protists. The rotary motor model used by bacteria uses the protons of an electrochemical gradient in order to move their flagella. Torque in the flagella of bacteria is created by particles that conduct protons around the base of the flagellum. The direction of rotation of the flagella in bacteria comes from the occupancy of the proton channels along the perimeter of the flagellar motor.{{cite journal |author = Brady, Richard M. |title= Torque and switching in the bacterial flagellar motor. An electrostatic model|journal= Biophysical Journal |volume= 64 |issue= 4| year = 1993 | pages = 961–973|bibcode = 1993BpJ....64..961B |doi = 10.1016/S0006-3495(93)81462-0 |pmid= 7684268|pmc= 1262414}}

Ciliates generally have hundreds to thousands of cilia that are densely packed together in arrays. During movement, an individual cilium deforms using a high-friction power stroke followed by a low-friction recovery stroke. Since there are multiple cilia packed together on an individual organism, they display collective behavior in a metachronal rhythm. This means the deformation of one cilium is in phase with the deformation of its neighbor, causing deformation waves that propagate along the surface of the organism. These propagating waves of cilia are what allow the organism to use the cilia in a coordinated manner to move. A typical example of a ciliated microorganism is the Paramecium, a one-celled, ciliated protozoan covered by thousands of cilia. The cilia beating together allow the Paramecium to propel through the water at speeds of 500 micrometers per second.{{cite journal|last=Lauga|first=Eric|author2=Thomas R Powers|title=The hydrodynamics of swimming microorganisms|journal=Reports on Progress in Physics|date=25 August 2009|volume=72|issue=9|pages=096601|doi=10.1088/0034-4885/72/9/096601|bibcode=2009RPPh...72i6601L|arxiv=0812.2887|s2cid=3932471}}

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File:Flagellum base diagram-en.svg|Bacterial flagellum rotated by a molecular motor at its base

File:Salmon spermatozoa illustration.png|Salmon spermatozoa

File:Инфузория туфелька поедает бактерии!.gif|Paramecium, a predatory ciliate, feeding on bacteria

File:Oxytricha trifallax.jpg|The ciliate Oxytricha trifallax with cilia clearly visible

File:Collection Penard MHNG Specimen 05bis-1-1 Amoeba proteus.tif|Amoeba with ingested diatoms

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File:Lichen rock.jpeg on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.]]

File:Littoraria irrorata.jpg, Littoraria irrorata, covered in lichen. This snail farms intertidal ascomycetous fungi]]

{{see also|Marine fungi|Mycoplankton|Evolution of fungi}}

Over 1500 species of fungi are known from marine environments.{{cite journal|last=Hyde|first=K.D. |author2=E.B.J. Jones |author3=E. Leaño |author4=S.B. Pointing |author5=A.D. Poonyth |author6=L.L.P. Vrijmoed|title=Role of fungi in marine ecosystems|journal=Biodiversity and Conservation|year=1998|volume=7|issue=9|pages=1147–1161|doi=10.1023/A:1008823515157|bibcode=1998BiCon...7.1147H |s2cid=22264931 }} These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, and other substrata.Kirk, P.M., Cannon, P.F., Minter, D.W. and Stalpers, J. "Dictionary of the Fungi". Edn 10. CABI, 2008 Spores of many species have special appendages which facilitate attachment to the substratum.{{cite journal|last=Hyde|first=K.D.|author2=E.B.J. Jones|title=Spore attachment in marine fungi|journal=Botanica Marina|year=1989|volume=32|issue=3|pages=205–218|doi=10.1515/botm.1989.32.3.205|s2cid=84879817}} Marine fungi can also be found in sea foam and around hydrothermal areas of the ocean.{{cite journal | vauthors=Le Calvez T, Burgaud G, Mahé S, Barbier G, Vandenkoornhuyse P | title=Fungal diversity in deep-sea hydrothermal ecosystems | journal=Applied and Environmental Microbiology | volume=75 | issue=20 | pages=6415–21 | date=October 2009 | pmid=19633124 | pmc=2765129 | doi=10.1128/AEM.00653-09| bibcode=2009ApEnM..75.6415L }} A diverse range of unusual secondary metabolites is produced by marine fungi.{{cite journal|last=San-Martín|first=A. |author2=S. Orejanera |author3=C. Gallardo |author4=M. Silva |author5=J. Becerra |author6=R. Reinoso |author7=M.C. Chamy |author8=K. Vergara |author9=J. Rovirosa|title=Steroids from the marine fungus Geotrichum sp|journal=Journal of the Chilean Chemical Society|year=2008|volume=53|issue=1|pages=1377–1378|doi=10.4067/S0717-97072008000100011 |doi-access=free }}

Mycoplankton are saprotropic members of the plankton communities of marine and freshwater ecosystems.Jones, E.B.G., Hyde, K.D., & Pang, K.-L., eds. (2014). [https://books.google.com/books?id=mXfnBQAAQBAJ Freshwater fungi: and fungal-like organisms]. Berlin/Boston: De Gruyter.{{cite book |year=2012 |editor1-last=Jones |editor1-first=E.B.G. |editor2-last=Pang |editor2-first=K.-L. |title=Marine Fungi, and Fungal-like Organisms |url=http://www.degruyter.com/view/product/177990 |series=Marine and Freshwater Botany |location=Berlin, Boston |publisher=De Gruyter |publication-date=August 2012 |isbn=978-3-11-026406-7 |access-date=3 September 2015 |ref=jones2012 |doi=10.1515/9783110264067}} They are composed of filamentous free-living fungi and yeasts associated with planktonic particles or phytoplankton.{{Cite journal|title = Distribution and Diversity of Planktonic Fungi in the West Pacific Warm Pool|journal = PLOS ONE|volume = 9|issue = 7|pages = e101523|doi = 10.1371/journal.pone.0101523.s001|pmid = 24992154|pmc = 4081592|bibcode = 2014PLoSO...9j1523W|last1 = Wang|first1 = Xin|last2 = Singh|first2 = Purnima|last3 = Gao|first3 = Zheng|last4 = Zhang|first4 = Xiaobo|last5 = Johnson|first5 = Zackary I.|last6 = Wang|first6 = Guangyi|year = 2014|doi-access = free}} Similar to bacterioplankton, these aquatic fungi play a significant role in heterotrophic mineralization and nutrient cycling.{{cite book |last1=Wang |first1=G. |last2=Wang |first2=X. |last3=Liu |first3=X. |last4=Li |first4=Q. |editor-last=Raghukumar |editor-first=C. |title=Biology of marine fungi |location=Berlin, Heidelberg |publisher=Springer-Verlag |year=2012 |pages=71–88 |chapter=Diversity and biogeochemical function of planktonic fungi in the ocean |isbn=978-3-642-23341-8 |chapter-url=https://www.springer.com/us/book/9783642233418 |access-date=3 September 2015 |doi=10.1007/978-3-642-23342-5|s2cid=39378040 }} While mostly microscopic, some mycoplankton can be up to 20 mm in diameter and over 50 mm in length.{{Cite journal|title = Fungi and Macroaggregation in Deep-Sea Sediments|journal = Microbial Ecology|date = 2007-11-11|issn = 0095-3628|pages = 168–177|volume = 56|issue = 1|doi = 10.1007/s00248-007-9334-y|pmid = 17994287|first1 = Samir|last1 = Damare|first2 = Chandralata|last2 = Raghukumar|s2cid = 21288251}}

A typical milliliter of seawater contains about 10³ to 10⁴ fungal cells.{{Cite journal|title = Seaweed resistance to microbial attack: A targeted chemical defense against marine fungi|journal = Proceedings of the National Academy of Sciences|date = 2003-06-10|issn = 0027-8424|pmc = 165804|pmid = 12756301|pages = 6916–6921|volume = 100|issue = 12|doi = 10.1073/pnas.1131855100|first1 = Julia|last1 = Kubanek|first2 = Paul R.|last2 = Jensen|first3 = Paul A.|last3 = Keifer|first4 = M. Cameron|last4 = Sullards|first5 = Dwight O.|last5 = Collins|first6 = William|last6 = Fenical|bibcode = 2003PNAS..100.6916K|doi-access = free}} This number is greater in coastal ecosystems and estuaries due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 metres, with a vertical profile that depends on how abundant phytoplankton is.{{Cite journal|title = Molecular characterization of the spatial diversity and novel lineages of mycoplankton in Hawaiian coastal waters|journal = The ISME Journal|date = 2009-07-30|issn = 1751-7362|pages = 111–120|volume = 4|issue = 1|doi = 10.1038/ismej.2009.87|pmid = 19641535|first1 = Zheng|last1 = Gao|first2 = Zackary I.|last2 = Johnson|first3 = Guangyi|last3 = Wang|doi-access = free}}{{Cite journal|title = Identification of Habitat-Specific Biomes of Aquatic Fungal Communities Using a Comprehensive Nearly Full-Length 18S rRNA Dataset Enriched with Contextual Data|journal = PLOS ONE|date = 2015-07-30|pmc = 4520555|pmid = 26226014|pages = e0134377|volume = 10|issue = 7|doi = 10.1371/journal.pone.0134377|first1 = Katrin|last1 = Panzer|first2 = Pelin|last2 = Yilmaz|first3 = Michael|last3 = Weiß|first4 = Lothar|last4 = Reich|first5 = Michael|last5 = Richter|first6 = Jutta|last6 = Wiese|first7 = Rolf|last7 = Schmaljohann|first8 = Antje|last8 = Labes|first9 = Johannes F.|last9 = Imhoff|bibcode = 2015PLoSO..1034377P|doi-access = free}} This profile changes between seasons due to changes in nutrient availability.Gutierrez, Marcelo H; Pantoja, Silvio; Quinones, Renato a and Gonzalez, Rodrigo R. First record of flamentous fungi in the coastal upwelling ecosystem off central Chile. Gayana (Concepc.) [online]. 2010, vol.74, n.1, pp. 66-73. {{ISSN|0717-6538}}.

Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by turbulence and oxygen generated by photosynthetic organisms.{{Cite book|title = Plankton Dynamics of Indian Waters|last = Sridhar|first = K.R.|publisher = Pratiksha Publications|year = 2009|location = Jaipur, India|pages = 133–148|chapter = 10. Aquatic fungi – Are they planktonic?}}

Marine fungi can be classified as:

• Lower fungi – adapted to marine habitats (zoosporic fungi, including mastigomycetes: oomycetes and chytridiomycetes)
• Higher fungi – filamentous, modified to planktonic lifestyle (hyphomycetes, ascomycetes, basidiomycetes). Most mycoplankton species are higher fungi.

Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens are found in marine environments.{{cite web |website=Marine Fungi |url=http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm |title=Species of Higher Marine Fungi |archive-url=https://web.archive.org/web/20130422084649/http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm |archive-date=22 April 2013 |publisher=University of Southern Mississippi |access-date=2012-02-05}} Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.{{cite journal |last=Hawksworth |first=D.L. |date=2000 |journal=Fungal Diversity |volume=5 |pages=1–7 |url=http://www.fungaldiversity.org/fdp/sfdp/FD_5_1-7.pdf |title=Freshwater and marine lichen-forming fungi |access-date=2012-02-06}} Some lichens live a long time; one species has been dated at 8,600 years.{{cite web |title=Lichens |url=https://www.nps.gov/glac/learn/nature/lichens.htm |publisher=National Park Service, US Department of the Interior, Government of the United States|date=22 May 2016 |access-date=4 April 2018}} However their lifespan is difficult to measure because what defines the same lichen is not precise.{{cite web |url=http://www.earthlife.net/lichens/growth.html|title=The Earth Life Web, Growth and Development in Lichens|date=14 February 2020 |publisher=earthlife.net}} Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.

The sea snail Littoraria irrorata damages plants of Spartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonise the plant. The snail then eats the fungal growth in preference to the grass itself.{{cite journal |vauthors=Silliman BR, Newell SY |year=2003 |title=Fungal farming in a snail |journal=PNAS |volume=100 |issue=26 |pages=15643–15648 |doi=10.1073/pnas.2535227100 |pmid=14657360 |pmc=307621 |bibcode=2003PNAS..10015643S |doi-access=free}}

According to fossil records, fungi date back to the late Proterozoic era 900-570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.{{cite journal |vauthors=Yuan X, Xiao S |year=2005 |title=Lichen-Like Symbiosis 600 Million Years Ago |journal=Science |volume=308 |issue=5724 |pages=1017–1020 |doi=10.1126/science.1111347 |pmid=15890881 |bibcode=2005Sci...308.1017Y |s2cid=27083645}} It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the Paleozoic era (390 million years ago).{{Cite book |title=Marine Fungi: and Fungal-like Organisms |url=https://books.google.com/books?id=RcF97cHppPsC |publisher=Walter de Gruyter |date=2012-08-31 |isbn=9783110264067 |first1=E. B. Gareth |last1=Jones |first2=Ka-Lai |last2=Pang}}

{{see also|Microanimal|Ichthyoplankton}}

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| video2 = [https://www.youtube.com/watch?v=wje4W2XiZus Gastrotrichs: Four-day-old grandmothers]

| video3 = [https://www.youtube.com/watch?v=U3PLUeD_JAg Rotifers: Charmingly bizarre and often ignored]

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As juveniles, animals develop from microscopic stages, which can include spores, eggs and larvae. At least one microscopic animal group, the parasitic cnidarian Myxozoa, is unicellular in its adult form, and includes marine species. Other adult marine microanimals are multicellular. Microscopic adult arthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marine crustaceans include some copepods, cladocera and tardigrades (water bears). Some marine nematodes and rotifers are also too small to be recognised with the naked eye, as are many loricifera, including the recently discovered anaerobic species that spend their lives in an anoxic environment.{{cite journal |author=Janet Fang |date=6 April 2010 |title=Animals thrive without oxygen at sea bottom |journal=Nature |volume=464 |doi=10.1038/464825b |pmid=20376121 |pages=825 |issue=7290|bibcode=2010Natur.464..825F |doi-access= |s2cid=4340458 }}{{cite web|url=http://www.sciencenews.org/view/generic/id/58154/description/Multicelled_animals_may_live_oxygen-free|title=Briny deep basin may be home to animals thriving without oxygen|work=Science News|date=2013-09-23}} Copepods contribute more to the secondary productivity and carbon sink of the world oceans than any other group of organisms.

