marine viruses

{{plankton sidebar|trophic}}

{{further|introduction to viruses|viral evolution}}

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.{{cite book |editor=Mahy WJ |editor2= Van Regenmortel MHV |title=Desk Encyclopedia of General Virology |publisher=Academic Press |location=Oxford |year=2009 |pages=28 |isbn=978-0-12-375146-1}} They are found wherever there is life and have probably existed since living cells first evolved.{{cite journal

|author=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 origins of viruses in the evolutionary history of life are unclear because they do not form fossils. Molecular techniques are used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose.{{cite journal |author=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}} Some viruses 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.

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.{{cite journal |vauthors=Koonin EV, Starokadomskyy P |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 |year=2016 |volume=59 |pages=125–134 |pmid=26965225 |doi=10.1016/j.shpsc.2016.02.016 |pmc=5406846}} 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{{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=7 March 2016 |last1=Koonin |first1=E. V. |last2=Starokadomskyy |first2=P. |doi=10.1016/j.shpsc.2016.02.016 |pmid=26965225 |volume=59 |pmc=5406846 |pages=125–34}} and as "organisms at the edge of life".{{cite journal|author = 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}}

The existence of viruses in the ocean was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of uncultured viral samples.{{cite journal | vauthors = Wommack KE, Hill RT, Muller TA, Colwell RR | title = Effects of sunlight on bacteriophage viability and structure | journal = Applied and Environmental Microbiology | volume = 62 | issue = 4 | pages = 1336–41 | date = April 1996 | pmid = 8919794 | pmc = 167899 | doi = 10.1128/AEM.62.4.1336-1341.1996 | bibcode = 1996ApEnM..62.1336W }} Marine viruses, although microscopic and essentially unnoticed by scientists until recently, are the most abundant and diverse biological entities in the ocean. Viruses have an estimated abundance of 10³⁰ in the ocean, or between 10⁶ and 10¹¹ viruses per millilitre. Quantification of marine viruses was originally performed using transmission electron microscopy but has been replaced by epifluorescence or flow cytometry.{{cite journal | vauthors = Marie D, Brussaard CP, Thyrhaug R, Bratbak G, Vaulot D | title = Enumeration of marine viruses in culture and natural samples by flow cytometry | journal = Applied and Environmental Microbiology | volume = 65 | issue = 1 | pages = 45–52 | date = January 1999 | doi = 10.1128/AEM.65.1.45-52.1999 | pmid = 9872758 | pmc = 90981 | bibcode = 1999ApEnM..65...45M }}

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File:Phage injecting its genome into bacteria.svg

{{further|cyanophage|phage ecology}}

Bacteriophages, often contracted to phages, are viruses that parasitize bacteria for replication. As aptly named, marine phages parasitize marine bacteria, such as cyanobacteria.{{cite journal | last = Mann | first = 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 diverse group of viruses which are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. There are up to ten times more phages in the oceans than there are bacteria,{{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 | date = March 2000 | pmid = 10704475 | pmc = 98987 | doi = 10.1128/MMBR.64.1.69-114.2000 }} reaching levels of 250 million bacteriophages per millilitre of seawater.{{cite journal | vauthors = Bergh O, Børsheim KY, Bratbak G, Heldal M | s2cid = 4271861 | title = High abundance of viruses found in aquatic environments | journal = Nature | volume = 340 | issue = 6233 | pages = 467–68 | date = August 1989 | pmid = 2755508 | doi = 10.1038/340467a0 | bibcode = 1989Natur.340..467B }} These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins, which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and there are phages that can replicate three hundred phages twenty minutes after injection.Shors pp. 595–97

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

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File:Process of phage adsorption onto a bacterium.webp

File:Adsorption of a cyanophage onto a marine prochlorococcus.webps onto a marine Prochlorococcus}} (a) Slice (~20 nm) through a reconstructed tomogram of P-SSP7 phage incubated with MED4, imaged at ~86 min post-infection. FC and EC show full-DNA capsid phage and empty capsid phage, respectively.
(b) same image visualised by highlighting the cell wall in orange, the plasma membrane in light yellow, the thylakoid membrane in green, carboxysomes in cyan, the polyphosphate body in blue, adsorbed phages on the sides or top of the cell in red, and cytoplasmic granules (probably mostly ribosomes) in light purple.{{cite journal | last1 = Murata | first1 = K. | last2 = Zhang | first2 = Q. | last3 = Galaz-Montoya | first3 = J.G. | last4 = Fu | first4 = C. | last5 = Coleman | first5 = M.L. | last6 = Osburne | first6 = M.S. | last7 = Schmid | first7 = M.F. | last8 = Sullivan | first8 = M.B. | last9 = Chisholm | first9 = S.W. | last10 = Chiu | first10 = W. | year = 2017 | title = Visualizing adsorption of cyanophage P-SSP7 onto marine Prochlorococcus | journal = Scientific Reports | volume = 7 | page = 44176 | doi = 10.1038/srep44176 | pmid = 28281671 | pmc = 5345008 | bibcode = 2017NatSR...744176M }} {{align|center|scale bar: 200 nm}}]]

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Bacteria defend themselves from bacteriophages by producing enzymes that destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells.{{cite journal | vauthors = Bickle TA, Krüger DH | title = Biology of DNA restriction | journal = Microbiological Reviews | volume = 57 | issue = 2 | pages = 434–50 | date = June 1993 | pmid = 8336674 | pmc = 372918 | doi = 10.1128/MMBR.57.2.434-450.1993 }} Bacteria also contain a system that uses CRISPR sequences to retain fragments of the genomes of viruses that the bacteria have come into contact with in the past, which allows them to block the virus's replication through a form of RNA interference.{{cite journal | vauthors = Barrangou R, Fremaux C, Deveau H, Richards M, Boyaval P, Moineau S, Romero DA, Horvath P | s2cid = 3888761 | title = CRISPR provides acquired resistance against viruses in prokaryotes | journal = Science | volume = 315 | issue = 5819 | pages = 1709–12 | date = March 2007 | pmid = 17379808 | doi = 10.1126/science.1138140 | bibcode = 2007Sci...315.1709B | hdl = 20.500.11794/38902 | hdl-access = free }}{{cite journal | vauthors = Brouns SJ, Jore MM, Lundgren M, Westra ER, Slijkhuis RJ, Snijders AP, Dickman MJ, Makarova KS, Koonin EV, van der Oost J | title = Small CRISPR RNAs guide antiviral defense in prokaryotes | journal = Science | volume = 321 | issue = 5891 | pages = 960–64 | date = August 2008 | pmid = 18703739 | pmc = 5898235 | doi = 10.1126/science.1159689 | bibcode = 2008Sci...321..960B }} This genetic system provides bacteria with acquired immunity to infection.{{cite journal | vauthors = Mojica FJ, Rodriguez-Valera F | title = The discovery of CRISPR in archaea and bacteria | journal = The FEBS Journal | volume = 283 | issue = 17 | pages = 3162–69 | date = September 2016 | pmid = 27234458 | doi = 10.1111/febs.13766 | hdl = 10045/57676 | s2cid = 42827598 | hdl-access = free }}

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

File:Lytic cycle.png, the reproductive cycle of the bacteriophage, has six stages:

→ attachment: the phage attaches itself to the surface of the host cell

→ penetration: the phage injects its DNA through the cell membrane

→ transcription: the host cell's DNA is degraded and the cell's metabolism is directed to initiate phage biosynthesis

→ biosynthesis: the phage DNA replicates inside the cell

→ maturation: the replicated material assembles into fully formed viral phages

→ lysis: the newly formed phages are released from the infected cell (which is itself destroyed in the process) to seek out new host cells [https://coliphages.com/index.php/reproduction/ How do bacteriophages reproduce?] University of Barcelona. Retrieved 12 July 2020. ]]