File:Copepod 2.jpg|Over 10,000 marine species are copepods, small, often microscopic crustaceans

File:Gastrotrich.jpg|Darkfield photo of a gastrotrich, 0.06-3.0 mm long, a worm-like animal living between sediment particles

File:Pliciloricus enigmatus.jpg|Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel

File:The Rotifer Notholca sp (cropped).jpg|Rotifers, usually 0.1–0.5 mm long, may look like protists but are multicellular and belong to the Animalia

File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2.png|Tardigrades (water bears), about 0.5 mm long, are among the most resilient animals known

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File:Seawifs global biosphere.jpg and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.]]

{{main|Marine primary production}}

{{see also|Evolution of photosynthesis}}

Primary producers are the autotroph organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in the food chain for heterotroph organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which are chemotrophs, making their own food by gathering around hydrothermal vents and cold seeps and using chemosynthesis. However most marine primary production comes from organisms which use photosynthesis on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water and carbon dioxide{{cite book |last1=Campbell |first1=Neil A. |last2=Reece |first2=Jane B. |last3=Urry |first3=Lisa Andrea |last4=Cain |first4=Michael L. |last5=Wasserman |first5=Steven Alexander |last6=Minorsky |first6=Peter V. |last7=Jackson |first7=Robert Bradley |title=Biology |edition=8th |year=2008 |publisher=Pearson – Benjamin Cummings|location=San Francisco |isbn=978-0-321-54325-7}}{{rp|186–187}} into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.{{rp|1242}} Marine primary producers are important because they underpin almost all marine animal life by generating most of the oxygen and food that provide other organisms with the chemical energy they need to exist.

The principal marine primary producers are cyanobacteria, algae and marine plants. The oxygen released as a by-product of photosynthesis is needed by nearly all living things to carry out cellular respiration. In addition, primary producers are influential in the global carbon and water cycles. They stabilize coastal areas and can provide habitats for marine animals. The term division has been traditionally used instead of phylum when discussing primary producers, but the International Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.{{Cite book |year=2012 |editor-last=McNeill |editor-first=J. |display-editors=etal|title=International Code of Nomenclature for algae, fungi, and plants (Melbourne Code), Adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011 |edition=electronic |publisher=International Association for Plant Taxonomy |url=http://www.iapt-taxon.org/nomen/main.php?page=art3 |access-date=2017-05-14}}

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| caption2 = The tiny cyanobacterium Prochlorococcus is a major contributor to atmospheric oxygen

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| caption3 = NASA image of a large bloom of Nodularia cyanobacteria swirling in the Baltic Sea[https://phys.org/news/2019-11-oxygen-ocean-disrupt-fundamental-biological.html Changes in oxygen concentrations in our ocean can disrupt fundamental biological cycles] Phys.org, 25 November 2019.

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Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular to filamentous and include colonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.{{cite book|editor-last1=Walsh|editor-first1=Patrick J.|editor-last2=Smith|editor-first2=Sharon|editor-last3=Fleming|editor-first3=Lora|editor-first4=Helena |editor-last4=Solo-Gabriele |editor-first5=William H.|editor-last5= Gerwick| name-list-style = vanc |title=Oceans and Human Health: Risks and Remedies from the Seas|chapter-url={{google books |plainurl=y |id=LMZPqW-PmFYC|page=271}}|date=2 September 2011|chapter=Cyanobacteria and cyanobacterial toxins|pages=271–296|publisher=Academic Press|isbn=978-0-08-087782-2}} In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of the phytoplankton.

The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3 billion years ago.{{Cite web |url=http://www.astrobio.net/news-exclusive/the-rise-of-oxygen/ |title=The Rise of Oxygen |website=Astrobiology Magazine |date=30 July 2003 |language=en-US |access-date=2016-04-06 |archive-url=https://web.archive.org/web/20150403153610/http://www.astrobio.net/news-exclusive/the-rise-of-oxygen/ |archive-date=2015-04-03 |url-status=dead}}{{cite journal|vauthors=Flannery DT, Walter RM |title=Archean tufted microbial mats and the Great Oxidation Event: new insights into an ancient problem |journal=Australian Journal of Earth Sciences |date=2012 |volume=59 |issue=1 |pages=1–11 |doi=10.1080/08120099.2011.607849 |bibcode=2012AuJES..59....1F |s2cid=53618061}} The release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction of oxygen-intolerant organisms, a dramatic change which redirected the evolution of the major animal and plant species.{{cite web |url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-url=https://web.archive.org/web/20120329092237/http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-date=29 March 2012 |title=Understand the evolutionary mechanisms and environmental limits of life |access-date=13 July 2009 |last=Rothschild |first=Lynn |author-link=Lynn J. Rothschild |date=September 2003 |url-status=dead |publisher=NASA}}

The tiny (0.6 μm) marine cyanobacterium Prochlorococcus, discovered in 1986, forms today an important part of the base of the ocean food chain and accounts for much of the photosynthesis of the open ocean{{cite journal |vauthors=Nadis S |title=The cells that rule the seas |journal=Scientific American |volume=289 |issue=6 |pages=52–53 |date=December 2003 |doi=10.1038/scientificamerican1203-52 |url=http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf |bibcode=2003SciAm.289f..52N |pmid=14631732 |access-date=2 June 2019 |archive-url=https://web.archive.org/web/20140419222251/http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf |archive-date=19 April 2014 |url-status=dead}} and an estimated 20% of the oxygen in the Earth's atmosphere.{{cite news |author=Joe Palca |date=12 June 2008 |title=The Most Important Microbe You've Never Heard Of |url=https://www.npr.org/templates/story/story.php?storyId=91448837 |website=npr.org}} It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.{{Cite journal |vauthors=Flombaum P, Gallegos JL, Gordillo RA, Rincon J, Zabala LL, Jiao N, Karl DM, Li WK, Lomas MW, Veneziano D, Vera CS, Vrugt JA, Martiny AC |title=Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus |journal=Proceedings of the National Academy of Sciences |volume=110 |issue=24 |pages=9824–9829 |year=2013 |pmid=23703908| pmc=3683724| bibcode=2013PNAS..110.9824F| doi=10.1073/pnas.1307701110 |doi-access=free}}

Originally, biologists thought cyanobacteria was algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the same Kingdom as algae. Most authorities exclude all prokaryotes, and hence cyanobacteria from the definition of algae.{{cite book |last=Nabors |first=Murray W. |title=Introduction to Botany |year=2004 |publisher=Pearson Education, Inc |location=San Francisco, CA |isbn=978-0-8053-4416-5}}{{cite encyclopedia |editor1-last=Allaby |editor1-first=M. |year=1992 |encyclopedia=The Concise Dictionary of Botany |publisher=Oxford University Press |location=Oxford |title=Algae}}

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Algae is an informal term for a widespread and diverse group of photosynthetic protists which are not necessarily closely related and are thus polyphyletic. Marine algae can be divided into six groups: green, red and brown algae, euglenophytes, dinoflagellates and diatoms.

Dinoflagellates and diatoms are important components of marine algae and have their own sections below. Euglenophytes are a phylum of unicellular flagellates with only a few marine members.

Not all algae are microscopic. Green, red and brown algae all have multicellular macroscopic forms that make up the familiar seaweeds. Green algae, an informal group, contains about 8,000 recognised species.{{cite journal |vauthors=Guiry MD |title=How many species of algae are there? |journal=Journal of Phycology |volume=48 |issue=5 |pages=1057–63 |date=October 2012 |pmid=27011267 |doi=10.1111/j.1529-8817.2012.01222.x| bibcode=2012JPcgy..48.1057G |s2cid=30911529}} Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds. Red algae, a (disputed) phylum contains about 7,000 recognised species,{{Cite web |url=http://www.algaebase.org/browse/taxonomy/?id=97240 |title=Algaebase |last1=Guiry |first1=M.D. |last2=Guiry |first2=G.M. |date=2016 |website=www.algaebase.org|access-date=20 November 2016}} mostly multicellular and including many notable seaweeds.{{cite book |title=Seaweeds |publisher=Life Series. Natural History Museum, London |year=2002 |isbn=978-0-565-09175-0 |author=D. Thomas}} Brown algae form a class containing about 2,000 recognised species,{{Cite book |url=https://books.google.com/books?id=s1P855ZWc0kC&pg=166 |title=Algae: an introduction to phycology |last1=Van den Hoek |first1=Christiaan |last2=Mann |first2=David |last3=Jahns |first3=H.M. |date=1995 |publisher=Cambridge University Press |isbn=9780521316873 |oclc=443576944|page=166}} mostly multicellular and including many seaweeds such as kelp.

Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as microalgae or macroalgae.

Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostly unicellular species which exist as individuals or in chains or groups, though some are multicellular. Microalgae are important components of the marine protists discussed above, as well as the phytoplankton discussed below. They are very diverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.{{cite web |last=Starckx |first=Senne |date=31 October 2012 |url=http://www.flanderstoday.eu/current-affairs/place-sun |title=A place in the sun |quote=Algae is the crop of the future, according to researchers in Geel |archive-url=https://web.archive.org/web/20160304105525/http://www.flanderstoday.eu/current-affairs/place-sun |archive-date=4 March 2016 |work=Flanders Today |access-date=8 December 2012}} Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

Unicellular organisms are usually microscopic. There are exceptions. Mermaid's wineglass, a genus of subtropical green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying cell biology.{{cite journal |vauthors=Mandoli DF |year=1998 |title=Elaboration of Body Plan and Phase Change during Development of Acetabularia: How Is the Complex Architecture of a Giant Unicell Built? |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=49 |pages=173–198 |doi=10.1146/annurev.arplant.49.1.173 |pmid=15012232 |s2cid=6241264}} Another single-celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.{{cite journal |author1=Pierre Madl |author2=Maricela Yip |title=Literature Review of Caulerpa taxifolia |journal=BUFUS-Info |volume=19 |issue=31 |year=2004 |url=http://biophysics.sbg.ac.at/ct/caulerpa.htm |access-date=11 October 2019 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008200554/http://biophysics.sbg.ac.at/ct/caulerpa.htm |url-status=dead}}

File:Chlamydomonas globosa - 400x (13263097835).jpg|Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left

File:Инфузории Ophridium versatile.jpg|Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate{{cite journal|last1 = Duval|first1 = B.|last2 = Margulis|first2 = L.|year = 1995|title = The microbial community of Ophrydium versatile colonies: endosymbionts, residents, and tenants|journal = Symbiosis|volume = 18|pages = 181–210|pmid = 11539474 }}

File:Centric diatom.jpg|Centric diatom

File:Dinoflagellates.jpg|Dinoflagellates

File:Mixed phytoplankton community.png|Colonial algal chains

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Macroalgae are the larger, multicellular and more visible types of algae, commonly called seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a holdfast. Like microalgae, macroalgae (seaweeds) can be regarded as marine protists since they are not true plants. But they are not microorganisms, so they are not within the scope of this article.

File:Plankton collage.jpg, archaea, algae, protozoa and animals.|alt=Six relatively large variously shaped organisms with dozens of small light-colored dots all against a dark background. Some of the organisms have antennae that are longer than their bodies.]]

File:Super Blooms.ogv

{{see also|Marine protists}}

{{further|Bacterioplankton|Ichthyoplankton|Mycoplankton|Phycosphere}}

Plankton (from Greek for wanderers) are a diverse group of organisms that live in the water column of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.{{cite book

| last = Lalli

| first = C.

|author2=Parsons, T.

| year = 1993

| title = Biological Oceanography: An Introduction

| publisher = Butterworth-Heinemann

| isbn = 0-7506-3384-0}} Plankton are defined by their ecological niche, not by any phylogenetic or taxonomic classification. They are a crucial source of food for many marine animals, from forage fish to whales. Plankton can be divided into a plant-like component and an animal component.

[[File:Colony of Chaetoceros socialis.jpg|thumb|upright=1.1| Phytoplankton – such as this colony of Chaetoceros socialis – naturally gives off red fluorescent light which dissipates excess solar energy they cannot consume through photosynthesis. This glow can be detected by satellites as an indicator of how efficiently ocean phytoplankton is photosynthesising.[https://science.nasa.gov/science-news/science-at-nasa/2009/28may_redglow Eerie red glow traces ocean plant health] NASA Science, 28 May 2009.

{{cite journal | last1 = Behrenfeld | first1 = M.J. | last2 = Westberry | first2 = T.K. | last3 = Boss | first3 = E.S. | last4 = O'Malley | first4 = R.T. | last5 = Siegel | first5 = D.A. | last6 = Wiggert | first6 = J.D. | last7 = Franz | first7 = B.A. | last8 = McLain | first8 = C.R. | last9 = Feldman | first9 = G.C. | last10 = Doney | first10 = S.C. | last11 = Moore | first11 = J.K. | year = 2009 | title = Satellite-detected fluorescence reveals global physiology of ocean phytoplankton | journal = Biogeosciences | volume = 6 | issue = 5| pages = 779–794 | doi = 10.5194/bg-6-779-2009 | bibcode = 2009BGeo....6..779B | doi-access = free | hdl = 1912/2854 | hdl-access = free }}]]

Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek for plant). They are autotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton perform three crucial functions: they generate nearly half of the world atmospheric oxygen, they regulate ocean and atmospheric carbon dioxide levels, and they form the base of the marine food web. When conditions are right, blooms of phytoplankton algae can occur in surface waters. Phytoplankton are r-strategists which grow rapidly and can double their population every day. The blooms can become toxic and deplete the water of oxygen. However, phytoplankton numbers are usually kept in check by the phytoplankton exhausting available nutrients and by grazing zooplankton.Wassilieff, Maggy (2006) [https://teara.govt.nz/en/plankton/page-1 "Plankton – Plant plankton"], Te Ara – the Encyclopedia of New Zealand. Accessed: 2 November 2019.

Phytoplankton consist mainly of microscopic photosynthetic eukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some are protists.Lindsey, R., Scott, M. and Simmon, R. (2010) [https://earthobservatory.nasa.gov/features/Phytoplankton "What are phytoplankton"]. NASA Earth Observatory. Phytoplankton include cyanobacteria (above), diatoms, various other types of algae (red, green, brown, and yellow-green), dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chlorophytes, prasinophytes, and silicoflagellates. They form the base of the primary production that drives the ocean food web, and account for half of the current global primary production, more than the terrestrial forests.{{cite journal | last1 = Field | first1 = C. B. | last2 = Behrenfeld | first2 = M. J. | last3 = Randerson | first3 = J. T. | last4 = Falwoski | first4 = P. G. | year = 1998 | title = Primary production of the biosphere: Integrating terrestrial and oceanic components | journal = Science | volume = 281 | issue = 5374| pages = 237–240 | doi = 10.1126/science.281.5374.237 | pmid = 9657713 | bibcode = 1998Sci...281..237F | url = https://escholarship.org/uc/item/9gm7074q }}

File:Phytoplankton Lake Chuzenji.jpg|Phytoplankton are the foundation of the ocean food chain

File:Phytopla.jpg|They come in many shapes and sizes.

File:Phytoplankton - the foundation of the oceanic food chain.jpg|Colonial phytoplankton

File:Prochlorococcus marinus 2.jpg|The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production

File:Cyanobacterial Scum.JPG|Green cyanobacteria scum washed up on a rock in California

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Diatoms form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 per cent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live,{{cite journal | last1 = Treguer | first1 = P. | last2 = Nelson | first2 = D. M. | last3 = Van Bennekom | first3 = A. J. | last4 = Demaster | first4 = D. J. | last5 = Leynaert | first5 = A. | last6 = Queguiner | first6 = B. | year = 1995 | title = The Silica Balance in the World Ocean: A Reestimate | journal = Science | volume = 268 | issue = 5209| pages = 375–9 | pmid = 17746543 | doi = 10.1126/science.268.5209.375 | bibcode = 1995Sci...268..375T | s2cid = 5672525 }} and contribute nearly half of the organic material found in the oceans.

File:Diatoms (248 05) Various diatoms.jpg|Diatoms are one of the most common types of phytoplankton

File:Diatom Helipelta metil.jpg|Their protective shells (frustles) are made of silicon

File:Diatom - Triceratium favus.jpg

File:Diatom2.jpg|They come in many shapes and sizes

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Diatoms are enclosed in protective silica (glass) shells called frustules. Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes. The frustules of dead diatoms drift to the ocean floor where, over millions of years, they can build up as much as half a mile deep.{{Cite web|url=https://www.kcl.ac.uk/sspp/departments/geography/people/academic/drake/Research/The-Sahara-Megalakes-Project/Lake-Megachad.aspx|title=King's College London – Lake Megachad|website=www.kcl.ac.uk|language=en-GB|access-date=2018-05-05}}

File:Diatom algae Amphora sp.jpg|Silicified frustule of a pennate diatom with two overlapping halves

File:Fjouenne sbrmvr012w 20070924163039 small.jpg|Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel{{cite journal|last1 = Arsenieff|first1 = L.|last2 = Simon|first2 = N.|last3 = Rigaut-Jalabert|first3 = F.|last4 = Le Gall|first4 = F.|last5 = Chaffron|first5 = S.|last6 = Corre|first6 = E.|last7 = Com|first7 = E.|last8 = Bigeard|first8 = E.|last9 = Baudoux|first9 = A.C.|year = 2018|title = First Viruses Infecting the Marine Diatom Guinardia delicatula|journal = Frontiers in Microbiology|volume = 9|page = 3235|doi = 10.3389/fmicb.2018.03235 |pmid = 30687251|pmc = 6334475|doi-access = free}}

File:Ископаемая диатомовая водоросль.jpg|Fossil diatom

File:Pinnularia major.jpg|There are over 100,000 species of diatoms which account for 50% of the ocean's primary production

File:Structure of diatom frustules.png

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Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell covered with ornate circular plates or scales called coccoliths. The coccoliths are made from calcium carbonate. The calcite shells are important to the marine carbon cycle.Rost, B. and Riebesell, U. (2004) "Coccolithophores and the biological pump: responses to environmental changes". In: Coccolithophores: From Molecular Processes to Global Impact, pages 99–125, Springer. {{isbn|9783662062784}}. The term coccolithophore derives from the Greek for a seed carrying stone, referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white.Wassilieff, Maggy (2006) [https://teara.govt.nz/en/photograph/5130/a-coccolithophore "A coccolithophore"], Te Ara – the Encyclopedia of New Zealand. Accessed: 2 November 2019.

File:9Calcidiscus leptoporus, diploid, SEM, showing coccoliths.tif|Coccolithophores have plates called coccoliths

File:Coccolithus pelagicus.jpg|Coccolithus pelagicus ssp. braarudii

File:JRYSEM-247-05-azurapl.jpg|Syracosphaera azureaplaneta, named after the BBC documentary series
The Blue Planet

File:Emiliania huxleyi.jpg|The coccolithophore Emiliania huxleyi

File:Cwall99 lg.jpg|Algae bloom of Emiliania huxleyi off the southern coast of England

File:Discoaster surculus 01 (cropped).jpg|Extinct fossil

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File:Model of the energy generating mechanism in marine bacteria.jpg .{{cite journal |last1=DeLong |first1=E.F. |last2=Beja |first2=O. |year=2010 |title=The light-driven proton pump proteorhodopsin enhances bacterial survival during tough times |journal=PLOS Biology |volume=8 |issue=4 |id=e1000359 |doi=10.1371/journal.pbio.1000359 |pages=e1000359 |pmid=20436957 |pmc=2860490 |doi-access=free }}]]

{{main|Microbial rhodopsin}}

Phototrophic metabolism relies on one of three energy-converting pigments: chlorophyll, bacteriochlorophyll, and retinal. Retinal is the chromophore found in rhodopsins. The significance of chlorophyll in converting light energy has been written about for decades, but phototrophy based on retinal pigments is just beginning to be studied.{{cite journal | last1 = Gómez-Consarnau | first1 = L. | last2 = Raven | first2 = J.A. | last3 = Levine | first3 = N.M. | last4 = Cutter | first4 = L.S. | last5 = Wang | first5 = D. | last6 = Seegers | first6 = B. | last7 = Arístegui | first7 = J. | last8 = Fuhrman | first8 = J.A. | last9 = Gasol | first9 = J.M. | last10 = Sañudo-Wilhelmy | first10 = S.A. | year = 2019 | title = Microbial rhodopsins are major contributors to the solar energy captured in the sea | journal = Science Advances | volume = 5| issue = 8| page = eaaw8855| doi = 10.1126/sciadv.aaw8855 | pmid = 31457093 | pmc = 6685716 | bibcode = 2019SciA....5.8855G | doi-access = free }}

File:Salt ponds SF Bay (dro!d).jpg in salt evaporation ponds coloured purple by bacteriorhodopsin{{cite journal|last1=Oren|first1=Aharon|title=Molecular ecology of extremely halophilic Archaea and Bacteria|journal=FEMS Microbiology Ecology|volume=39|issue=1|year=2002|pages=1–7|issn=0168-6496|doi=10.1111/j.1574-6941.2002.tb00900.x|pmid=19709178|doi-access=free|bibcode=2002FEMME..39....1O }}]]

In 2000 a team of microbiologists led by Edward DeLong made a crucial discovery in the understanding of the marine carbon and energy cycles. They discovered a gene in several species of bacteria{{cite journal | last1 = Béja | first1 = O. | last2 = Aravind | first2 = L. | last3 = Koonin | first3 = E.V. | last4 = Suzuki | first4 = M.T. | last5 = Hadd | first5 = A. | last6 = Nguyen | first6 = L.P. | last7 = Jovanovich | first7 = S.B. | last8 = Gates | first8 = C.M. | last9 = Feldman | first9 = R.A. | last10 = Spudich | first10 = J.L. | last11 = Spudich | first11 = E.N. | year = 2000 | title = Bacterial rhodopsin: evidence for a new type of phototrophy in the sea | journal = Science | volume = 289 | issue = 5486| pages = 1902–1906 | doi = 10.1126/science.289.5486.1902 | pmid=10988064| bibcode = 2000Sci...289.1902B | s2cid = 1461255 }}{{cite web |url=http://academy.asm.org/index.php/news-views/interviews-with-fellows/366-ed-delong |title=Interviews with Fellows: Ed Delong |work=American Academy of Microbiology |access-date=2 July 2016 |archive-url=https://web.archive.org/web/20160807061814/http://academy.asm.org/index.php/news-views/interviews-with-fellows/366-ed-delong |archive-date=7 August 2016 |url-status=dead }} responsible for production of the protein rhodopsin, previously unheard of in bacteria. These proteins found in the cell membranes are capable of converting light energy to biochemical energy due to a change in configuration of the rhodopsin molecule as sunlight strikes it, causing the pumping of a proton from inside out and a subsequent inflow that generates the energy.Bacteria with Batteries, Popular Science, January 2001, Page 55. The archaeal-like rhodopsins have subsequently been found among different taxa, protists as well as in bacteria and archaea, though they are rare in complex multicellular organisms.{{cite journal | last1 = Béja | first1 = O. | last2 = Aravind | first2 = L. | last3 = Koonin | first3 = E.V. | last4 = Suzuki | first4 = M.T. | last5 = Hadd | first5 = A. | last6 = Nguyen | first6 = L.P. | last7 = Jovanovich | first7 = S.B. | last8 = Gates | first8 = C.M. | last9 = Feldman | first9 = R.A. | last10 = Spudich | first10 = J.L. | last11 = Spudich | first11 = E.N. | year = 2000 | title = Bacterial rhodopsin: evidence for a new type of phototrophy in the sea | journal = Science | volume = 289 | issue = 5486| pages = 1902–1906 | doi = 10.1126/science.289.5486.1902 | pmid = 10988064 | bibcode = 2000Sci...289.1902B }}{{cite journal|last1=Boeuf|first1=Dominique|last2=Audic|first2=Stéphane|last3=Brillet-Guéguen|first3=Loraine|last4=Caron|first4=Christophe|last5=Jeanthon|first5=Christian|title=MicRhoDE: a curated database for the analysis of microbial rhodopsin diversity and evolution|journal=Database|volume=2015|year=2015|pages=bav080|issn=1758-0463|doi=10.1093/database/bav080|pmid=26286928|pmc=4539915}}{{cite book|last1=Yawo|first1=Hiromu|last2=Kandori|first2=Hideki|last3=Koizumi|first3=Amane|title=Optogenetics: Light-Sensing Proteins and Their Applications|url=https://books.google.com/books?id=5M3WCQAAQBAJ&pg=PA3|access-date=30 September 2015|date=5 June 2015|publisher=Springer|isbn=978-4-431-55516-2|pages=3–4}}

Research in 2019 shows these "sun-snatching bacteria" are more widespread than previously thought and could change how oceans are affected by global warming. "The findings break from the traditional interpretation of marine ecology found in textbooks, which states that nearly all sunlight in the ocean is captured by chlorophyll in algae. Instead, rhodopsin-equipped bacteria function like hybrid cars, powered by organic matter when available — as most bacteria are — and by sunlight when nutrients are scarce."[https://pressroom.usc.edu/a-tiny-marine-microbe-could-play-a-big-role-in-climate-change/ A tiny marine microbe could play a big role in climate change] University of Southern California, Press Room, 8 August 2019.

There is an astrobiological conjecture called the Purple Earth hypothesis which surmises that original life forms on Earth were retinal-based rather than chlorophyll-based, which would have made the Earth appear purple instead of green.{{Cite journal |last1=DasSarma |first1=Shiladitya |last2=Schwieterman |first2=Edward W. |date=11 October 2018 |title=Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/early-evolution-of-purple-retinal-pigments-on-earth-and-implications-for-exoplanet-biosignatures/63A1AD8AF544BEEF4C6D4A2D53130327 |journal=International Journal of Astrobiology |volume=20 |issue=3 |pages=241–250 |doi=10.1017/S1473550418000423 |issn=1473-5504 |arxiv=1810.05150 |bibcode=2018arXiv181005150D |s2cid=119341330}}{{cite journal |last1=Sparks |first1=William B. |last2=DasSarma |first2=S. |last3=Reid |first3=I. N. |title=Evolutionary Competition Between Primitive Photosynthetic Systems: Existence of an early purple Earth? |journal=American Astronomical Society Meeting Abstracts |volume=38 |page=901 |bibcode=2006AAS...209.0605S |date=December 2006}}

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During the 1930s Alfred C. Redfield found similarities between the composition of elements in phytoplankton and the major dissolved nutrients in the deep ocean.{{cite book |last1=Redfield |first1=Alfred C. |year=1934 |chapter=On the Proportions of Organic Derivatives in Sea Water and their Relation to the Composition of Plankton |pages=176–92 |editor1-last=Johnstone |editor1-first=James |editor2-last=Daniel |editor2-first=Richard Jellicoe |title=James Johnstone Memorial Volume |location=Liverpool |publisher=University Press of Liverpool |oclc=13993674}} Redfield proposed that the ratio of carbon to nitrogen to phosphorus (106:16:1) in the ocean was controlled by the phytoplankton's requirements, as phytoplankton subsequently release nitrogen and phosphorus as they remineralize. This ratio has become known as the Redfield ratio, and is used as a fundamental principle in describing the stoichiometry of seawater and phytoplankton evolution.{{cite journal |last1=Arrigo |first1=Kevin R. |title=Marine microorganisms and global nutrient cycles |journal=Nature |volume=437 |issue=7057 |pages=349–55 |year=2005 |pmid=16163345 |doi=10.1038/nature04159 |bibcode=2005Natur.437..349A|s2cid=62781480 }}

However, the Redfield ratio is not a universal value and can change with things like geographical latitude.{{cite journal | last1 = Martiny | first1 = A.C. | last2 = Pham | first2 = C.T. | last3 = Primeau | first3 = F.W. | last4 = Vrugt | first4 = J.A. | last5 = Moore | first5 = J.K. | last6 = Levin | first6 = S.A. | last7 = Lomas | first7 = M.W. | year = 2013 | title = Strong latitudinal patterns in the elemental ratios of marine plankton and organic matter | url = http://www.escholarship.org/uc/item/68n582hp| journal = Nature Geoscience | volume = 6 | issue = 4| pages = 279–283 | doi = 10.1038/ngeo1757 | bibcode = 2013NatGe...6..279M | s2cid = 5677709 }} Based on allocation of resources, phytoplankton can be classified into three different growth strategies: survivalist, bloomer and generalist. Survivalist phytoplankton has a high N:P ratio (>30) and contains an abundance of resource-acquisition machinery to sustain growth under scarce resources. Bloomer phytoplankton has a low N:P ratio (<10), contains a high proportion of growth machinery and is adapted to exponential growth. Generalist phytoplankton has similar N:P to the Redfield ratio and contain relatively equal resource-acquisition and growth machinery.