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Microbes drive the nutrient transformations that sustain Earth's ecosystems,{{cite journal | last1 = Falkowski | first1 = P.G. | last2 = Fenchel | first2 = T. | last3 = Delong | first3 = E.F. | s2cid = 2844984 | year = 2008 | title = The microbial engines that drive Earth's biogeochemical cycles | journal = Science | volume = 320 | issue = 5879| pages = 1034–1039 | doi = 10.1126/science.1153213 | pmid = 18497287 | bibcode = 2008Sci...320.1034F }} and the viruses that infect these microbes modulate both microbial population size and diversity.{{cite journal | last1 = Brum | first1 = J.R. | last2 = Sullivan | first2 = M.B. | s2cid = 32998525 | year = 2015 | title = Rising to the challenge: accelerated pace of discovery transforms marine virology | journal = Nature Reviews Microbiology| volume = 13 | issue = 3| pages = 147–159 | doi = 10.1038/nrmicro3404 | pmid = 25639680 }} The cyanobacterium Prochlorococcus, the most abundant oxygenic phototroph on Earth, contributes a substantial fraction of global primary carbon production, and often reaches densities of over 100,000 cells per milliliter in oligotrophic and temperate oceans.{{cite journal | last1 = Bouman | first1 = H.A. | last2 = Ulloa | first2 = O. | last3 = Scanlan | first3 = D.J. | last4 = Zwirglmaier | first4 = K. | last5 = Li | first5 = W.K. | last6 = Platt | first6 = T. | last7 = Stuart | first7 = V. | last8 = Barlow | first8 = R. | last9 = Leth | first9 = O. | last10 = Clementson | first10 = L. | last11 = Lutz | first11 = V. | s2cid = 20738145 | year = 2006 | title = Oceanographic basis of the global surface distribution of Prochlorococcus ecotypes | journal = Science | volume = 312 | issue = 5775| pages = 918–921 | doi = 10.1126/science.1122692 | pmid = 16690867 | bibcode = 2006Sci...312..918B }} Hence, viral (cyanophage) infection and lysis of Prochlorococcus represent an important component of the global carbon cycle. In addition to their ecological role in inducing host mortality, cyanophages influence the metabolism and evolution of their hosts by co-opting and exchanging genes, including core photosynthesis genes.

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| caption1 = {{center|Tailed V22 phages of Alteromonas bacteria{{hsp}}{{cite journal |doi = 10.1128/mSystems.00217-20|title = Alteromonas Myovirus V22 Represents a New Genus of Marine Bacteriophages Requiring a Tail Fiber Chaperone for Host Recognition|year = 2020|last1 = Gonzalez-Serrano|first1 = Rafael|last2 = Dunne|first2 = Matthew|last3 = Rosselli|first3 = Riccardo|last4 = Martin-Cuadrado|first4 = Ana-Belen|last5 = Grosboillot|first5 = Virginie|last6 = Zinsli|first6 = Léa V.|last7 = Roda-Garcia|first7 = Juan J.|last8 = Loessner|first8 = Martin J.|last9 = Rodriguez-Valera|first9 = Francisco|journal = mSystems|volume = 5|issue = 3|pmid = 32518192|pmc = 7289586}}
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File:Caudovirales.svgs of different families of tailed phages: Myoviridae, Podoviridae and Siphoviridae}}]]

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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 dominate multiple depths and oceanic regions.{{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 }} These non-tailed phages also infect marine bacteria, and include 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, VCYϕ, 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 | doi=10.1038/nature25474| title=A major lineage of non-tailed dsDNA viruses as unrecognized killers of marine bacteria| year=2018| last1=Kauffman| first1=Kathryn M.| last2=Hussain| first2=Fatima A.| last3=Yang| first3=Joy| last4=Arevalo| first4=Philip| last5=Brown| first5=Julia M.| last6=Chang| first6=William K.| last7=Vaninsberghe| first7=David| last8=Elsherbini| first8=Joseph| last9=Sharma| first9=Radhey S.| last10=Cutler| first10=Michael B.| last11=Kelly| first11=Libusha| last12=Polz| first12=Martin F.| s2cid=4462007| journal=Nature| volume=554| issue=7690| pages=118–122| pmid=29364876| bibcode=2018Natur.554..118K}}[http://www.sci-news.com/biology/autolykiviridae-05664.html Scientists Find New Type of Virus in World's Oceans: Autolykiviridae], on: sci-news, 25 January 2018[https://www.sciencealert.com/entirely-unknown-family-of-infectious-viruses-discovered-in-the-ocean-parasites-tail-less-autolykiviridae Never-Before-Seen Viruses With Weird DNA Were Just Discovered in The Ocean], on: science^alert, 25 January 2018NCBI: [https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=2184034 Autolykiviridae] (family) – unclassified dsDNA viruses

As of September 2023, Halomonas phage vB HmeY H4907 is the first virus isolated from the deepest part of the ocean.{{cite journal |last1=Su |first1=Yue |last2=Zhang |first2=Wenjing |last3=Liang |first3=Yantao |last4=Wang |first4=Hongmin |last5=Liu |first5=Yundan |last6=Zheng |first6=Kaiyang |last7=Liu |first7=Ziqi |last8=Yu |first8=Hao |last9=Ren |first9=Linyi |last10=Shao |first10=Hongbing |last11=Sung |first11=Yeong Yik |last12=Mok |first12=Wen Jye |last13=Wong |first13=Li Lian |last14=Zhang |first14=Yu-Zhong |last15=McMinn |first15=Andrew |date=2023-09-20 |editor-last=Chao |editor-first=Day-Yu |title=Identification and genomic analysis of temperate Halomonas bacteriophage vB_HmeY_H4907 from the surface sediment of the Mariana Trench at a depth of 8,900 m |journal=Microbiology Spectrum |volume=11 |issue=5 |pages=e0191223 |doi=10.1128/spectrum.01912-23 |issn=2165-0497 |pmid=37728551 |pmc=10580944}}

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File:Evolution of the virus world.webp

{{further|Archaeal viruses}}

Archaean viruses replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.{{cite journal|vauthors=Lawrence CM, Menon S, Eilers BJ, Bothner B, Khayat R, Douglas T, Young MJ|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 |author=Prangishvili D, Forterre P, Garrett RA |s2cid=9915859 |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}} These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.{{cite journal|author=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}} Defences against these viruses involve RNA interference from repetitive DNA sequences within archaean genomes that are related to the genes of the viruses.{{cite journal | vauthors = Mojica FJ, Díez-Villaseñor C, García-Martínez J, Soria E | s2cid = 27481111 | title = Intervening sequences of regularly spaced prokaryotic repeats derive from foreign genetic elements | journal = Journal of Molecular Evolution | volume = 60 | issue = 2 | pages = 174–82 | date = February 2005 | pmid = 15791728 | doi = 10.1007/s00239-004-0046-3 | bibcode = 2005JMolE..60..174M }}{{cite journal | vauthors = Makarova KS, Grishin NV, Shabalina SA, Wolf YI, Koonin EV | title = A putative RNA-interference-based immune system in prokaryotes: computational analysis of the predicted enzymatic machinery, functional analogies with eukaryotic RNAi, and hypothetical mechanisms of action | journal = Biology Direct | volume = 1 | pages = 7 | date = March 2006 | pmid = 16545108 | pmc = 1462988 | doi = 10.1186/1745-6150-1-7 | doi-access = free }} Most archaea have CRISPR–Cas systems as an adaptive defence against viruses. These enable archaea to retain sections of viral DNA, which are then used to target and eliminate subsequent infections by the virus using a process similar to RNA interference.{{cite journal | vauthors = van der Oost J, Westra ER, Jackson RN, Wiedenheft B | title = Unravelling the structural and mechanistic basis of CRISPR-Cas systems | journal = Nature Reviews Microbiology| volume = 12 | issue = 7 | pages = 479–92 | date = July 2014 | pmid = 24909109 | pmc = 4225775 | doi = 10.1038/nrmicro3279 }}