The f-ratio is the fraction of total primary production fuelled by nitrate (as opposed to that fuelled by other nitrogen compounds such as ammonium). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production.{{cite journal | doi=10.1038/282677a0 | last1=Eppley | first1=R.W. |last2=Peterson |first2=B.J. | year=1979 | title=Particulate organic matter flux and planktonic new production in the deep ocean | journal=Nature | volume=282 | pages=677–680 | issue=5740|bibcode = 1979Natur.282..677E | s2cid=42385900 }}

Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek for animal). They are heterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, foraminiferans, radiolarians, some dinoflagellates and marine microanimals. Macroscopic zooplankton (not generally covered in this article) include pelagic cnidarians, ctenophores, molluscs, arthropods and tunicates, as well as planktonic arrow worms and bristle worms.

Microzooplankton: major grazers of the plankton...

File:Copepod 2 with eggs.jpg|Copepods eat phytoplankton. This one is carrying eggs.

File:Tintinnid ciliate Favella.jpg|Tintinnid ciliate Favella

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Many species of protozoa (eukaryotes) and bacteria (prokaryotes) prey on other microorganisms; the feeding mode is evidently ancient, and evolved many times in both groups.{{cite journal |last1=Velicer |first1=Gregory J. |last2=Mendes-Soares |first2=Helena |title=Bacterial predators |journal=Cell |date=2007 |volume=19 |issue=2 |pages=R55–R56 |doi=10.1016/j.cub.2008.10.043 |pmid=19174136 |s2cid=5432036 |url=https://www.cell.com/current-biology/pdf/S0960-9822(08)01412-7.pdf|doi-access=free }}{{cite journal |author=Stevens, Alison N. P. |date=2010 |title=Predation, Herbivory, and Parasitism |journal=Nature Education Knowledge |volume=3 |issue=10 |page=36 |url=https://www.nature.com/scitable/knowledge/library/predation-herbivory-and-parasitism-13261134}}{{cite book | last1=Jurkevitch | first1=Edouard | last2=Davidov | first2=Yaacov | title=Predatory Prokaryotes | url=https://archive.org/details/predatoryprokary00jurk | url-access=limited | chapter=Phylogenetic Diversity and Evolution of Predatory Prokaryotes | publisher=Springer |date=2006 | isbn=978-3-540-38577-6 | doi=10.1007/7171_052 | pages=[https://archive.org/details/predatoryprokary00jurk/page/n17 11]–56}} Among freshwater and marine zooplankton, whether single-celled or multi-cellular, predatory grazing on phytoplankton and smaller zooplankton is common, and found in many species of nanoflagellates, dinoflagellates, ciliates, rotifers, a diverse range of meroplankton animal larvae, and two groups of crustaceans, namely copepods and cladocerans.{{cite journal | last1=Hansen | first1=Per Juel | last2=Bjørnsen | first2=Peter Koefoed | last3=Hansen | first3=Benni Winding | title=Zooplankton grazing and growth: Scaling within the 2-2,-μm body size range | journal=Limnology and Oceanography | volume=42 | issue=4 | year=1997 | doi=10.4319/lo.1997.42.4.0687 | pages=687–704| bibcode=1997LimOc..42..687H | doi-access=free }} summarizes findings from many authors.

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Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.Wassilieff, Maggy (2006) [http://www.TeAra.govt.nz/en/photograph/5138/radiolarian-fossils "Plankton - Animal plankton"], Te Ara - the Encyclopedia of New Zealand. Accessed: 2 November 2019.

File:Mikrofoto.de-Radiolarien 6.jpg|Like diatoms, radiolarians come in many shapes

File:Theocotylissa ficus Ehrenberg - Radiolarian (34638920262).jpg|Also like diatoms, radiolarian shells are usually made of silicate

File:Acantharian radiolarian Xiphacantha (Haeckel).jpg|However acantharian radiolarians have shells made from strontium sulfate crystals

File:Spherical radiolarian 2.jpg|Cutaway schematic diagram of a spherical radiolarian shell

File:Cladococcus abietinus.jpg| Cladococcus abietinus

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Like radiolarians, foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) of silica. Most forams are benthic, but about 40 species are planktic.{{cite book |first1=C. |last1=Hemleben |first2=O.R. |last2=Anderson |first3=M. |last3=Spindler |title=Modern Planktonic Foraminifera |url=https://books.google.com/books?id=NaHOmAEACAAJ |year=1989 |publisher=Springer-Verlag |isbn=978-3-540-96815-3}} They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.

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File:EB1911 Foraminifera - Section of Rotalia beccarii.jpg|section showing chambers of a spiral foram

File:Live Ammonia tepida.jpg|Live Ammonia tepida streaming granular ectoplasm for catching food

File:Planktic Foraminifera of the northern Gulf of Mexico.jpg|Group of planktonic forams

File:Nummulitids.jpg|Fossil nummulitid forams of various sizes from the Eocene

File:All Gizah Pyramids.jpg|The Egyptian pyramids were constructed from limestone that contained nummulites.[http://www.ucl.ac.uk/GeolSci/micropal/foram.html#histofstudy Foraminifera: History of Study], University College London. Retrieved: 18 November 2019.

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A number of forams are mixotrophic (see below). These have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates. Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.{{cite book |author=Finlay, B.J. |chapter=Microaerophily |editor=Marshall, K.C. |chapter-url= https://books.google.com/books?id=QvvlBwAAQBAJ&dq=%22The+symbiont-bearing+foraminifera+are+particularly+common+in+nutrient-poor+oceanic+waters%22&pg=PA22 |title=Advances in Microbial Ecology |volume=11 |page=22 |publisher=Springer Science & Business Media |year=2013 |isbn=978-1-4684-7612-5}} Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.{{Cite journal |title=Benthic Foraminifera of dysoxic sediments: chloroplast sequestration and functional morphology |year=1999 |last=Bernhard |first=J. M. |author2=Bowser, S.M. |journal=Earth-Science Reviews |volume=46 |issue=1 |pages=149–165 |doi=10.1016/S0012-8252(99)00017-3 |bibcode=1999ESRv...46..149B}}

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File:Amoeba engulfing diatom.ogv

Amoeba can be shelled (testate) or naked.

File:Cyphoderia ampulla - Testate amoeba - 160x (14997391862).jpg|Testate amoeba, Cyphoderia sp.

File:Arcella sp.jpg|Shell or test of a testate amoeba, Arcella sp.

File:Collection Penard MHNG Specimen 533-2-1 Pamphagus granulatus.tif|Xenogenic testate amoeba covered in diatoms (from [https://commons.wikimedia.org/wiki/Commons:P%C3%A9nard_project Penard's Amoeba Collection])

File:Chaos carolinense.jpg|Naked amoeba, Chaos sp.

File:Amoeba proteus 2.jpg|Naked amoeba showing food vacuoles and ingested diatom

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File:Oxytricha chlorelligera - 400x (10403483023).jpg|Oxytricha chlorelligera

File:Stylonychia putrina - 160x - II (13215594964).jpg|Stylonychia putrina

File:Holophyra ovum - 400x (9836710085).jpg|Holophyra ovum

File:Mesodinium rubrum.jpg|Mesodinium rubrum produce deep red blooms using enslaved chloroplasts from their algal prey{{hsp}}{{cite journal |doi=10.1073/pnas.1512538112 |title=Space station image captures a red tide ciliate bloom at high spectral and spatial resolution |year=2015 |last1=Dierssen |first1=Heidi |last2=McManus |first2=George B. |last3=Chlus |first3=Adam |last4=Qiu |first4=Dajun |last5=Gao |first5=Bo-Cai |last6=Lin |first6=Senjie |journal=Proceedings of the National Academy of Sciences|volume=112|issue=48|pages=14783–14787 |pmid=26627232 |pmc=4672822 |bibcode=2015PNAS..11214783D |doi-access=free}}

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File:Mikrofoto.de-Blepharisma japonicum 15.jpg|Blepharisma japonicum

File:Из жизни инфузорий.webm|Several taxa of ciliates interacting

File:Blepharisma americana.ogv|Blepharisma americanum swimming in a drop of pond water with other microorganisms

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{{see also|Mixotrophic dinoflagellate}}

A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.{{cite web |url=https://www.irishexaminer.com/lifestyle/outdoors/richard-collins/beware-the-mixotrophs--they-can-destroy-entire-ecosystems-in-a-matter-of-hours-430358.html |author=Richard Collins |date=2016-11-14 |title=Beware the mixotrophs – they can destroy entire ecosystems 'in a matter of hours' |work=Irish Examiner}} There are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and others that acquire them through kleptoplasty or by enslaving the entire phototrophic cell.{{cite web |url=https://phys.org/news/2017-08-microscopic-body-snatchers-infest-oceans.html |title=Microscopic body snatchers infest our oceans |date=2017-08-02 |website=Phys.org}}

The distinction between plants and animals often breaks down in very small organisms. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic.{{cite journal |author= Eiler A |title= Evidence for the Ubiquity of Mixotrophic Bacteria in the Upper Ocean: Implications and Consequences |journal=Appl Environ Microbiol |volume=72 |issue=12 |pages= 7431–7|date=December 2006|pmid=17028233 |doi=10.1128/AEM.01559-06|pmc=1694265|bibcode= 2006ApEnM..72.7431E }} They can take advantage of different environmental conditions.{{cite journal |vauthors=Katechakis A, Stibor H |title=The mixotroph Ochromonas tuberculata may invade and suppress specialist phago- and phototroph plankton communities depending on nutrient conditions |journal= Oecologia|volume= 148 |issue= 4|pages=692–701 |date=July 2006|pmid=16568278 |doi= 10.1007/s00442-006-0413-4|bibcode=2006Oecol.148..692K |s2cid=22837754 }}

Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarian biomass was mixotrophic.

Phaeocystis is an important algal genus found as part of the marine phytoplankton around the world. It has a polymorphic life cycle, ranging from free-living cells to large colonies.{{Cite journal|title = Phaeocystis blooms in the global ocean and their controlling mechanisms: a review|journal = Journal of Sea Research|date = 2005-01-01|pages = 43–66|volume = 53|series = Iron Resources and Oceanic Nutrients – Advancement of Global Environmental Simulations|issue = 1–2|doi = 10.1016/j.seares.2004.01.008|first1 = Véronique|last1 = Schoemann|first2 = Sylvie|last2 = Becquevort|first3 = Jacqueline|last3 = Stefels|first4 = Véronique|last4 = Rousseau|first5 = Christiane|last5 = Lancelot|citeseerx = 10.1.1.319.9563|bibcode = 2005JSR....53...43S}} It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms.{{cite web |url= http://www.phaeocystis.org/|title = Welcome to the Phaeocystis antarctica genome sequencing project homepage }} As a result, Phaeocystis is an important contributor to the marine carbon{{cite journal |title=Rapid and early export of Phaeocystis antarctica blooms in the Ross Sea, Antarctica |journal = Nature|pages = 595–598|volume = 404|issue = 6778|doi = 10.1038/35007061|pmid = 10766240|first1 = G. R.|last1 = DiTullio|first2 = J. M.|last2 = Grebmeier|author-link2=Jacqueline M. Grebmeier|first3 = K. R.|last3 = Arrigo|first4 = M. P.|last4 = Lizotte|first5 = D. H.|last5 = Robinson|first6 = A.|last6 = Leventer|first7 = J. P.|last7 = Barry|first8 = M. L.|last8 = VanWoert|first9 = R. B.|last9 = Dunbar|year = 2000|bibcode = 2000Natur.404..595D|s2cid = 4409009}} and sulfur cycles.{{Cite journal|title = DMSP-lyase activity in a spring phytoplankton bloom off the Dutch coast, related to Phaeocystis sp. abundance|journal = Marine Ecology Progress Series|date = 1995-07-20|pages = 235–243|volume = 123|doi = 10.3354/meps123235|first1 = Stefels|last1 = J|first2 = Dijkhuizen|last2 = L|first3 = Gieskes|last3 = WWC|url = https://pure.rug.nl/ws/files/62552225/DMSP_lyase_activity_in_a_spring_phytoplankton_bloom_off_the_Dutch_coast.pdf|bibcode = 1995MEPS..123..235S|doi-access = free}} Phaeocystis species are endosymbionts to acantharian radiolarians.{{cite journal|last1=Decelle|first1=Johan|last2=Simó|first2=Rafel|last3=Galí|first3=Martí|last4=Vargas|first4=Colomban de|last5=Colin|first5=Sébastien|last6=Desdevises|first6=Yves|last7=Bittner|first7=Lucie|last8=Probert|first8=Ian|last9=Not|first9=Fabrice|date=2012-10-30|title=An original mode of symbiosis in open ocean plankton|journal=Proceedings of the National Academy of Sciences|language=en|volume=109|issue=44|pages=18000–18005|doi=10.1073/pnas.1212303109|issn=0027-8424|pmid=23071304|pmc=3497740|bibcode=2012PNAS..10918000D|doi-access=free}}{{Cite journal|last1=Mars Brisbin|first1=Margaret|last2=Grossmann|first2=Mary M.|last3=Mesrop|first3=Lisa Y.|last4=Mitarai|first4=Satoshi|date=2018|title=Intra-host Symbiont Diversity and Extended Symbiont Maintenance in Photosymbiotic Acantharea (Clade F)|journal=Frontiers in Microbiology|language=en|volume=9|pages=1998|doi=10.3389/fmicb.2018.01998|pmid=30210473|pmc=6120437|issn=1664-302X|doi-access=free}}

File:Tintinnid ciliate Favella.jpg|Tintinnid ciliate Favella

File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|Euglena mutabilis, a photosynthetic flagellate

File:Stichotricha secunda - 400x (14974779356).jpg|Zoochlorellae (green) living inside the ciliate Stichotricha secunda

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{{see also|Mixotrophic dinoflagellate|Predatory dinoflagellate}}

Dinoflagellates are part of the algae group, and form a phylum of unicellular flagellates with about 2,000 marine species.{{cite journal|author=Gómez F |title=A checklist and classification of living dinoflagellates (Dinoflagellata, Alveolata) |journal=CICIMAR Oceánides |volume=27 |issue=1 |pages=65–140 |year=2012 |doi=10.37543/oceanides.v27i1.111 |doi-access=free }} The name comes from the Greek "dinos" meaning whirling and the Latin "flagellum" meaning a whip or lash. This refers to the two whip-like attachments (flagella) used for forward movement. Most dinoflagellates are protected with red-brown, cellulose armour. Like other phytoplankton, dinoflagellates are r-strategists which under right conditions can bloom and create red tides. Excavates may be the most basal flagellate lineage.