{{further|mycovirus}}

Mycoviruses, also known as mycophages, are viruses that infect fungi. The infection of fungal cells is different from that of animal cells. Fungi have a rigid cell wall made of chitin, so most viruses can get inside these cells only after trauma to the cell wall.{{cite book | vauthors = Dimmock NJ, Easton AJ, Leppard K | date = 2007 | title = Introduction to Modern Virology | url = https://archive.org/details/introductiontomo00dimm | url-access = limited | edition = Sixth | page = [https://archive.org/details/introductiontomo00dimm/page/n84 70] | publisher = Blackwell Publishing | isbn = 978-1-4051-3645-7 }}

• {{cite journal | last1 = See | last2 = Nerva | first2 = L. | last3 = Ciuffo | first3 = M. | last4 = Vallino | first4 = M. | last5 = Margaria | first5 = P. | last6 = Varese | first6 = G.C. | last7 = Gnavi | first7 = G. | last8 = Turina | first8 = M. | year = 2016 | title = Multiple approaches for the detection and characterization of viral and plasmid symbionts from a collection of marine fungi | journal = Virus Research | volume = 219 | pages = 22–38 | doi = 10.1016/j.virusres.2015.10.028 | pmid = 26546154| hdl = 2318/1527617| s2cid = 53417720 | hdl-access = free }}

File:Origin of eukaryotic viruses.webp

By 2015, about 40 viruses affecting marine protists had been isolated and examined, most of them viruses of microalgae.Tomaru Y, Kimura K and Nagasaki K (2015) [https://books.google.com/books?id=vgyhCgAAQBAJ&dq=%22Marine+Protist+Viruses%22&pg=PA502 "Marine Protist Viruses"]. In: Ohtsuka S, Suzaki T, Horiguchi T, Suzuki N, Not F (eds) Marine Protists pages 501–517. Springer, Tokyo. {{doi|10.1007/978-4-431-55130-0_20}}. {{ISBN|978-4-431-55130-0}}. The genomes of these marine protist viruses are highly diverse.{{cite journal | doi = 10.6064/2012/734023 | volume=2012 | title=Smaller Fleas: Viruses of Microorganisms | year=2012 | journal=Scientifica | pages=1–23 | last1 = Hyman | first1 = Paul | last2 = Abedon | first2 = Stephen T.| pmid=24278736 | pmc=3820453 | doi-access=free }}. 50px 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].

{{cite journal | last1 = Short | first1 = S.M. | year = 2012 | title = The ecology of viruses that infect eukaryotic algae | journal = Environmental Microbiology | volume = 14 | issue = 9| pages = 2253–2271 | doi = 10.1111/j.1462-2920.2012.02706.x | pmid = 22360532 | bibcode = 2012EnvMi..14.2253S }} Marine algae can be infected by viruses in the family Phycodnaviridae. These are large (100–560 kb) double-stranded DNA viruses with icosahedral shaped capsids. By 2014, 33 species divided into six genera had been identified within the family,{{cite web|title=Viral Zone|url=http://viralzone.expasy.org/all_by_species/145.html|publisher=ExPASy|access-date=15 June 2015}}{{cite web|last1=ICTV|title=Virus Taxonomy: 2014 Release|url=http://ictvonline.org/virusTaxonomy.asp|access-date=15 June 2015}} which belongs to a super-group of large viruses known as nucleocytoplasmic large DNA viruses. Evidence was published in 2014 suggesting some strains of Phycodnaviridae might infect humans rather than just algal species, as was previously believed.{{cite journal|year=2014|title=Chlorovirus ATCV-1 is part of the human oropharyngeal virome and is associated with changes in cognitive functions in humans and mice|journal=Proc Natl Acad Sci U S A|volume=111|issue=45|pages=16106–16111|doi=10.1073/pnas.1418895111|pmc=4234575|pmid=25349393|last1=Yolken|first1=RH|display-authors=etal|bibcode=2014PNAS..11116106Y|doi-access=free}} Most genera under this family enter the host cell by cell receptor endocytosis and replicate in the nucleus.

File:Virus cocco 2.jpg (arrowed), infecting an Emiliania huxleyi coccolithophore]]

Phycodnaviridae play important ecological roles by regulating the growth and productivity of their algal hosts. Algal species such Heterosigma akashiwo and the genus Chrysochromulina can form dense blooms which can be damaging to fisheries, resulting in losses in the aquaculture industry.{{cite journal |last1=Tomaru |first1=Yuji |last2=Shirai |first2=Yoko |last3=Nagasaki |first3=Keizo |s2cid=23152411 |date=2008-08-01 |title=Ecology, physiology and genetics of a phycodnavirus infecting the noxious bloom-forming raphidophyte Heterosigma akashiwo |journal=Fisheries Science |volume=74 |issue=4 |pages=701–711 |doi=10.1111/j.1444-2906.2008.01580.x |bibcode=2008FisSc..74..701T }} Heterosigma akashiwo virus (HaV) has been suggested for use as a microbial agent to prevent the recurrence of toxic red tides produced by this algal species.{{cite journal |last1=Nagasaki |first1=Keizo |last2=Tarutani |first2=Kenji |last3=Yamaguchi |first3=Mineo |date=1999-03-01 |title=Growth Characteristics of Heterosigma akashiwo Virus and Its Possible Use as a Microbiological Agent for Red Tide Control |journal=Applied and Environmental Microbiology |volume=65 |issue=3 |pages=898–902 |pmc=91120 |pmid=10049839|doi=10.1128/AEM.65.3.898-902.1999 |bibcode=1999ApEnM..65..898N }} The coccolithovirus Emiliania huxleyi virus 86, a giant double-stranded DNA virus, infects the ubiquitous coccolithophore Emiliania huxleyi.{{cite web|last1=ICTV|title=Virus Taxonomy: 2014 Release|url=http://ictvonline.org/virusTaxonomy.asp|access-date=15 June 2015}} This virus has one of the largest known genomes among marine viruses.[http://www.giantvirus.org/top.html Largest known viral genomes] Giantviruses.org. Accessed: 11 June 2020. Phycodnaviridae cause death and lysis of freshwater and marine algal species, liberating organic carbon, nitrogen and phosphorus into the water, providing nutrients for the microbial loop.{{cite book|url=https://books.google.com/books?id=AJ8nIkoB1o0C&q=Viruses:+major+parasites+in+the+freshwater+environment&pg=PA340|title=Freshwater Microbiology: Biodiversity and Dynamic Interactions of Microorganisms in the Aquatic Environment|last=Sigee|first=David|date=2005-09-27|publisher=John Wiley & Sons|isbn=9780470026472|language=en}}

The virus-to-prokaryote ratio, VPR, is often used as an indicator of the relationship between viruses and hosts. Studies have used VPR to indirectly infer virus impact on marine microbial productivity, mortality, and biogeochemical cycling.{{cite journal | vauthors = Wigington CH, Sonderegger DL, Brussaard CP, Buchan A, Finke JF, Fuhrman J, Lennon JT, Middelboe M, Suttle CA, Stock C, Wilson WH |date=2015-08-26|title=Re-examining the relationship between virus and microbial cell abundances in the global oceans | biorxiv = 10.1101/025544 |journal=bioRxiv|pages=025544|doi=10.1101/025544|doi-access=free}} However, in making these approximations, scientists assume a VPR of 10:1, the median observed VPR in the surface ocean. The actual VPR varies greatly depending on location, so VPR may not be the accurate proxy for viral activity or abundance as it has been treated.{{cite journal | vauthors = Parikka KJ, Le Romancer M, Wauters N, Jacquet S | s2cid = 3463306 | title = Deciphering the virus-to-prokaryote ratio (VPR): insights into virus-host relationships in a variety of ecosystems | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 92 | issue = 2 | pages = 1081–1100 | date = May 2017 | pmid = 27113012 | doi = 10.1111/brv.12271 }}