By trophic orientation dinoflagellates cannot be uniformly categorized. Some dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).{{Cite journal | last1 = Stoecker | first1 = D. K. | title = Mixotrophy among Dinoflagellates | doi = 10.1111/j.1550-7408.1999.tb04619.x | journal = The Journal of Eukaryotic Microbiology | volume = 46 | issue = 4 | pages = 397–401 | year = 1999 | s2cid = 83885629 | name-list-style = vanc}} Some species are endosymbionts of marine animals and other protists, and play an important part in the biology of coral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates are mixotrophic and could also be classified as phytoplankton.

The toxic dinoflagellate Dinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the red Myrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".

File:Gyrodinium dinoflagellate.jpg|Gyrodinium, one of the few naked dinoflagellates which lack armour

File:Protoperidinium dinoflagellate.jpg|The dinoflagellate Protoperidinium extrudes a large feeding veil to capture prey

File:Radiolarian - Podocyrtis (Lampterium) mitra Ehrenberg - 160x.jpg|Nassellarian radiolarians can be in symbiosis with dinoflagellates

File:Dinophysis acuta.jpg| The dinoflagellate Dinophysis acuta

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Dinoflagellates often live in symbiosis with other organisms. Many nassellarian radiolarians house dinoflagellate symbionts within their tests.{{Cite book |title=Handbook of the Protists |last1=Boltovskoy |first1=Demetrio |last2=Anderson |first2=O. Roger |last3=Correa |first3=Nancy M. |date=2017|publisher=Springer, Cham |isbn=9783319281476 |pages=731–763 |language=en |doi=10.1007/978-3-319-28149-0_19}} The nassellarian provides ammonium and carbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.{{Cite book |title=Radiolaria |last=Anderson |first=O. R.|publisher=Springer Science & Business Media|year=1983}} There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.{{Cite journal |last1=Gast|first1=R. J.|last2=Caron|first2=D. A.|date=1996-11-01|title=Molecular phylogeny of symbiotic dinoflagellates from planktonic foraminifera and radiolaria|journal=Molecular Biology and Evolution |language=en |volume=13 |issue=9|pages=1192–1197 |doi=10.1093/oxfordjournals.molbev.a025684|pmid=8896371|issn=0737-4038|doi-access=}}

Some dinoflagellates are bioluminescent. At night, ocean water can light up internally and sparkle with blue light because of these dinoflagellates.{{cite book |last1=Castro |first1=Peter |first2=Michael E. |last2=Huber |title=Marine Biology |publisher=McGraw Hill |year=2010 |isbn=978-0071113021 |pages=95 |edition=8th | name-list-style = vanc}}{{cite journal | vauthors = Hastings JW | title = Chemistries and colors of bioluminescent reactions: a review | journal = Gene | volume = 173 | issue = 1 Spec No | pages = 5–11 | year = 1996 | pmid = 8707056 | doi = 10.1016/0378-1119(95)00676-1}} Bioluminescent dinoflagellates possess scintillons, individual cytoplasmic bodies which contain dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.{{cite journal | vauthors = Haddock SH, Moline MA, Case JF | title = Bioluminescence in the sea | journal = Annual Review of Marine Science | volume = 2 | pages = 443–93 | date = 2009 | pmid = 21141672 | doi = 10.1146/annurev-marine-120308-081028 | bibcode = 2010ARMS....2..443H| s2cid = 3872860 }}

File:Ceratium tripos.jpg| Tripos muelleri is recognisable by its U-shaped horns

File:Archives de zoologie expérimentale et générale (1920) (20299351186).jpg|Oodinium, a genus of parasitic dinoflagellates, causes velvet disease in fish{{cite web|title=Protozoa Infecting Gills and Skin|url=http://www.merckvetmanual.com:80/mvm/index.jsp?cfile=htm/bc/170410.htm|publisher=The Merck Veterinary Manual|access-date= 4 November 2019|archive-url=https://web.archive.org/web/20160303221140/http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm%2Fbc%2F170410.htm|archive-date=3 March 2016 |url-status=dead}}

File:Karenia brevis.jpg|Karenia brevis produces red tides highly toxic to humans{{Cite journal |last1=Brand |first1=Larry E. |last2=Campbell |first2=Lisa |last3=Bresnan |first3=Eileen |title=Karenia: The biology and ecology of a toxic genus|journal=Harmful Algae |volume=14 |pages=156–178 |doi=10.1016/j.hal.2011.10.020 |year=2012 |pmid=36733478 |pmc=9891709|bibcode=2012HAlga..14..156B }}

File:Algal bloom(akasio) by Noctiluca in Nagasaki.jpg|Red tide

File:Noctiluca scintillans unica.jpg|Noctiluca scintillans, a bioluminescent dinoflagellate{{cite journal |last1=Buskey |first1=E.J. |year=1995 |title=Growth and bioluminescence of Noctiluca scintillans on varying algal diets |journal=Journal of Plankton Research |volume=17 |issue=1 |pages=29–40 |doi=10.1093/plankt/17.1.29}}

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File:Ötzi the Iceman - Dagger 2.png who lived during the Copper Age. The blade is made of chert containing radiolarians, calcispheres, calpionellids and a few sponge spicules. The presence of calpionellids, which are extinct, was used to date this dagger.{{cite journal |last1=Wierer |first1=U. |last2=Arrighi |first2=S. |last3=Bertola |first3=S. |last4=Kaufmann |first4=G. |last5=Baumgarten |first5=B. |last6=Pedrotti |first6=A. |last7=Pernter |first7=P. |last8=Pelegrin |first8=J. |year=2018 |title=The Iceman's lithic toolkit: Raw material, technology, typology and use |journal=PLOS ONE |volume=13 |issue=6| page=e0198292 |doi=10.1371/journal.pone.0198292 |pmid=29924811 |pmc=6010222 |bibcode=2018PLoSO..1398292W |doi-access=free}}]]

{{further|Marine sediments|microfossils|protist shells}}

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.

Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger expedition.Thomson, Charles Wyville (2014) [https://books.google.com/books?id=zcFkAwAAQBAJ&q=radiolarian+ooze Voyage of the Challenger : The Atlantic] Cambridge University Press, page235.

{{ISBN|9781108074759}}. A biogenic ooze is a pelagic sediment containing at least 30 per cent from the skeletal remains of marine organisms.

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File:Diatomaceous Earth BrightField.jpg|Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify)

File:Nanoplankton-fossil-sediment hg.jpg|Calcareous microfossils from marine sediment consisting mainly of star-shaped discoaster with a sprinkling of coccoliths

File:FMIB 47660 Shells from Globigerina Ooze.jpeg|Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

File:PSM V44 D483 Globigerina ooze.jpg|Illustration of a Globigerina ooze

File:MarmoCipollino FustoBasMassenzioRoma.jpg| Marble can contain protist microfossils of foraminiferans, coccolithophores, calcareous nannoplankton and algae, ostracodes, pteropods, calpionellids and bryozoa

File:Coober Pedy Opal Doublet.jpg| Opal can contain protist microfossils of diatoms, radiolarians, silicoflagellates and ebridiansHaq B.U. and Boersma A. (Eds.) (1998) [https://books.google.com/books?id=0XezCm7IwpUC&q=%22Introduction+to+Marine+Micropaleontology%22 Introduction to Marine Micropaleontology] Elsevier. {{ISBN|9780080534961}}

File:Carbonate-Silicate Cycle (Carbon Cycle focus).jpg|Carbonate–silicate cycle

File:Marine sediment thickness (map).jpg|File:Marine sediment thickness (legend).jpgThickness of marine sediments

File:Distribution of sediment types on the seafloor.png |Distribution of sediment types on the seafloor
Within each colored area, the type of material shown is what dominates, although other materials are also likely to be present.
For further information, [https://en.wikibooks.org/wiki/Historical_Geology/Marine_sediments see here]

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File:Archaea rock.jpg

{{see also|Seabed#Sediments|bioturbation|bioirrigation}}

Marine microbenthos are microorganisms that live in the benthic zone of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. The word benthos comes from Greek, meaning "depth of the sea". Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In shallow waters, seagrass meadows, coral reefs and kelp forests provide particularly rich habitats. In photic zones benthic diatoms dominate as photosynthetic organisms. In intertidal zones changing tides strongly control opportunities for microbenthos.

File:Elphidium-incertum hg.jpg|Elphidium a widespread abundant genus of benthic forams

File:FMIB 50025 Textilaria.jpeg|Heterohelix, an extinct genus of benthic forams

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Both foraminifera and diatoms have planktonic and benthic forms, that is, they can drift in the water column or live on sediment at the bottom of the ocean. Either way, their shells end up on the seafloor after they die. These shells are widely used as climate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable oxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water.Bruckner, Monica (2020) [https://serc.carleton.edu/microbelife/topics/proxies/index.html"Paleoclimatology: How Can We Infer Past Climates?"] SERC, Carleton College. Modified 23 July 2020. Retrieved 10 September 2020.

The sudden extinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.[https://www.livescience.com/oldest-climate-record-ever-cenozoic-era.html Earth barreling toward 'Hothouse' state not seen in 50 million years, epic new climate record shows] LiveScience, 10 September 2020.{{cite journal |doi-access=free |doi=10.1126/science.aba6853 |title=An astronomically dated record of Earth's climate and its predictability over the last 66 million years |journal=Science |date=11 September 2020 |volume=369 |issue=6509 |pages=1383–1387 |last1=Westerhold |first1=Thomas |last2=Marwan |first2=Norbert |last3=Drury |first3=Anna Joy |last4=Liebrand |first4=Diederik |last5=Agnini |first5=Claudia |last6=Anagnostou |first6=Eleni |last7=Barnet |first7=James S. K. |last8=Bohaty |first8=Steven M. |last9=De Vleeschouwer |first9=David |last10=Florindo |first10=Fabio |last11=Frederichs |first11=Thomas |last12=Hodell |first12=David A. |last13=Holbourn |first13=Ann E. |last14=Kroon |first14=Dick |last15=Lauretano |first15=Vittoria |last16=Littler |first16=Kate |last17=Lourens |first17=Lucas J. |last18=Lyle |first18=Mitchell |last19=Pälike |first19=Heiko |last20=Röhl |first20=Ursula |last21=Tian |first21=Jun |last22=Wilkens |first22=Roy H. |last23=Wilson |first23=Paul A. |last24=Zachos |first24=James C. |pmid=32913105 |bibcode=2020Sci...369.1383W |hdl=11577/3351324 |s2cid=221593388 |hdl-access=free}}

Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years.{{cite web |author=Bob Yirka |url=http://phys.org/news/2013-08-soil-beneath-ocean-harbor-bacteria.html |date=29 Aug 2013 |title=Soil beneath ocean found to harbor long lived bacteria, fungi and viruses |website=Phys.org}} These are slowly metabolizing and not in a dormant state. Some Actinomycetota found in Siberia are estimated to be half a million years old.{{Cite news |author=Ian Sample |title=The oldest living organisms: ancient survivors with a fragile future |url=http://www.guardian.co.uk/theobserver/2010/may/02/rachel-sussman-oldest-plants |work=The Guardian |date=2 May 2010}}{{Cite web |title=The oldest living thing in the world |url=https://www.itsokaytobesmart.com/post/91481365622/siberian-actinobacteria-oldest-living-thing |website=It's Okay to be Smart |author=Joe Hanson |access-date=2018-07-13 |archive-url=https://web.archive.org/web/20180713074804/https://www.itsokaytobesmart.com/post/91481365622/siberian-actinobacteria-oldest-living-thing |archive-date=2018-07-13 |url-status=dead}}{{Cite journal |title=Ancient bacteria show evidence of DNA repair |first1=Eske |last1=Willerslev |first2=Duane |last2=Froese |first3=David |last3=Gilichinsky |first4=Regin |last4=Rønn |first5=Michael |last5=Bunce |first6=Maria T. |last6=Zuber |first7=M. Thomas P.|last7=Gilbert |first8=Tina |last8=Brand |first9=Kasper |last9=Munch |first10=Rasmus |last10=Nielsen |first11=Mikhail |last11=Mastepanov |first12=Torben R. |last12=Christensen |first13=Martin B.|last13=Hebsgaard |first14=Sarah Stewart |last14=Johnson |date=4 September 2007 |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=36 |pages=14401–14405 |doi=10.1073/pnas.0706787104 |pmid=17728401 |pmc=1958816 |bibcode=2007PNAS..10414401J |doi-access=free}}

{{main|Marine microbiomes}}

File:Types of microbial symbioses.jpg Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].]]

{{further|marine microbial symbiosis|holobiont}}

The concept of the holobiont was initially defined by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation as an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit.{{Cite book |url=https://mitpress.mit.edu/books/symbiosis-source-evolutionary-innovation |title=Symbiosis as a Source of Evolutionary Innovation |last1=Margulis |first1=Lynn|last2=Fester |first2=René |date=1991 |publisher=MIT Press |isbn=9780262132695}} The components of a holobiont are individual species or bionts, while the combined genome of all bionts is the hologenome.{{Cite journal |last1=Bordenstein|first1=Seth R. |last2=Theis |first2=Kevin R. |date=2015 |title=Host Biology in Light of the Microbiome: Ten Principles of Holobionts and Hologenomes |journal=PLOS Biol |volume=13 |issue=8 |pages=e1002226 |pmc=4540581 |pmid=26284777 |doi=10.1371/journal.pbio.1002226 |issn=1545-7885 |doi-access=free}}

The concept has subsequently evolved since this original definition,Theis, K.R., Dheilly, N.M., Klassen, J.L., Brucker, R.M., Baines, J.F., Bosch, T.C., Cryan, J.F., Gilbert, S.F., Goodnight, C.J., Lloyd, E.A. and Sapp, J. (2016) "Getting the hologenome concept right: an eco-evolutionary framework for hosts and their microbiomes". Msystems, 1(2). {{doi|10.1128/mSystems.00028-16}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]. with the focus moving to the microbial species associated with the host. Thus the holobiont includes the host, virome, microbiome, and other members, all of which contribute in some way to the function of the whole.Bulgarelli, D., Schlaeppi, K., Spaepen, S., Van Themaat, E.V.L. and Schulze-Lefert, P. (2013) "Structure and functions of the bacterial microbiota of plants". Annual review of plant biology, 64: 807–838. {{doi|10.1146/annurev-arplant-050312-120106}}.{{Cite journal |last1=Vandenkoornhuyse |first1=Philippe |last2=Quaiser |first2=Achim |last3=Duhamel |first3=Marie |last4=Le Van |first4=Amandine |last5=Dufresne |first5=Alexis |date=2015-06-01 |title=The importance of the microbiome of the plant holobiont |journal=The New Phytologist |volume=206 |issue=4 |pages=1196–1206 |doi=10.1111/nph.13312 |issn=1469-8137 |pmid=25655016 |doi-access=free}} A holobiont typically includes a eukaryote host and all of the symbiotic viruses, bacteria, fungi, etc. that live on or inside it.Sánchez-Cañizares, C., Jorrín, B., Poole, P.S. and Tkacz, A. (2017) "Understanding the holobiont: the interdependence of plants and their microbiome". Current Opinion in Microbiology, 38: 188–196. {{doi|10.1016/j.mib.2017.07.001}}.

However, there is controversy over whether holobionts can be viewed as single evolutionary units.Douglas, A.E. and Werren, J.H. (2016) "Holes in the hologenome: why host-microbe symbioses are not holobionts". MBio, 7(2). {{doi|10.1128/mBio.02099-15}}.

Reef-building corals are well-studied holobionts that include the coral itself (a eukaryotic invertebrate within class Anthozoa), photosynthetic dinoflagellates called zooxanthellae (Symbiodinium), and associated bacteria and viruses.Knowlton, N. and Rohwer, F. (2003) "Multispecies microbial mutualisms on coral reefs: the host as a habitat". The American Naturalist, 162(S4): S51-S62. {{doi|10.1086/378684}}. Co-evolutionary patterns exist for coral microbial communities and coral phylogeny.{{Cite journal |last1=Pollock |first1=F. Joseph |last2=McMinds |first2=Ryan |last3=Smith |first3=Styles |last4=Bourne |first4=David G. |last5=Willis |first5=Bette L. |last6=Medina |first6=Mónica |last7=Thurber |first7=Rebecca Vega |last8=Zaneveld |first8=Jesse R. |date=2018-11-22 |title=Coral-associated bacteria demonstrate phylosymbiosis and cophylogeny |journal=Nature Communications |volume=9 |issue=1 |page=4921 |doi=10.1038/s41467-018-07275-x |pmid=30467310 |pmc=6250698 |bibcode=2018NatCo...9.4921P |issn=2041-1723 |doi-access=free}}

File:Trophic connections of the coral holobiont in the planktonic food web.jpg| Coral holobiontThompson, J.R., Rivera, H.E., Closek, C.J. and Medina, M. (2015) "Microbes in the coral holobiont: partners through evolution, development, and ecological interactions". Frontiers in cellular and infection microbiology, 4: 176. {{doi|10.3389/fcimb.2014.00176}}.

File:The sponge holobiont.webp| Sponge holobiontPita, L., Rix, L., Slaby, B.M., Franke, A. and Hentschel, U. (2018) "The sponge holobiont in a changing ocean: from microbes to ecosystems". Microbiome, 6(1): 46. {{doi|10.1186/s40168-018-0428-1}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

File:Processes within the seagrass holobiont.webp| Seagrass holobiontUgarelli, K., Chakrabarti, S., Laas, P. and Stingl, U. (2017) "The seagrass holobiont and its microbiome". Microorganisms, 5(4): 81. {{doi|10.3390/microorganisms5040081}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

File:Climate change stressors and rhodolith holobiont fitness.webp| {{center|Climate change and the rhodolith holobiontCavalcanti, G.S., Shukla, P., Morris, M., Ribeiro, B., Foley, M., Doane, M.P., Thompson, C.C., Edwards, M.S., Dinsdale, E.A. and Thompson, F.L. (2018) "Rhodoliths holobionts in a changing ocean: host-microbes interactions mediate coralline algae resilience under ocean acidification". BMC Genomics, 19(1): 1–13. {{doi|10.1186/s12864-018-5064-4}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}}

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File:Marine export production.jpg}}]]

{{further|marine food web}}

Marine microorganisms play central roles in the marine food web.

The viral shunt pathway is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms.{{Cite journal |last1=Wilhelm |first1=Steven W. |last2=Suttle |first2=Curtis A. |date=1999 |title=Viruses and Nutrient Cycles in the Sea |journal=BioScience |volume=49 |issue=10 |pages=781–788 |doi=10.2307/1313569 |issn=1525-3244 |jstor=1313569 |doi-access=free}} Viral shunting helps maintain diversity within the microbial ecosystem by preventing a single species of marine microbe from dominating the micro-environment.{{cite journal |last1=Weinbauer |first1=Markus G. |display-authors=etal |year=2007 |title=Synergistic and antagonistic effects of viral lysis and protistan grazing on bacterial biomass, production and diversity |journal=Environmental Microbiology |volume=9 |issue=3| pages=777–788 |doi=10.1111/j.1462-2920.2006.01200.x |pmid=17298376 |bibcode=2007EnvMi...9..777W}} The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.Robinson, Carol, and Nagappa Ramaiah. "Microbial heterotrophic metabolic rates constrain the microbial carbon pump." The American Association for the Advancement of Science, 2011.

File:Microbial Loop.jpg}}]]

File:Roles of fungi in the marine carbon cycle.jpg

File:Viral shunt.jpg facilitates the flow of dissolved organic matter (DOM) and particulate organic matter (POM) through the marine food web]]

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| video1 = [https://www.youtube.com/watch?v=xFQ_fO2D7f0 The secret Life of plankton]

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File:Pelagibacter.jpg|Pelagibacter ubique, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.

File:Marine snow.jpg| {{center|Marine snow is the shower of organic particles that falls from upper waters to the deep ocean[https://oceanservice.noaa.gov/facts/marinesnow.html What is marine snow?] NOAA National Ocean Service. Updated:06/25/18. It is a major exporter of carbon.}}

File:Size and classification of marine particles.png Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 3.0 International License].
Adapted from Simon et al., 2002.Simon, M., Grossart, H., Schweitzer, B. and Ploug, H. (2002) "Microbial ecology of organic aggregates in aquatic ecosystems". Aquatic microbial ecology, 28: 175–211. {{doi|10.3354/ame028175}}.}}]]

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| image1 = High Rise black smoker.jpg

| alt1 =

| caption1 = Black smoker in the High Rise portion of the Endeavour Hydrothermal Vents.

| image2 = Alvinella pompejana01.jpg

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| caption2 = Bacteria can be beneficial. This Pompeii worm, an extremophile found only at hydrothermal vents, has a protective cover of symbiotic bacteria.

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{{see also|Sea ice microbial communities|Hydrothermal vent microbial communities}}

Sea ice microbial communities (SIMCO) refer to groups of microorganisms living within and at the interfaces of sea ice at the poles. The ice matrix they inhabit has strong vertical gradients of salinity, light, temperature and nutrients. Sea ice chemistry is most influenced by the salinity of the brine which affects the pH and the concentration of dissolved nutrients and gases. The brine formed during the melting sea ice creates pores and channels in the sea ice in which these microbes can live. As a result of these gradients and dynamic conditions, a higher abundance of microbes are found in the lower layer of the ice, although some are found in the middle and upper layers.{{cite journal|last1=Boetius|first1=Antje|last2=Anesio|first2=Alexandre M.|last3=Deming|first3=Jody W.|last4=Mikucki|first4=Jill A.|last5=Rapp|first5=Josephine Z.|date=1 November 2015|title=Microbial ecology of the cryosphere: sea ice and glacial habitats|journal=Nature Reviews Microbiology|volume=13|issue=11|pages=677–690|doi=10.1038/nrmicro3522|pmid=26344407|s2cid=20798336}}

Hydrothermal vents are located where the tectonic plates are moving apart and spreading. This allows water from the ocean to enter into the crust of the earth where it is heated by the magma. The increasing pressure and temperature forces the water back out of these openings, on the way out, the water accumulates dissolved minerals and chemicals from the rocks that it encounters. Vents can be characterized by temperature and chemical composition as diffuse vents which release clear relatively cool water usually below 30 °C, as white smokers which emit milky coloured water at warmer temperatures, about 200-330 °C, and as black smokers which emit water darkened by accumulated precipitates of sulfide at hot temperatures, about 300-400 °C.{{Cite journal|last1=Lutz|first1=Richard A.|last2=Kennish|first2=Michael J.|date=1993|title=Ecology of deep-sea hydrothermal vent communities: A review|journal=Reviews of Geophysics|language=en|volume=31|issue=3|pages=211|doi=10.1029/93rg01280|issn=8755-1209|bibcode=1993RvGeo..31..211L}}

Hydrothermal vent microbial communities are microscopic unicellular organisms that live and reproduce in the chemically distinct area around hydrothermal vents. These include organisms in microbial mats, free floating cells, and bacteria in endosymbiotic relationships with animals. Because there is no sunlight at these depths, energy is provided by chemosynthesis where symbiotic bacteria and archaea form the bottom of the food chain and are able to support a variety of organisms such as giant tube worms and Pompeii worms. These organisms utilize this symbiotic relationship in order to utilize and obtain the chemical energy that is released at these hydrothermal vent areas.{{Cite journal|last1=Kádár|first1=Enikõ|last2=Costa|first2=Valentina|last3=Santos|first3=Ricardo S.|last4=Powell|first4=Jonathan J.|date=July 2006|title=Tissue partitioning of micro-essential metals in the vent bivalve Bathymodiolus azoricus and associated organisms (endosymbiont bacteria and a parasite polychaete) from geochemically distinct vents of the Mid-Atlantic Ridge|journal=Journal of Sea Research|volume=56|issue=1|pages=45–52|doi=10.1016/j.seares.2006.01.002|issn=1385-1101|bibcode=2006JSR....56...45K}} Chemolithoautotrophic bacteria can derive nutrients and energy from the geological activity at a hydrothermal vent to fix carbon into organic forms.{{cite journal |last1=Anderson |first1=Rika E. |last2=Brazelton |first2=William J. |last3=Baross |first3=John A. |title=Is the Genetic Landscape of the Deep Subsurface Biosphere Affected by Viruses? |journal=Frontiers in Microbiology |date=2011 |volume=2 |page=219 |doi=10.3389/fmicb.2011.00219 |pmid=22084639 |pmc=3211056 |doi-access=free }}

Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.{{Cite journal|last1=Anderson|first1=Rika E.|last2=Brazelton|first2=William J.|last3=Baross|first3=John A.|date=2011|title=Is the genetic landscape of the deep subsurface biosphere affected by viruses?|journal=Frontiers in Microbiology|volume=2|pages=219|doi=10.3389/fmicb.2011.00219|issn=1664-302X|pmc=3211056|pmid=22084639|doi-access=free}} Viruses are the most abundant life in the ocean, harboring the greatest reservoir of genetic diversity.{{Cite journal|last=Suttle|first=Curtis A.|date=September 2005|title=Viruses in the sea|journal=Nature|volume=437|issue=7057|pages=356–361|doi=10.1038/nature04160|pmid=16163346|issn=0028-0836|bibcode=2005Natur.437..356S|s2cid=4370363}} As their infections are often fatal, they constitute a significant source of mortality and thus have widespread influence on biological oceanographic processes, evolution and biogeochemical cycling within the ocean.{{Cite journal|last=Suttle|first=Curtis A.|date=October 2007|title=Marine viruses — major players in the global ecosystem|journal=Nature Reviews Microbiology|language=En|volume=5|issue=10|pages=801–812|doi=10.1038/nrmicro1750|pmid=17853907|s2cid=4658457|issn=1740-1526}} Evidence has been found however to indicate that viruses found in vent habitats have adopted a more mutualistic than parasitic evolutionary strategy in order to survive the extreme and volatile environment they exist in.{{Cite journal|last1=Anderson|first1=Rika E.|last2=Sogin|first2=Mitchell L.|last3=Baross|first3=John A.|date=2014-10-03|title=Evolutionary Strategies of Viruses, Bacteria and Archaea in Hydrothermal Vent Ecosystems Revealed through Metagenomics|journal=PLOS ONE|volume=9|issue=10|pages=e109696|doi=10.1371/journal.pone.0109696|issn=1932-6203|pmc=4184897|pmid=25279954|bibcode=2014PLoSO...9j9696A|doi-access=free}} Deep-sea hydrothermal vents were found to have high numbers of viruses indicating high viral production.{{Cite journal|last1=Ortmann|first1=Alice C.|last2=Suttle|first2=Curtis A.|date=August 2005|title=High abundances of viruses in a deep-sea hydrothermal vent system indicates viral mediated microbial mortality|journal=Deep Sea Research Part I: Oceanographic Research Papers|volume=52|issue=8|pages=1515–1527|doi=10.1016/j.dsr.2005.04.002|issn=0967-0637|bibcode=2005DSRI...52.1515O}} Like in other marine environments, deep-sea hydrothermal viruses affect abundance and diversity of prokaryotes and therefore impact microbial biogeochemical cycling by lysing their hosts to replicate.{{Cite journal|author-link=Mya Breitbart|last=Breitbart|first=Mya|date=2012-01-15|title=Marine Viruses: Truth or Dare|journal=Annual Review of Marine Science|language=en|volume=4|issue=1|pages=425–448|doi=10.1146/annurev-marine-120709-142805|pmid=22457982|issn=1941-1405|bibcode=2012ARMS....4..425B}} However, in contrast to their role as a source of mortality and population control, viruses have also been postulated to enhance survival of prokaryotes in extreme environments, acting as reservoirs of genetic information. The interactions of the virosphere with microorganisms under environmental stresses is therefore thought to aide microorganism survival through dispersal of host genes through horizontal gene transfer.{{Cite journal|last1=Goldenfeld|first1=Nigel|last2=Woese|first2=Carl|date=January 2007|title=Biology's next revolution|journal=Nature|volume=445|issue=7126|pages=369|doi=10.1038/445369a|pmid=17251963|issn=0028-0836|arxiv=q-bio/0702015|bibcode=2007Natur.445..369G|s2cid=10737747}}

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File:Picoplancton fluorescence Pacific.jpg in the ocean, much of which cannot be effectively studied because they resist attempts at laboratory culture}}]]

{{see also|deep biosphere|rare biosphere|microbial dark matter|SLiME|marine cryptic interactions}}

The deep biosphere is that part of the biosphere that resides below the first few meters of the surface. It extends at least 5 kilometers below the continental surface and 10.5 kilometers below the sea surface, with temperatures that may exceed 100 °C.