File:Marine virus-host interactions.jpg

Marine invertebrates are susceptible to viral diseases.{{cite journal | last1 = TJohnson | first1 = P.T. | s2cid = 30161955 | year = 1984 | title = Viral diseases of marine invertebrates | journal = Helgoländer Meeresuntersuchungen | volume = 37 | issue = 1–4| pages = 65–98 | doi = 10.1007/BF01989296 | bibcode = 1984HM.....37...65J | doi-access = free }}Renault T (2011) [https://books.google.com/books?id=8uXDYcIf1fwC&dq=Viruses+infecting+marine+molluscs&pg=PA153 "Viruses infecting marine molluscs"] In: Hurst CJ (Ed) Studies in Viral Ecology, Volume 2: Animal Host Systems, John Wiley & Sons. {{ISBN|9781118024584}}.{{cite journal | last1 = Arzul | first1 = I. | last2 = Corbeil | first2 = S. | last3 = Morga | first3 = B. | last4 = Renault | first4 = T. | year = 2017 | title = Viruses infecting marine molluscs | url = https://archimer.ifremer.fr/doc/00371/48206/48319.pdf| journal = Journal of Invertebrate Pathology | volume = 147 | pages = 118–135 | doi = 10.1016/j.jip.2017.01.009 | pmid = 28189502 | bibcode = 2017JInvP.147..118A }} Sea star wasting disease is a disease of starfish and several other echinoderms that appears sporadically, causing mass mortality of those affected.Dawsoni, Solaster. "Sea Star Species Affected by Wasting Syndrome." Pacificrockyintertidal.org Seastarwasting.org (n.d.): n. pag. Ecology and Evolutionary Biology. Web. There are around 40 different species of sea stars that have been affected by this disease. In 2014 it was suggested that the disease is associated with a single-stranded DNA virus now known as the sea star-associated densovirus (SSaDV); however, sea star wasting disease is not fully understood.{{cite web|url=https://www.eeb.ucsc.edu/pacificrockyintertidal/data-products/sea-star-wasting/|title=Sea Star Wasting Syndrome {{!}} MARINe|website=eeb.ucsc.edu|language=en|access-date=2018-06-03}}

Fish are particularly prone to infections with rhabdoviruses, which are distinct from, but related to rabies virus. At least nine types of rhabdovirus cause economically important diseases in species including salmon, pike, perch, sea bass, carp and cod. The symptoms include anaemia, bleeding, lethargy and a mortality rate that is affected by the temperature of the water. In hatcheries the diseases are often controlled by increasing the temperature to {{Convert|15-18|C}}.{{cite book |author1=Murphy, FA |author2=Gibbs, EPJ |author3=Horzinek, MC |author4=Studdart MJ |title=Veterinary Virology |publisher=Academic Press |location=Boston |year=1999 |isbn=978-0-12-511340-3}}{{rp|442–443}} Like all vertebrates, fish suffer from herpes viruses. These ancient viruses have co-evolved with their hosts and are highly species-specific.{{rp|324}} In fish, they cause cancerous tumours and non-cancerous growths called hyperplasia.{{rp|325}}

In 1984, infectious salmon anemia (ISAv) was discovered in Norway in an Atlantic salmon hatchery. Eighty per cent of the fish in the outbreak died. ISAv, a viral disease, is now a major threat to the viability of Atlantic salmon farming.[http://www.fis.com/fis/worldnews/worldnews.asp?l=e&ndb=1&id=30796 New Brunswick to help Chile beat disease] Fish Information and Services As the name implies, it causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA and can become infected with viruses. Management strategies include developing a vaccine and improving genetic resistance to the disease.[http://www.dfo-mpo.gc.ca/aquaculture/sheet_feuillet/aquatic_salmon-saumon-eng.htm Fact Sheet - Atlantic Salmon Aquaculture Research] {{webarchive |url=https://web.archive.org/web/20101229044828/http://www.dfo-mpo.gc.ca/aquaculture/sheet_feuillet/aquatic_salmon-saumon-eng.htm |date=December 29, 2010 }} Fisheries and Oceans Canada. Retrieved 12 May 2009.

Marine mammals are also susceptible to marine viral infections. In 1988 and 2002, thousands of harbour seals were killed in Europe by phocine distemper virus.{{cite journal | vauthors = Hall AJ, Jepson PD, Goodman SJ, Harkonen T | year = 2006 | title = Phocine distemper virus in the North and European Seas – data and models, nature and nurture | journal = Biological Conservation | volume = 131 | issue = 2| pages = 221–29 | doi=10.1016/j.biocon.2006.04.008| bibcode = 2006BCons.131..221H }} Many other viruses, including caliciviruses, herpesviruses, adenoviruses and parvoviruses, circulate in marine mammal populations.

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Most viruses 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 belong to the 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|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}}

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

File:Electron microscopic image of a mimivirus - journal.ppat.1000087.g007 crop.png| {{center|The giant mimivirus}}

File:CroV TEM (cropped).jpg| Cryo-electron micrograph of the CroV giant virus {{cite journal | last1 = Xiao | first1 = C. | last2 = Fischer | first2 = M.G. | last3 = Bolotaulo | first3 = D.M. | last4 = Ulloa-Rondeau | first4 = N. | last5 = Avila | first5 = G.A. | last6 = Suttle | first6 = C.A. | year = 2017 | title = Cryo-EM reconstruction of the Cafeteria roenbergensis virus capsid suggests novel assembly pathway for giant viruses | journal = Scientific Reports | volume = 7 | issue = 1| page = 5484 | doi = 10.1038/s41598-017-05824-w | pmid = 28710447 | bibcode = 2017NatSR...7.5484X | pmc = 5511168 }}
scale bar=0.2 μm

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The discovery and subsequent characterization of giant viruses has triggered some debate concerning their evolutionary origins. 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 |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|url-access=subscription }}{{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 | s2cid = 206655792 | 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 | url = https://escholarship.org/content/qt0kf9t6gn/qt0kf9t6gn.pdf?t=oruwia| doi-access = free | osti = 1434812 }}{{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}}

{{further|Virophages}}

Virophages are small, double-stranded DNA viruses that rely on the co-infection of giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for their own replication. One of the characteristics of virophages is that they have a parasitic relationship with the co-infecting virus. Their dependence upon the giant virus for replication often results in the deactivation of the giant viruses. The virophage may improve the recovery and survival of the host organism. Unlike other satellite viruses, virophages have a parasitic effect on their co-infecting virus. Virophages have been observed to render a giant virus inactive and thereby improve the condition of the host organism.

All known virophages are grouped into the family Lavidaviridae (from "large virus dependent or associated" + -viridae).{{cite journal |last1=Duponchel |first1=S |last2=Fischer |first2=MG |title=Viva lavidaviruses! Five features of virophages that parasitize giant DNA viruses. |journal=PLOS Pathogens |date=March 2019 |volume=15 |issue=3 |pages=e1007592 |doi=10.1371/journal.ppat.1007592 |pmid=30897185 |pmc=6428243 |doi-access=free }} The first virophage was discovered in a cooling tower in Paris in 2008. It was discovered with its co-infecting giant virus, Acanthamoeba castellanii mamavirus (ACMV). The virophage was named Sputnik and its replication relied entirely on the co-infection of ACMV and its cytoplasmic replication machinery. Sputnik was also discovered to have an inhibitory effect on ACMV and improved the survival of the host. Other characterised virophages include Sputnik 2, Sputnik 3, Zamilon and Mavirus.{{cite journal |author=Fischer MG, Suttle CA |s2cid=206530677 |title=A virophage at the origin of large DNA transposons |journal=Science |volume=332 |issue=6026 |pages=231–4 |date=April 2011 |pmid=21385722 |doi=10.1126/science.1199412 |bibcode=2011Sci...332..231F }}{{cite journal |author=Fischer MG, Hackl |s2cid=4458402 |title=Host genome integration and giant virus-induced reactivation of the virophage mavirus |journal=Nature |volume=540 |issue=7632 |pages=288–91 |date=December 2016 |pmid=27929021 |doi=10.1038/nature20593 |bibcode=2016Natur.540..288F }}