Above the surface living organisms consume organic matter and oxygen. Lower down, these are not available, so they make use of "edibles" (electron donors) such as hydrogen released from rocks by various chemical processes, methane, reduced sulfur compounds and ammonium. They "breathe" electron acceptors such as nitrates and nitrites, manganese and iron oxides, oxidized sulfur compounds and carbon dioxide.

There is very little energy at greater depths, and metabolism can be up to a million times slower than at the surface. Cells may live for thousands of years before dividing and there is no known limit to their age. The subsurface accounts for about 90% of the biomass in bacteria and archaea, and 15% of the total biomass for the biosphere. Eukaryotes are also found, mostly microscopic, but including some multicellular life. Viruses are also present and infect the microbes.

File:Subsurface life environments.jpg

In 2018, researchers from the Deep Carbon Observatory announced that life forms, including 70% of the bacteria and archaea on Earth, totaling a biomass of 23 billion tonnes carbon, live up to {{nowrap|{{convert|4.8|km|mi|abbr=on}}}} deep underground, including {{nowrap|{{convert|2.5|km|mi|abbr=on}}}} below the seabed.{{cite news |author=Deep Carbon Observatory |title=Life in deep Earth totals 15 to 23 billion tons of carbon – hundreds of times more than humans |url=https://www.eurekalert.org/pub_releases/2018-12/tca-lid120318.php |date=10 December 2018 |work=EurekAlert! |access-date=11 December 2018}}{{cite news |last=Dockrill |first=Peter |title=Scientists Reveal a Massive Biosphere of Life Hidden Under Earth's Surface |url=https://www.sciencealert.com/scientists-lift-lid-on-massive-biosphere-of-life-hidden-under-earth-s-surface |date=11 December 2018 |work=Science Alert |access-date=11 December 2018}}{{cite news |last=Gabbatiss |first=Josh |title=Massive 'deep life' study reveals billions of tonnes of microbes living far beneath Earth's surface |url=https://www.independent.co.uk/news/science/deep-life-microbes-underground-bacteria-earth-surface-carbon-observatory-science-study-a8677521.html |date=11 December 2018 |work=The Independent |access-date=11 December 2018}} In 2019 microbial organisms were discovered living {{Convert|7900|ft}} below the surface, breathing sulfur and eating rocks such as pyrite as their regular food source.{{cite journal |doi=10.1080/01490451.2019.1641770 |volume=36 |title='Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory |year=2019 |journal=Geomicrobiology Journal |pages=859–872 |last1=Lollar |first1=Garnet S. |last2=Warr |first2=Oliver |last3 = Telling |first3=Jon |last4=Osburn |first4=Magdalena R. |last5=Sherwood Lollar |first5=Barbara| issue=10 |bibcode=2019GmbJ...36..859L |s2cid=199636268}}{{cite web |url=https://deepcarbon.net/worlds-oldest-groundwater-supports-life-through-water-rock-chemistry |title=World's Oldest Groundwater Supports Life Through Water-Rock Chemistry |archive-url=https://web.archive.org/web/20190910013319/https://deepcarbon.net/worlds-oldest-groundwater-supports-life-through-water-rock-chemistry |archive-date=10 September 2019 |date=29 July 2019 |website=deepcarbon.net |publisher=Deep Carbon Observatory}}[https://www.nbcnews.com/mach/science/strange-life-forms-found-deep-mine-point-vast-underground-galapagos-ncna1050906 Strange life-forms found deep in a mine point to vast 'underground Galapagos'], By Corey S. Powell, 7 Sept. 2019, nbcnews.com. This discovery occurred in the oldest known water on Earth.[http://thescienceexplorer.com/nature/oldest-water-earth-found-deep-within-canadian-shield Oldest Water on Earth Found Deep Within the Canadian Shield], 14 December 2016, Maggie Romuld

File:13C and 15N incorporation in representative microbial cells.webp

In 2020 researchers reported they had found what could be the longest-living life forms ever: aerobic microorganisms which had been in quasi-suspended animation for up to 101.5 million years. The microorganisms were found in organically poor sediments {{convert|68.9|m|ft|abbr=off}} below the seafloor in the South Pacific Gyre (SPG), "the deadest spot in the ocean".{{cite news |last=Wu |first=Katherine J. |title=These Microbes May Have Survived 100 Million Years Beneath the Seafloor – Rescued from their cold, cramped and nutrient-poor homes, the bacteria awoke in the lab and grew. |newspaper=The New York Times |url=https://www.nytimes.com/2020/07/28/science/microbes-100-million-years-old.html |date=28 July 2020 |access-date=31 July 2020 }}{{cite journal |author=Morono, Yuki |display-authors=et al. |title=Aerobic microbial life persists in oxic marine sediment as old as 101.5 million years |date=28 July 2020 |journal=Nature Communications |volume=11 |number=3626 |page=3626 |doi=10.1038/s41467-020-17330-1 |pmid=32724059 |pmc=7387439 |bibcode=2020NatCo..11.3626M }}

File:Unidentified spherical algal microorganism.jpg]]

To date biologists have been unable to culture in the laboratory the vast majority of microorganisms. This applies particularly to bacteria and archaea, and is due to a lack of knowledge or ability to supply the required growth conditions.{{cite journal |last1=Filee |first1=J. |last2=Tetart |first2=F. |last3=Suttle |first3=C. A. |last4=Krisch |first4=H. M. |title=Marine T4-type bacteriophages, a ubiquitous component of the dark matter of the biosphere|journal=Proceedings of the National Academy of Sciences |volume=102 |issue=35 |year=2005 |pages=12471–12476 |issn=0027-8424 |doi=10.1073/pnas.0503404102 |pmid=16116082 |pmc=1194919 |bibcode=2005PNAS..10212471F |doi-access=free}}{{cite news |author=University of Tennessee at Knoxville |title=Study: Microbial dark matter dominates Earth's environments |url=https://www.eurekalert.org/pub_releases/2018-09/uota-smd092518.php |date=25 September 2018 |work=Eurekalert! |access-date=26 September 2018|author-link=University of Tennessee at Knoxville }} The term microbial dark matter has come to be used to describe microorganisms scientists know are there but have been unable to culture, and whose properties therefore remain elusive. Microbial dark matter is unrelated to the dark matter of physics and cosmology, but is so-called for the difficulty in effectively studying it. It is hard to estimate its relative magnitude, but the accepted gross estimate is that less than one per cent of microbial species in a given ecological niche is culturable. In recent years effort is being put to decipher more of the microbial dark matter by means of learning their genome DNA sequence from environmental samples{{cite journal |last1=Hedlund |first1=Brian P. |last2=Dodsworth |first2=Jeremy A. |last3=Murugapiran |first3=Senthil K. |last4=Rinke |first4=Christian |last5=Woyke |first5=Tanja |title=Impact of single-cell genomics and metagenomics on the emerging view of extremophile "microbial dark matter" |journal=Extremophiles |volume=18 |issue=5 |year=2014 |pages=865–875 |issn=1431-0651 |doi=10.1007/s00792-014-0664-7 |pmid=25113821 |s2cid=16888890}} and then by gaining insights to their metabolism from their sequenced genome, promoting the knowledge required for their cultivation.

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File:Microbial species present in the three domains of life.png| {{center|Estimates of microbial species counts in the three domains of life}} Bacteria are the oldest and most biodiverse group, followed by Archaea and Fungi (the most recent groups). In 1998, before awareness of the extent of microbial life had gotten underway, Robert M. May{{cite journal |last1=May |first1=R.M. |author-link=Robert May, Baron May of Oxford |year=1988 |title=How many species are there on earth? |journal=Science |volume=241 |issue=4872| pages=1441–1449 |pmid=17790039 |doi=10.1126/science.241.4872.1441 |bibcode=1988Sci...241.1441M |s2cid=34992724}} estimated there were 3 million species of living organisms on the planet. But in 2016, Locey and Lennon{{cite journal |last1=Locey |first1=K.J. |last2=Lennon |first2=J.T. |year=2016 |title=Scaling laws predict global microbial diversity |journal=Proceedings of the National Academy of Sciences |volume=113 |issue=21| pages=5970–5975 |doi=10.1073/pnas.1521291113 |pmid=27140646 |pmc=4889364 |bibcode=2016PNAS..113.5970L |doi-access=free}} estimated the number of microorganism species could be as high as 1 trillion.{{cite journal |last1=Vitorino |first1=L.C. |last2=Bessa |first2=L.A. |year=2018 |title=Microbial diversity: the gap between the estimated and the known |journal=Diversity |volume=10 |issue=2| page=46 |doi=10.3390/d10020046 |doi-access=free}} 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

File:Known, estimated and unknown microbial diversity.webp| {{center|Microbial diversity}} Comparative representation of the known and estimated (small box) and the yet unknown (large box) microbial diversity, which applies to both marine and terrestrial microorganisms. The text boxes refer to factors that adversely affect the knowledge of the microbial diversity that exists on the planet.

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{{see also|niskin bottle|plankton net|video plankton recorder|continuous plankton recorder}}

File:Plankton sampling methods.png

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| caption2 = High volumes of plankton samples can also be analysed rapidly with sequencing techniques.

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{{further|microscopy|flow cytometry|Sanger sequencing|DNA barcoding|high throughput biology|microbial DNA barcoding|algae DNA barcoding|Bacterioplankton counting methods}}

Traditionally, the phylogeny of microorganisms was inferred and their taxonomy was established based on studies of morphology. However, developments in molecular phylogenetics have allowed evolutionary relationship of species to be established by analyzing deeper characteristics, such as their DNA and protein sequences, for example ribosomal DNA.{{cite journal|vauthors=Olsen GJ, Woese CR, Overbeek R|year=1994|title=The winds of (evolutionary) change: breathing new life into microbiology|url= |journal=Journal of Bacteriology|volume=176|issue=1|pages=1–6|doi=10.2172/205047|pmc=205007|pmid=8282683}} The lack of easily accessible morphological features, such as those present in animals and plants, particularly hampered early efforts at classifying bacteria and archaea. This resulted in erroneous, distorted and confused classification, an example of which, noted Carl Woese, is Pseudomonas whose etymology ironically matched its taxonomy, namely "false unit".{{Cite journal | last1 = Woese | first1 = CR | title = Bacterial evolution | journal = Microbiological Reviews | volume = 51 | issue = 2 | pages = 221–71 | year = 1987 | doi = 10.1128/MMBR.51.2.221-271.1987 | pmid = 2439888 | pmc = 373105}} Many bacterial taxa have been reclassified or redefined using molecular phylogenetics.

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| footer = It would be difficult to consistently separate out these two microbes using images alone. However, if their barcodes are aligned to each other and their bases are coloured to see them more clearly, it becomes easy to see which bases are different between these two microbes. In this manner, millions of different kinds of microbes can be distinguished.Collins, R. Eric (2019) [https://oceanexplorer.noaa.gov/explorations/19gulfofalaska/background/microbes/microbes.html The Small and Mighty – Microbial Life] NOAA: Ocean Exploration and Research, Gulf of Alaska Seamounts 2019 Expedition. {{PD-notice}}

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| caption2 = Marinomonas arctica, a bacterium which grows inside Arctic sea ice at subzero temperatures

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File:Methods used for phytoplankton studies.png, mass spectrometry, and nuclear magnetic resonance spectroscopy.Käse L, Geuer JK. (2018) [https://link.springer.com/chapter/10.1007/978-3-319-93284-2_5 "Phytoplankton Responses to Marine Climate Change – An Introduction"]. In Jungblut S., Liebich V., Bode M. (Eds) YOUMARES 8–Oceans Across Boundaries: Learning from each other, pages 55–72, Springer. {{doi|10.1007/978-3-319-93284-2_5}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].]]

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| video1 = [https://www.youtube.com/watch?v=VBmzwM76V0o Microbes don't actually look like anything] – Meet the Microcosmos

| video2 = [https://www.youtube.com/watch?v=TiPw1byKuzk How to Identify Microbes] – Meet the Microcosmos

| video3 = [https://www.youtube.com/watch?v=HbwV-EzP67Q&ab_channel=JourneytotheMicrocosmos Differential interference contrast (DIC)] – Meet the Microcosmos

}}

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Recent developments in molecular sequencing have allowed for the recovery of genomes in situ, directly from environmental samples and avoiding the need for culturing. This has led for example, to a rapid expansion in knowledge of the diversity of bacterial phyla. These techniques are genome-resolved metagenomics and single-cell genomics.