Most of these virophages were discovered by analyzing metagenomic data sets. In metagenomic analysis, DNA sequences are run through multiple bioinformatic algorithms which pull out certain important patterns and characteristics. In these data sets are giant viruses and virophages. They are separated by looking for sequences around 17 to 20 kbp long which have similarities to already sequenced virophages. These virophages can have linear or circular double-stranded DNA genomes.{{cite journal|last1=Katzourakis|first1=Aris|last2=Aswad|first2=Amr|date=2014|title=The origins of giant viruses, virophages and their relatives in host genomes|journal=BMC Biology|volume=12|pages=2–3|doi=10.1186/s12915-014-0051-y|pmc=4096385|pmid=25184667 |doi-access=free }} Virophages in culture have icosahedral capsid particles that measure around 40 to 80 nanometers long.{{cite journal|last1=Krupovic|first1=Mart|last2=Kuhn|first2=Jens|last3=Fischer|first3=Metthias|s2cid=14196910|date=Fall 2015|title=A classification system for virophages and satellite viruses|url=https://link.springer.com/content/pdf/10.1007%2Fs00705-015-2622-9.pdf|journal=Archives of Virology|volume=161|issue=1|pages=233–247|via=Springer|doi=10.1007/s00705-015-2622-9|pmid=26446887|doi-access=free}} Virophage particles are so small that electron microscopy must be used to view these particles. Metagenomic sequence-based analyses have been used to predict around 57 complete and partial virophage genomes{{cite journal|last1=Roux|first1=Simon|last2=Chan|first2=Leong-Keat|last3=Egan|first3=Rob|last4=Malmstrom|first4=Rex R.|last5=McMahon|first5=Katherine D.|last6=Sullivan|first6=Matthew B.|date=2017|title=Ecogenomics of virophages and their giant virus hosts assessed through time series metagenomics|journal=Nature Communications|language=En|volume=8|issue=1|pages=858|doi=10.1038/s41467-017-01086-2|pmid=29021524|pmc=5636890|issn=2041-1723|bibcode=2017NatCo...8..858R}} and in December 2019 to identify 328 high-quality (complete or near-complete) genomes from diverse habitats including the human gut, plant rhizosphere, and terrestrial subsurface, from 27 distinct taxonomic clades.{{cite journal|last1=Paez-Espino|first1=David|last2=Zhou|first2=Jinglie|last3=Roux|first3=Simon|last4=Nayfach|first4=Stephen|last5=Pavlopoulos|first5=Georgios A.|last6=Schulz|first6=Frederik|last7=McMahon|first7=Katherine D.|last8=Walsh|first8=David|last9=Woyke|first9=Tanja|last10=Ivanova|first10=Natalia N.|last11=Eloe-Fadrosh|first11=Emiley A.|last12=Tringe|first12=Susannah G.|last13=Kyrpides|first13=Nikos C.|date=2019-12-10|title=Diversity, evolution, and classification of virophages uncovered through global metagenomics|journal=Microbiome|language=En|volume=7|issue=1|pages=157| doi=10.1186/s40168-019-0768-5|pmid=31823797|pmc=6905037 |doi-access=free }}

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| caption1 = The giant virus CroV attacks C.{{nbsp}}roenbergensis.

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| caption3 = A Mavirus virophage (lower left) alongside a giant CroV{{cite journal | doi = 10.1371/journal.ppat.1007592 | pmid=30897185 | pmc=6428243 | volume=15 | title=Viva lavidaviruses! Five features of virophages that parasitize giant DNA viruses | year=2019 | journal=PLOS Pathogens| page=e1007592 | last1 = Duponchel | first1 = S | last2 = Fischer | first2 = MG| issue=3 | 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].

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A giant marine virus CroV infects and causes the death by lysis of the marine zooflagellate Cafeteria roenbergensis.{{cite journal |author1=Fischer, M. G. |author2=Allen, M. J. |author3=Wilson, W. H. |author4=Suttle, C. A. |year=2010 |title=Giant virus with a remarkable complement of genes infects marine zooplankton |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=45 |pages=19508–19513 |bibcode=2010PNAS..10719508F |doi=10.1073/pnas.1007615107 |pmc=2984142 |pmid=20974979|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:2272841/component/escidoc:2272838/PNAS_107_2010_19508.pdf |doi-access=free }} This impacts coastal ecology because Cafeteria roenbergensis feeds on bacteria found in the water. When there are low numbers of Cafeteria roenbergensis due to extensive CroV infections, the bacterial populations rise exponentially.{{cite journal |author1=Matthias G. Fischer |author2=Michael J. Allen |author3=William H. Wilson |author4=Curtis A. Suttle |title=Giant virus with a remarkable complement of genes infects marine zooplankton|journal=Proceedings of the National Academy of Sciences|year=2010|doi=10.1073/pnas.1007615107 |volume=107 |issue=45 |pages=19508–19513 |bibcode=2010PNAS..10719508F |pmid=20974979 |pmc=2984142|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:2272841/component/escidoc:2272838/PNAS_107_2010_19508.pdf|doi-access=free }} The impact of CroV on natural populations of C. roenbergensis remains unknown; however, the virus has been found to be very host specific, and does not infect other closely related organisms.{{cite journal |author1=Massana, Ramon |author2=Javier Del Campo |author3=Christian Dinter |author4=Ruben Sommaruga |s2cid=30191542 |year=2007 |title=Crash of a population of the marine heterotrophic flagellate Cafeteria roenbergensis by viral infection |journal=Environmental Microbiology |volume=9 |issue=11 |pages=2660–2669 |doi=10.1111/j.1462-2920.2007.01378.x|pmid=17922751 |bibcode=2007EnvMi...9.2660M }} Cafeteria roenbergensis is also infected by a second virus, the Mavirus virophage, during co-infection with CroV. This virus interferes with the replication of CroV, which leads to the survival of C. roenbergensis cells. Mavirus is able to integrate into the genome of cells of C. roenbergensis and thereby confer immunity to the population.

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Although marine viruses have only recently been studied extensively, they are already known to hold critical roles in many ecosystem functions and cycles.Herndl G, Briand F, eds. (2003). Ecology of marine viruses. CIESM Workshop Monograph 21 [https://www.researchgate.net/publication/367150361] Marine viruses offer a number of important ecosystem services and are essential to the regulation of marine ecosystems.{{cite book | vauthors = Shors T | date = 2008 | title = Understanding Viruses | publisher = Jones and Bartlett Publishers | page = 5| isbn = 978-0-7637-2932-5 }} Marine bacteriophages and other viruses appear to influence biogeochemical cycles globally, provide and regulate microbial biodiversity, cycle carbon through marine food webs, and are essential in preventing bacterial population explosions.{{cite book|title=Phages: their role in bacterial pathogenesis and biotechnology|url=https://archive.org/details/phagestheirrolei0000unse|url-access=registration|publisher=ASM Press|year=2005|isbn=978-1-55581-307-9| veditors = Waldor MK, Friedman DI, Adhya SL |location=Washington DC|pages=[https://archive.org/details/phagestheirrolei0000unse/page/450 450]}}

File:Cycling of marine phytoplankton.png.{{cite book | doi=10.1007/978-3-319-93284-2_5| chapter=Phytoplankton Responses to Marine Climate Change – an Introduction| title=YOUMARES 8 – Oceans Across Boundaries: Learning from each other| year=2018| last1=Käse| first1=Laura| last2=Geuer| first2=Jana K.| pages=55–71| isbn=978-3-319-93283-5| s2cid=134263396}}}}]]

{{main|Viral shunt}}

The dominant hosts for viruses in the ocean are marine microorganisms, such as bacteria.{{cite journal | vauthors = Mann NH | title = The third age of phage | journal = PLOS Biology | volume = 3 | issue = 5 | pages = e182 | date = May 2005 | pmid = 15884981 | pmc = 1110918 | doi = 10.1371/journal.pbio.0030182 | doi-access = free }} Bacteriophages are harmless to plants and animals, and are essential to the regulation of marine and freshwater ecosystems{{cite journal | vauthors = Weitz JS, Wilhelm SW |title=An ocean of viruses |journal=The Scientist |volume=27 |issue=7 |pages=35–39 |year=2013 |url=https://www.the-scientist.com/?articles.view/articleNo/36120/title/An-Ocean-of-Viruses/ }} are important mortality agents of phytoplankton, the base of the foodchain in aquatic environments.{{cite journal | vauthors = Suttle CA | s2cid = 4370363 | title = Viruses in the sea | journal = Nature | volume = 437 | issue = 7057 | pages = 356–61 | date = September 2005 | pmid = 16163346 | doi = 10.1038/nature04160 | bibcode = 2005Natur.437..356S }} They infect and destroy bacteria in aquatic microbial communities, and are one of the most important mechanisms of recycling carbon and nutrient cycling in marine environments. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth, in a process known as the viral shunt.{{cite journal | vauthors = Wilhelm SW, Suttle CA |title=Viruses and nutrient cycles in the sea: viruses play critical roles in the structure and function of aquatic food webs |journal=BioScience |volume=49 |issue=10 |pages=781–88 |year=1999 |doi=10.2307/1313569|jstor=1313569 |doi-access=free }}

In this way, marine viruses are thought to play an important role in nutrient cycles by increasing the efficiency of the biological pump. Viruses cause lysis of living cells, that is, they break the cell membranes down. This releases compounds such as amino acids and nucleic acids, which tend to be recycled near the surface.

Viral activity also enhances the ability of the biological pump to sequester carbon in the deep ocean.{{cite journal | vauthors = Suttle CA | s2cid = 4658457 | title = Marine viruses – major players in the global ecosystem | journal = Nature Reviews Microbiology| volume = 5 | issue = 10 | pages = 801–12 | date = October 2007 | pmid = 17853907 | doi = 10.1038/nrmicro1750 }} Lysis releases more indigestible carbon-rich material like that found in cell walls, which is likely exported to deeper waters. Thus, the material that is exported to deeper waters by the viral shunt is probably more carbon rich than the material from which it was derived.{{cite journal|author=Suttle CA|s2cid=4658457|title=Marine viruses—major players in the global ecosystem|journal= Nature Reviews Microbiology|volume=5|issue=10|pages=801–12|date=October 2007|pmid=17853907|doi=10.1038/nrmicro1750}}{{cite journal | vauthors = Suttle CA | s2cid = 4370363 | title = Viruses in the sea | journal = Nature | volume = 437 | issue = 7057 | pages = 356–61 | date = September 2005 | pmid = 16163346 | doi = 10.1038/nature04160 | bibcode = 2005Natur.437..356S }} By increasing the amount of respiration in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by about three gigatonnes of carbon per year. Lysis of bacteria by viruses has been shown to also enhance nitrogen cycling and stimulate phytoplankton growth.{{cite journal | vauthors = Shelford EJ, Suttle CA |title=Virus-mediated transfer of nitrogen from heterotrophic bacteria to phytoplankton |journal=Biogeosciences |volume=15 |issue=3 |pages=809–15 |year=2018 |url=https://www.biogeosciences.net/15/809/2018/bg-15-809-2018.html |doi=10.5194/bg-15-809-2018|bibcode=2018BGeo...15..809S |doi-access=free }}

The viral shunt pathway is a mechanism that prevents (prokaryotic and eukaryotic) 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. 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:Viral shunt.jpg facilitates the flow of dissolved organic matter (DOM) and particulate organic matter (POM) through the marine food web.]]

File:Marine connections between the living and the nonliving.png

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Viruses are the most abundant biological entity in marine environments. On average there are about ten million of them in one milliliter of seawater.{{cite journal | vauthors = Dávila-Ramos S, Castelán-Sánchez HG, Martínez-Ávila L, Sánchez-Carbente MD, Peralta R, Hernández-Mendoza A, Dobson AD, Gonzalez RA, Pastor N, Batista-García RA | title = A Review on Viral Metagenomics in Extreme Environments | journal = Frontiers in Microbiology | volume = 10 | pages = 2403 | date = 2019 | pmid = 31749771 | pmc = 6842933 | doi = 10.3389/fmicb.2019.02403 | doi-access = free }} Most of these viruses are bacteriophages infecting heterotrophic bacteria and cyanophages infecting cyanobacteria. Viruses easily infect microorganisms in the microbial loop due to their relative abundance compared to microbes.{{cite journal|last=Fuhrman|first=Jed A.|s2cid=1260399|date=1999|title=Marine viruses and their biogeochemical and ecological effects|journal=Nature|volume=399|issue=6736|pages=541–548|doi=10.1038/21119|pmid=10376593|issn=0028-0836|bibcode=1999Natur.399..541F}} Prokaryotic and eukaryotic mortality contribute to carbon nutrient recycling through cell lysis. There is evidence as well of nitrogen (specifically ammonium) regeneration. This nutrient recycling helps stimulates microbial growth.Tsai, An-Yi, Gwo-Ching Gong, and Yu-Wen Huang. "Importance of the Viral Shunt in Nitrogen Cycling in Synechococcus Spp. Growth in Subtropical Western Pacific Coastal Waters." Terrestrial, Atmospheric & Oceanic Sciences25.6 (2014). As much as 25% of the primary production from phytoplankton in the global oceans may be recycled within the microbial loop through viral shunting.{{cite journal | last1 = Wilhelm | first1 = Steven W. | last2 = Suttle | first2 = Curtis A. | year = 1999 | title = Viruses and nutrient cycles in the sea: viruses play critical roles in the structure and function of aquatic food webs | journal = BioScience | volume = 49 | issue = 10| pages = 781–788 | doi = 10.2307/1313569 | jstor = 1313569 | doi-access = free }}

Viruses act as "regulators" of the global carbon cycle because they impact the material cycles and energy flows of food webs and the microbial loop. The average contribution of viruses to the Earth ecosystem carbon cycle is 8.6%, of which its contribution to marine ecosystems (1.4%) is less than its contribution to terrestrial (6.7%) and freshwater (17.8%) ecosystems. Over the past 2,000 years, anthropogenic activities and climate change have gradually altered the regulatory role of viruses in ecosystem carbon cycling processes. This has been particularly conspicuous over the past 200 years due to rapid industrialization and the attendant population growth.{{cite journal | last1=Gao | first1=Yang | last2=Lu | first2=Yao | last3=Dungait | first3=Jennifer A. J. | last4=Liu | first4=Jianbao | last5=Lin | first5=Shunhe | last6=Jia | first6=Junjie | last7=Yu | first7=Guirui | title=The "Regulator" Function of Viruses on Ecosystem Carbon Cycling in the Anthropocene | journal=Frontiers in Public Health | publisher=Frontiers Media SA | volume=10 | date=2022-03-29 | issn=2296-2565 | doi=10.3389/fpubh.2022.858615 | doi-access=free | page=| pmid=35425734 | pmc=9001988 | bibcode=2022FrPH...1058615G }} 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:Viral impacts on ecosystem carbon cycles.jpg of bacteria to the ecosystem dissolved organic carbon (DOC) pool. Freshwater ecosystems are regulated in a manner similar to marine ecosystems, and are not shown separately. The microbial loop is an important supplement to the classic food chain, wherein dissolved organic matter is ingested by heterotrophic "planktonic" bacteria during secondary production. These bacteria are then consumed by protozoa, copepods and other organisms, and eventually returned to the classical food chain.]]