The new sequencing technologies and the accumulation of sequence data have resulted in a paradigm shift, highlighted both the ubiquity of microbial communities in association within higher organisms and the critical roles of microbes in ecosystem health.Lozupone, C.A., Stombaugh, J.I., Gordon, J.I., Jansson, J.K. and Knight, R. (2012) "Diversity, stability and resilience of the human gut microbiota". Nature, 489(7415): 220–230. {{doi|10.1038/nature11550}}. These new possibilities have revolutionized microbial ecology, because the analysis of genomes and metagenomes in a high-throughput manner provides efficient methods for addressing the functional potential of individual microorganisms as well as of whole communities in their natural habitats.Venter, J.C., Remington, K., Heidelberg, J.F., Halpern, A.L., Rusch, D., Eisen, J.A., Wu, D., Paulsen, I., Nelson, K.E., Nelson, W. and Fouts, D.E. (2004) "Environmental genome shotgun sequencing of the Sargasso Sea". Science, 304(5667): 66–74. {{doi|10.1126/science.1093857}}.Liu, L., Li, Y., Li, S., Hu, N., He, Y., Pong, R., Lin, D., Lu, L. and Law, M. (2012) "Comparison of next-generation sequencing systems". BioMed Research International, 2012: 251364. {{doi|10.1155/2012/251364}}.Berg, Gabriele; Daria Rybakova, Doreen Fischer, Tomislav Cernava, Marie-Christine Champomier Vergès, Trevor Charles, Xiaoyulong Chen, Luca Cocolin, Kellye Eversole, Gema Herrero Corral, Maria Kazou, Linda Kinkel, Lene Lange, Nelson Lima, Alexander Loy, James A. Macklin, Emmanuelle Maguin, Tim Mauchline, Ryan McClure, Birgit Mitter, Matthew Ryan, Inga Sarand, Hauke Smidt, Bettina Schelkle, Hugo Roume, G. Seghal Kiran, Joseph Selvin, Rafael Soares Correa de Souza, Leo van Overbeek, Brajesh K. Singh, Michael Wagner, Aaron Walsh, Angela Sessitsch and Michael Schloter (2020) "Microbiome definition re-visited: old concepts and new challenges". Microbiome, 8(103): 1–22. {{doi|10.1186/s40168-020-00875-0}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].

File:DNA sequencing technologies used in marine metagenomics.jpg and contig generation. Previously unknown genes, pathways and even whole genomes are being discovered. These genome-editing technologies are used to retrieve and modify valuable microorganisms for production, particularly in marine metagenomics. Organisms may be cultivable or uncultivable. Metagenomics is providing especially valuable information for uncultivable samples.{{cite journal | last1 = Kodzius | first1 = R. | last2 = Gojobori | first2 = T. | year = 2015 | title = Marine metagenomics as a source for bioprospecting | journal = Marine Genomics | volume = 24 | pages = 21–30 | doi = 10.1016/j.margen.2015.07.001 | pmid = 26204808 | bibcode = 2015MarGn..24...21K | doi-access = free | hdl = 10754/567056 | hdl-access = free }} 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].]]

{{further|Omics|environmental DNA|metabarcoding|metagenomics|metatranscriptomics}}

Omics is a term used informally to refer to branches of biology whose names end in the suffix -omics, such as genomics, proteomics, metabolomics, and glycomics. Marine Omics has recently emerged as a field of research of its own.{{cite book |editor-last1=Giuliano |editor-first1=Laura |editor-last2=Barbier |editor-first2=Michele |editor-last3=Briand |editor-first3=Frederic |title=Special Issue: Marine Omics |date=September 2010 |publisher=Wiley-Blackwell |series=Microbial Biotechnology, Vol. 3, n°5 |pages=489–621 |url=https://ami-journals.onlinelibrary.wiley.com/toc/17517915/2010/3/5}} Omics aims at collectively characterising and quantifying pools of biological molecules that translate into the structure, function, and dynamics of an organism or organisms. For example, functional genomics aims at identifying the functions of as many genes as possible of a given organism. It combines different

-omics techniques such as transcriptomics and proteomics with saturated mutant collections.{{cite journal |last1=Holtorf |first1=Hauke |author-link3=Ralf Reski |last2=Guitton |first2=Marie-Christine |last3=Reski |first3=Ralf |year=2002 |title=Plant functional genomics |journal=Naturwissenschaften |volume=89 |issue=6| pages=235–249 |doi=10.1007/s00114-002-0321-3 |pmid=12146788 |bibcode=2002NW.....89..235H |s2cid=7768096}}{{cite journal |vauthors=Cain AK, Barquist L, Goodman AL, Paulsen IT, Parkhill J, van Opijnen T |date=2020 |title=A decade of advances in transposon-insertion sequencing |journal=Nature Reviews Genetics |volume=21 |issue=9 |pages=526–540 |doi=10.1038/s41576-020-0244-x|pmid=32533119 |pmc=7291929 |hdl=10033/622374 |hdl-access=free }}

Many omes beyond the original genome have become useful and have been widely adopted in recent years by research scientists. The suffix -omics can provide an easy shorthand to encapsulate a field; for example, an interactomics study is reasonably recognisable as relating to large-scale analyses of gene-gene, protein-protein, or protein-ligand interactions, while proteomics has become established as a term for studying proteins on a large scale.

Any given omics technique, used just by itself, cannot adequately disentangle the intricacies of a host microbiome. Multi-omics approaches are needed to satisfactorily unravel the complexities of the host-microbiome interactions.Zhang, W., Li, F. and Nie, L. (2010) "Integrating multiple 'omics' analysis for microbial biology: application and methodologies". Microbiology, 156(2): 287–301. {{doi|10.1099/mic.0.034793-0}}. For instance, metagenomics, metatranscriptomics, metaproteomics and metabolomics methods are all used to provide information on the metagenome.Singh, R.P. and Kothari, R. (2017) [https://books.google.com/books?id=kbszDwAAQBAJ&dq=%22The+omics+era+and+host+microbiomes%22&pg=PA3 "The omics era and host microbiomes"]. In: Singh RP, Kothari R, Koringa PG and Singh SP (Eds.) Understanding Host-Microbiome Interactions – An Omics Approach, pages 3–12, Springer. {{ISBN|9789811050503}}. {{doi|10.1007/978-981-10-5050-3_1}}

File:Meta-omics data based biogeochemical modeling.jpg Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}} A schematic conceptual framework for marine biogeochemical modeling from environmental, imaging, and meta-omics data.Sunagawa, S., Coelho, L. P., Chaffron, S., Kultima, J. R., Labadie, K., Salazar, G., et al. (2015). "Ocean plankton. Structure and function of the global ocean microbiome". Science, 348: 1261359. {{doi|10.1126/science.1261359}}. A semi-automatic computational pipeline is schematized for combining biomarkers with biogeochemical data Guidi, L., Chaffron, S., Bittner, L., Eveillard, D., Larhlimi, A., Roux, S., et al. (2016). "Plankton networks driving carbon export in the oligotrophic ocean". Nature, 532: 465–470. {{doi|10.1038/nature16942}}. that can be incorporated into classic biogeochemical models Guyennon, A., Baklouti, M., Diaz, F., Palmieri, J., Beuvier, J., Lebaupin-Brossier, C., et al. (2015). "New insights into the organic carbon export in the Mediterranean Sea from 3-D modelling". Biogeosciences, 11: 425–442. {{doi|10.5194/bg-12-7025-2015}}. for creating a next generation of biogeochemical trait-based meta-omics models by considering their respective traits. Such novel meta-omics-enabled approaches aim to improve the monitoring and prediction of ocean processes while respecting the complexity of the planktonic system.Louca, S., Parfrey, L. W., and Doebeli, M. (2016). "Decoupling function and taxonomy in the global ocean microbiome". Science, 353: 1272–1277. {{doi|10.1126/science.aaf4507}}.Coles, V. J., Stukel, M. R., Brooks, M. T., Burd, A., Crump, B. C., Moran, M. A., et al. (2017). "Ocean biogeochemistry modeled with emergent trait-based genomics". Science, 358: 1149–1154. {{doi|10.1126/science.aan5712}}.]]

File:Omics data and marine phytoplankton.jpg Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].}} As an example of how omics data can be used with marine phytoplankton to inform Earth system science, metatranscriptome sequences from natural phytoplankton communities were used to help identify physiological traits (cellular concentration of ribosomes and their rRNAs) underpinning adaptation to environmental conditions (temperature). A mechanistic phytoplankton cell model was used to test the significance of the identified physiological trait for cellular stoichiometry. Environmental selection in a trait‐based global marine ecosystem model was then linking emergent growth and cellular allocation strategies to large‐scale patterns in light, nutrients and temperature in the surface marine environment. Global predictions of cellular resource allocation and stoichiometry (N:P ratio) were consistent with patterns in metatranscriptome dataToseland A, Daines SJ and Clark JR et al. (2013) "The impact of temperature on marine phytoplankton resource allocation and metabolism". Nature Climate Change, 3: 979– 984. {{doi|10.1038/nclimate1989}}. and latitudinal patterns in the elemental ratios of marine plankton and organic matter.Martiny AC, Pham CTA, Primeau FW, Vrugt JA, Moore JK, Levin SA and Lomas MW (2013) "Strong latitudinal patterns in the elemental ratios of marine plankton and organic matter". Nature Geoscience, 6: 279–283. {{doi|10.1038/ngeo1757}}. The three‐dimensional view of ribosome shows rRNA in dark blue and dark red. Lighter colours represent ribosomal proteins. Bands above show temperature‐dependent abundance of the eukaryotic ribosomal protein S14.]]

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See...

• Brüwer, J.D. and Buck-Wiese, H. (2018) [https://library.oapen.org/bitstream/handle/20.500.12657/22929/1007232.pdf?sequence=1#page=73 "Reading the Book of Life–Omics as a Universal Tool Across Disciplines"]. In: YOUMARES 8 – Oceans Across Boundaries: Learning from each other, pages 73–82. Springer. {{ISBN|9783319932842}}.

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{{see also|Human impact on marine life}}

File:Microorganisms and climate change.png

In marine environments, microbial primary production contributes substantially to CO₂ sequestration. Marine microorganisms also recycle nutrients for use in the marine food web and in the process release CO₂ to the atmosphere. Microbial biomass and other organic matter (remnants of plants and animals) are converted to fossil fuels over millions of years. By contrast, burning of fossil fuels liberates greenhouse gases in a small fraction of that time. As a result, the carbon cycle is out of balance, and atmospheric CO₂ levels will continue to rise as long as fossil fuels continue to be burnt.

{{Quote box

|title = Scientists' warning to humanity{{hsp}}

|quote = Microorganisms have key roles in carbon and nutrient cycling, animal (including human) and plant health, agriculture and the global food web. Microorganisms live in all environments on Earth that are occupied by macroscopic organisms, and they are the sole life forms in other environments, such as the deep subsurface and ‘extreme’ environments. Microorganisms date back to the origin of life on Earth at least 3.8 billion years ago, and they will likely exist well beyond any future extinction events... Unless we appreciate the importance of microbial processes, we fundamentally limit our understanding of Earth's biosphere and response to climate change and thus jeopardize efforts to create an environmentally sustainable future.

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Marine microorganisms known as cyanobacteria first emerged in the oceans during the Precambrian era roughly 2 billion years ago. Over eons, the photosynthesis of marine microorganisms generated by oxygen has helped shape the chemical environment in the evolution of plants, animals and many other life forms. Marine microorganisms were first observed in 1675 by Dutch lensmaker Antonie van Leeuwenhoek.

{{portal|Marine life|Oceans}}

{{div col|colwidth=20em}}

• Marine life
• Deep chlorophyll maximum
• Hawaii Ocean Time-series
• International Census of Marine Microbes
• Marine microbial symbiosis
• Microbial biogeography
• Microbial communities
• Microbial ecology
• Microbial food web
• Microbial loop
• Microbial oxidation of sulfur
• Sulfate-reducing microorganisms
• Microbially induced sedimentary structure
• Microbiology of oxygen minimum zones
• Oceanic carbon cycle
• Ooid
• Picoeukaryote
• Roseobacter

{{div col end}}

{{reflist}}

• Haq B.U. and Boersma A. (Eds.) (1998) [https://books.google.com/books?id=0XezCm7IwpUC&q=%22Introduction+to+Marine+Micropaleontology%22 Introduction to Marine Micropaleontology] Elsevier. {{ISBN|9780080534961}}
• Kirchman, David L. (2018) [https://books.google.com/books?id=XdZjDwAAQBAJ&pg=PP1 Processes in Microbial Ecology] Oxford University Press. {{ISBN|9780192506474}}
• Helmreich, Stefan (2009) [https://books.google.com/books?id=u1rXKH-SRHYC&q=%22Alien+Ocean%22 Alien Ocean: Anthropological Voyages in Microbial Seas] University of California Press. {{ISBN|9780520942608}}
• Munn, Colin B. (2019) [https://books.google.com/books?id=KCsWBAAAQBAJ&q=%22Marine+Microbiology%22 Marine Microbiology: Ecology & Applications] CRC Press. {{ISBN|9780429592362}}
• Stal LJ and Cretoiu MS (2016) [https://books.google.com/books?id=MJ1PDAAAQBAJ&q=%22The+Marine+Microbiome%22 The Marine Microbiome: An Untapped Source of Biodiversity and Biotechnological Potential] Springer. {{ISBN|9783319330006}}
• Middelboe M and Corina Brussaard CPD (Eds.) [https://www.mdpi.com/books/pdfview/book/484 Marine Viruses 2016], MDPI. {{ISBN|978-3-03842-621-9}} [https://www.mdpi.com/books/pdfdownload/book/484 Download PDF]
• Ohtsuka S, Suzaki T, Horiguchi T, Suzuki N and Not F (Eds.) (2015) [https://books.google.com/books?id=vgyhCgAAQBAJ&q=%22Marine+protists%22 Marine Protists: Diversity and Dynamics] Springer. {{ISBN|9784431551300}}
• Johnson MD and V. Moeller HV (2019) [https://books.google.com/books?id=gO7CDwAAQBAJ&q=%22Mixotrophy+in+Protists%22 Mixotrophy in Protists: From Model Systems to Mathematical Models], 2nd Edition, Frontiers Media. {{ISBN|9782889631483}}
• [http://www.radiolaria.org/arts.htm Radiolarian art]

{{microorganisms|state=expanded}}

Category:Microorganisms

Category:Marine organisms

Category:Planktology

Category:Biological oceanography

Category:Marine biology

Category:Marine microorganisms