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Microorganisms make up about 70% of the marine biomass.{{cite journal | last1 = Bar-On | first1 = YM | last2 = Phillips | first2 = R | last3 = Milo | first3 = R | year = 2018 | title = The biomass distribution on Earth | journal = PNAS | volume = 115 | issue = 25| pages = 6506–6511 | doi = 10.1073/pnas.1711842115 | pmid = 29784790 | pmc = 6016768 | bibcode = 2018PNAS..115.6506B | doi-access = free }} It is estimated viruses kill 20% of the microorganism 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 |accessdate=2014-12-19}} Scientists are exploring the potential of marine cyanophages to be used to prevent or reverse eutrophication. The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

Marine bacteriophages often contain auxiliary metabolic genes, host-derived genes thought to sustain viral replication by supplementing host metabolism during viral infection.{{cite journal |last1=Breitbart |first1=Mya |last2=Thompson |first2=Luke |last3=Suttle |first3=Curtis |last4=Sullivan |first4=Matthew |date=2007-06-01|title=Exploring the Vast Diversity of Marine Viruses|journal=Oceanography|volume=20|issue=2|pages=135–139|doi=10.5670/oceanog.2007.58 |url=https://tos.org/oceanography/assets/docs/20-2_breitbart.pdf|doi-access=free |bibcode=2007Ocgpy..20b.135B }}  These genes can impact multiple biogeochemical cycles, including carbon, phosphorus, sulfur, and nitrogen.{{cite journal | vauthors = Hurwitz BL, U'Ren JM | title = Viral metabolic reprogramming in marine ecosystems | journal = Current Opinion in Microbiology | volume = 31 | pages = 161–168 | date = June 2016 | pmid = 27088500 | doi = 10.1016/j.mib.2016.04.002 }}{{cite journal | vauthors = Hurwitz BL, Hallam SJ, Sullivan MB | title = Metabolic reprogramming by viruses in the sunlit and dark ocean | journal = Genome Biology | volume = 14 | issue = 11 | pages = R123 | date = November 2013 | pmid = 24200126 | pmc = 4053976 | doi = 10.1186/gb-2013-14-11-r123 | doi-access = free }}{{cite journal | vauthors = Anantharaman K, Duhaime MB, Breier JA, Wendt KA, Toner BM, Dick GJ | s2cid = 692770 | title = Sulfur oxidation genes in diverse deep-sea viruses | journal = Science | volume = 344 | issue = 6185 | pages = 757–60 | date = May 2014 | pmid = 24789974 | doi = 10.1126/science.1252229 | hdl = 1912/6700 | bibcode = 2014Sci...344..757A | hdl-access = free }}{{cite journal | vauthors = Roux S, Hawley AK, Torres Beltran M, Scofield M, Schwientek P, Stepanauskas R, Woyke T, Hallam SJ, Sullivan MB | title = Ecology and evolution of viruses infecting uncultivated SUP05 bacteria as revealed by single-cell- and meta-genomics | journal = eLife | volume = 3 | pages = e03125 | date = August 2014 | pmid = 25171894 | pmc = 4164917 | doi = 10.7554/elife.03125 | doi-access = free }}

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.{{cite journal|author=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 viruses played a central role in early evolution, before bacteria, archaea and eukaryotes diversified, at the time of the last universal common ancestor of life on Earth.{{cite journal |author=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.

{{see also|Mangrove virome}}

Marine coastal habitats sit at the interface between the land and the ocean. It is likely that RNA viruses play significant roles in these environments.{{cite journal | last1 = Culley | first1 = A.I. | last2 = Lang | first2 = A.S. | last3 = Suttle | first3 = C.A. | s2cid = 20194876 | year = 2006 | title = Metagenomic analysis of coastal RNA virus communities | journal = Science | volume = 312 | issue = 5781| pages = 1795–1798 | doi = 10.1126/science.1127404 | pmid = 16794078 | bibcode = 2006Sci...312.1795C }}

File:Viral–bacterial dynamics in the ocean surface.png (SML) of the ocean and beyond. DOM = dissolved organic matter, UV = ultraviolet.{{cite journal | doi = 10.3390/v11020191 | volume=11 | title=The Virioneuston: A Review on Viral–Bacterial Associations at Air–Water Interfaces | journal=Viruses | page=191 | last1 = Rahlff | first1 = Janina| year=2019 | issue=2 | pmid=30813345 | pmc=6410083 | 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].]]

{{see also|Sea surface microlayer}}

Marine surface habitats sit at the interface between the atmosphere and the ocean. The biofilm-like habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as neuston. Viruses in the microlayer, the so-called virioneuston, have recently become of interest to researchers as enigmatic biological entities in the boundary surface layers with potentially important ecological impacts. Given this vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area, it is likely to have profound implications for marine biogeochemical cycles, on the microbial loop and gas exchange, as well as the marine food web structure, the global dispersal of airborne viruses originating from the sea surface microlayer, and human health.

Marine viral activity presents a potential explanation of the paradox of the plankton proposed by George Hutchinson in 1961.{{cite journal| vauthors = Hutchinson GE |date=1961 |title=The Paradox of the Plankton | jstor= 2458386 |journal=The American Naturalist |volume=95 |issue=882 |pages=137–145 |doi=10.1086/282171 |bibcode=1961ANat...95..137H |s2cid=86353285 }} The paradox of the plankton is that many plankton species have been identified in small regions in the ocean where limited resources should create competitive exclusion, limiting the number of coexisting species. Marine viruses could play a role in this effect, as viral infection increases as potential contact with hosts increases. Viruses could therefore control the populations of plankton species that grow too abundant, allowing a wide diversity of species to coexist.

{{see also|marine sediments}}

Marine bacteriophages play an important role in deep sea ecosystems. There are between 5x10¹² and 1x10¹³ phages per square metre in deep sea sediments and their abundance closely correlates with the number of prokaryotes found in the sediments. They are responsible for the death of 80% of the prokaryotes found in the sediments, and almost all of these deaths are caused by cell lysis (bursting). This allows nitrogen, carbon, and phosphorus from the living cells to be converted into dissolved organic matter and detritus, contributing to the high rate of nutrient turnover in deep sea sediments. Because of the importance of deep sea sediments in biogeochemical cycles, marine bacteriophages influence the carbon, nitrogen and phosphorus cycles. More research needs to be done to more precisely elucidate these influences.{{cite journal | vauthors = Danovaro R, Dell'Anno A, Corinaldesi C, Magagnini M, Noble R, Tamburini C, Weinbauer M | s2cid = 4331430 | title = Major viral impact on the functioning of benthic deep-sea ecosystems | journal = Nature | volume = 454 | issue = 7208 | pages = 1084–7 | date = August 2008 | pmid = 18756250 | doi = 10.1038/nature07268 | bibcode = 2008Natur.454.1084D }}

{{further|hydrothermal vent microbial community}}

Viruses are 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|bibcode=2011FrMic...2..219A }} Viruses are the most abundant life in the ocean, harboring the greatest reservoir of genetic diversity.{{cite journal|last=Suttle|first=Curtis A.|s2cid=4370363|date=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}} 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.|s2cid=4658457|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|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|s2cid=10737747|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}}

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{{further|Marine food web#Polar microorganisms}}

In addition to varied topographies and in spite of an extremely cold climate, the polar aquatic regions are teeming with microbial life. Even in sub-glacial regions, cellular life has adapted to these extreme environments where perhaps there are traces of early microbes on Earth. As grazing by macrofauna is limited in most of these polar regions, viruses are being recognised for their role as important agents of mortality, thereby influencing the biogeochemical cycling of nutrients that, in turn, impact community dynamics at seasonal and spatial scales.{{cite journal |doi = 10.3390/v11020189|doi-access = free|title = Viruses of Polar Aquatic Environments|year = 2019|last1 = Yau|first1 = Sheree|last2 = Seth-Pasricha|first2 = Mansha|journal = Viruses|volume = 11|issue = 2|page = 189|pmid = 30813316|pmc = 6410135}} 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]. The polar regions are characterised by truncated food webs, and the role of viruses in ecosystem function is likely to be even greater than elsewhere in the marine food web, yet their diversity is still relatively under-explored, and the way in which they affect polar communities is not well understood,{{cite journal |doi = 10.1016/j.tim.2010.11.002|title = Are low temperature habitats hot spots of microbial evolution driven by viruses?|year = 2011|last1 = Anesio|first1 = Alexandre M.|last2 = Bellas|first2 = Christopher M.|journal = Trends in Microbiology|volume = 19|issue = 2|pages = 52–57|pmid = 21130655}} particularly in nutrient cycling.{{cite journal |doi = 10.1126/science.1173645|title = No Place Too Cold|year = 2009|last1 = Laybourn-Parry|first1 = Johanna|journal = Science|volume = 324|issue = 5934|pages = 1521–1522|pmid = 19541982|bibcode = 2009Sci...324.1521L|s2cid = 33598792}}{{cite journal |doi = 10.1126/science.1179287|title = High Diversity of the Viral Community from an Antarctic Lake|year = 2009|last1 = López-Bueno|first1 = Alberto|last2 = Tamames|first2 = Javier|last3 = Velázquez|first3 = David|last4 = Moya|first4 = Andrés|last5 = Quesada|first5 = Antonio|last6 = Alcamí|first6 = Antonio|journal = Science|volume = 326|issue = 5954|pages = 858–861|pmid = 19892985|bibcode = 2009Sci...326..858L|s2cid = 32607904}}{{cite journal |doi = 10.1007/s00792-007-0134-6|title = Bacteriophage in polar inland waters|year = 2008|last1 = Säwström|first1 = Christin|last2 = Lisle|first2 = John|last3 = Anesio|first3 = Alexandre M.|last4 = Priscu|first4 = John C.|last5 = Laybourn-Parry|first5 = Johanna|journal = Extremophiles|volume = 12|issue = 2|pages = 167–175|pmid = 18188502| bibcode=2008Extmo..12..167S |s2cid = 2927907| url=https://figshare.com/articles/journal_contribution/22867211 }}

Viruses are highly host specific.{{cite journal | vauthors = Leggett HC, Buckling A, Long GH, Boots M | title = Generalism and the evolution of parasite virulence | language = en | journal = Trends in Ecology & Evolution | volume = 28 | issue = 10 | pages = 592–6 | date = October 2013 | pmid = 23968968 | doi = 10.1016/j.tree.2013.07.002 | bibcode = 2013TEcoE..28..592L }} A marine virus is more likely to infect cooccurring organisms, those that live in the same region the virus lives in.{{cite journal | vauthors = Flores CO, Valverde S, Weitz JS | title = Multi-scale structure and geographic drivers of cross-infection within marine bacteria and phages | journal = The ISME Journal | volume = 7 | issue = 3 | pages = 520–32 | date = March 2013 | pmid = 23178671 | pmc = 3578562 | doi = 10.1038/ismej.2012.135 | bibcode = 2013ISMEJ...7..520F }} Therefore, biogeography is an important factor in a virion's ability to infect.

Knowledge of this variation in viral populations across spatiotemporal and other environmental gradients is supported by viral morphology, as determined by transmission electron microscopy (TEM).  Non-tailed viruses appear to be dominant in multiple depths and oceanic regions, followed by the Caudovirales 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 }} However, viruses belonging to families Corticoviridae,{{cite journal | vauthors = Krupovic M, Bamford DH | title = Putative prophages related to lytic tailless marine dsDNA phage PM2 are widespread in the genomes of aquatic bacteria | journal = BMC Genomics | volume = 8 | pages = 236 | date = July 2007 | pmid = 17634101 | pmc = 1950889 | doi = 10.1186/1471-2164-8-236 | doi-access = free }} Inoviridae{{cite journal | vauthors = Xue H, Xu Y, Boucher Y, Polz MF | title = High frequency of a novel filamentous phage, VCY φ, within an environmental Vibrio cholerae population | journal = Applied and Environmental Microbiology | volume = 78 | issue = 1 | pages = 28–33 | date = January 2012 | pmid = 22020507 | pmc = 3255608 | doi = 10.1128/AEM.06297-11 | bibcode = 2012ApEnM..78...28X }} and Microviridae{{cite journal | vauthors = Roux S, Krupovic M, Poulet A, Debroas D, Enault F | title = Evolution and diversity of the Microviridae viral family through a collection of 81 new complete genomes assembled from virome reads | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e40418 | year = 2012 | pmid = 22808158 | pmc = 3394797 | doi = 10.1371/journal.pone.0040418 | bibcode = 2012PLoSO...740418R | doi-access = free }} are also known to infect diverse marine bacteria. Metagenomic evidence suggests that microviruses (icosahedral ssDNA phages) are particularly prevalent in marine habitats.

Metagenomic approaches to assess viral diversity are often limited by a lack of reference sequences, leaving many sequences unannotated.{{cite journal | vauthors = Hurwitz BL, Sullivan MB | title = The Pacific Ocean virome (POV): a marine viral metagenomic dataset and associated protein clusters for quantitative viral ecology | journal = PLOS ONE | volume = 8 | issue = 2 | pages = e57355 | date = 2013 | pmid = 23468974 | pmc = 3585363 | doi = 10.1371/journal.pone.0057355 | bibcode = 2013PLoSO...857355H | doi-access = free }}  However, viral contigs are generated through direct sequencing of a viral fraction, typically generated after 0.02-um filtration of a marine water sample, or through bioinformatics approaches to identify viral contigs or viral genomes from a microbial metagenome.  Novel tools to identify putative viral contigs, such as VirSorter{{cite journal | vauthors = Roux S, Enault F, Hurwitz BL, Sullivan MB | title = VirSorter: mining viral signal from microbial genomic data | journal = PeerJ | volume = 3 | pages = e985 | date = 2015-05-28 | pmid = 26038737 | pmc = 4451026 | doi = 10.7717/peerj.985 | doi-access = free }} and VirFinder,{{cite journal | vauthors = Ren J, Ahlgren NA, Lu YY, Fuhrman JA, Sun F | title = VirFinder: a novel k-mer based tool for identifying viral sequences from assembled metagenomic data | journal = Microbiome | volume = 5 | issue = 1 | pages = 69 | date = July 2017 | pmid = 28683828 | pmc = 5501583 | doi = 10.1186/s40168-017-0283-5 | doi-access = free }} allow for the assessment of patterns of viral abundance, host range, and functional content of marine bacteriophage.{{cite journal | vauthors = Paez-Espino D, Eloe-Fadrosh EA, Pavlopoulos GA, Thomas AD, Huntemann M, Mikhailova N, Rubin E, Ivanova NN, Kyrpides NC | s2cid = 4466854 | title = Uncovering Earth's virome | journal = Nature | volume = 536 | issue = 7617 | pages = 425–30 | date = August 2016 | pmid = 27533034 | doi = 10.1038/nature19094 | bibcode = 2016Natur.536..425P | url = http://www.escholarship.org/uc/item/4zh090xt }}{{cite journal | vauthors = Coutinho FH, Silveira CB, Gregoracci GB, Thompson CC, Edwards RA, Brussaard CP, Dutilh BE, Thompson FL | title = Marine viruses discovered via metagenomics shed light on viral strategies throughout the oceans | journal = Nature Communications | volume = 8 | pages = 15955 | date = July 2017 | pmid = 28677677 | pmc = 5504273 | doi = 10.1038/ncomms15955 | bibcode = 2017NatCo...815955C }}

{{portal|Marine life|Oceans|Viruses}}

• Human viruses in water
• Varidnaviria

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{{reflist}}

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Category:Viruses

Category:Marine organisms

Category:Planktology

Category:Biological oceanography

Category:Marine biology