Protist shell

Cellular life likely originated as single-celled prokaryotes (including modern bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes include organisms such as plants, animals, fungi and "protists". Protists are usually single-celled and microscopic. They can be heterotrophic, meaning they obtain nutrients by consuming other organisms, or autotrophic, meaning they produce their own food through photosynthesis or chemosynthesis, or mixotrophic, meaning they produce their own food through a mixture of those methods.

The term protist came into use historically to refer to a group of biologically similar organisms; however, modern research has shown it to be a paraphyletic group that does not contain all descendants of a common ancestor. As such it does not constitute a clade and is not currently in formal scientific use. Nonetheless, the term continues to be used informally to refer to those eukaryotes that cannot be classified as plants, fungi or animals.

Most protists are too small to be seen with the naked eye. They 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}} Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers 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 |pmid=11755083 | url=http://download.bioon.com.cn/view/upload/month_0803/20080326_daa08a6fdb5d38e3a0d8VBrocN3WtOdR.attach.pdf | doi=10.1016/S0966-842X(01)02257-0}} As eukaryotes, protists possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Many protists are asexual but can reproduce rapidly through mitosis or by fragmentation; others (including foraminifera) may reproduce either sexually or asexually.{{Cite book|url=https://www.worldcat.org/oclc/9276403|title=Foraminifera: notes for a short course organized by M.A. Buzas and B.K. Sen Gupta: prepared for the short course on foraminifera sponsored by the Paleontological Society, held at New Orleans, Louisiana, October 17, 1982|date=1982|publisher=University of Tennessee, Dept. of Geological Sciences|others=Thomas W. Broadhead, Paleontological Society|isbn=0-910249-05-9|location=[Knoxville, Tenn.]|oclc=9276403}}

In contrast to the cells of bacteria and archaea, the cells of protists and other 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, and some marine protists are neither single-celled nor microscopic, such as seaweed.

File:20110123 185042 Diatom.jpg

{{see also|Biogenic silica}}

Although silicon is readily available in the form of silicates, very few organisms use it directly. Diatoms, radiolaria, and siliceous sponges use biogenic silica as a structural material for their skeletons. In more advanced plants, the silica phytoliths (opal phytoliths) are rigid microscopic bodies occurring in the cell; some plants, including rice, need silica for their growth.{{cite book |chapter=Silicon |page=856 |isbn=978-0-444-53181-0 |title=Studies in Natural Products Chemistry |volume=35 |first=Atta-ur- |last=Rahman|year=2008 }}{{cite journal |last1=Exley |first1=C. |title=Silicon in life:A bioinorganic solution to bioorganic essentiality |journal=Journal of Inorganic Biochemistry |volume=69 |pages=139–144 |date=1998 |doi=10.1016/S0162-0134(97)10010-1 |issue=3}}{{cite journal |last1=Epstein |first1=Emanuel |title=SILICON |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=50 |date=1999 |pmid=15012222 |doi=10.1146/annurev.arplant.50.1.641 |pages=641–664}} Silica has been shown to improve plant cell wall strength and structural integrity in some plants.{{cite journal |doi=10.1094/PHYTO.2002.92.10.1095 |pmid=18944220 |title=Silicon-Induced Cell Wall Fortification of Rice Leaves: A Possible Cellular Mechanism of Enhanced Host Resistance to Blast |journal=Phytopathology |volume=92| issue=10 |pages=1095–103 |year=2002 |last1=Kim |first1=Sang Gyu |last2=Kim |first2=Ki Woo |last3=Park |first3=Eun Woo |last4=Choi |first4=Doil|doi-access=free }}

{{main|diatom frustule}}

{{see also|diatomaceous earth}}

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,[https://www.livescience.com/46250-teasing-apart-the-diatom-genome.html The Air You're Breathing? A Diatom Made That] 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.

Diatoms are enclosed in protective silica (glass) shells called frustules. The beautifully engineered and intricate structure of many of these frustules is such that they are often referred to as "jewels of the sea".Ireland, T., [https://www.rsb.org.uk/biologist-features/jewels-of-the-sea "Engineering with algae"]. Biologist, 63(5): 10. Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes.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. 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}}

Diatoms uses silicon in the biogenic silica (BSiO₂) form,{{Cite journal|last1=Bidle|first1=Kay D.|last2=Manganelli|first2=Maura|last3=Azam|first3=Farooq|date=2002-12-06|title=Regulation of Oceanic Silicon and Carbon Preservation by Temperature Control on Bacteria|url=https://www.science.org/doi/10.1126/science.1076076|journal=Science|language=en|volume=298|issue=5600|pages=1980–1984|doi=10.1126/science.1076076|issn=0036-8075|pmid=12471255|bibcode=2002Sci...298.1980B|s2cid=216994|url-access=subscription}} which is taken up by the silicon transport protein to be predominantly used in constructing these protective cell wall structures.{{Cite journal|last1=Durkin|first1=Colleen A.|last2=Koester|first2=Julie A.|last3=Bender|first3=Sara J.|last4=Armbrust|first4=E. Virginia|date=2016|title=The evolution of silicon transporters in diatoms|url= |journal=Journal of Phycology|language=en|volume=52|issue=5|pages=716–731|doi=10.1111/jpy.12441|issn=1529-8817|pmc=5129515|pmid=27335204}} Silicon enters the ocean in a dissolved form such as silicic acid or silicate.{{Cite journal|last1=Dugdale|first1=R. C.|last2=Wilkerson|first2=F. P.|date=2001-12-30|title=Sources and fates of silicon in the ocean: the role of diatoms in the climate and glacial cycles|journal=Scientia Marina|volume=65|issue=S2|pages=141–152|doi=10.3989/scimar.2001.65s2141|issn=1886-8134|doi-access=free}} Since diatoms are one of the main users of these forms of silicon, they contribute greatly to the concentration of silicon throughout the ocean. Silicon forms a nutrient-like profile in the ocean due to the diatom productivity in shallow depths, which means there is less concentration of silicon in the upper ocean and more concentration of silicon in the deep ocean.

Diatom productivity in the upper ocean contribute to the amount of silicon exported to the lower ocean.{{Cite journal|last1=Baines|first1=Stephen B.|last2=Twining|first2=Benjamin S.|last3=Brzezinski|first3=Mark A.|last4=Krause|first4=Jeffrey W.|last5=Vogt|first5=Stefan|last6=Assael|first6=Dylan|last7=McDaniel|first7=Hannah|date=December 2012|title=Significant silicon accumulation by marine picocyanobacteria|url=https://www.nature.com/articles/ngeo1641|journal=Nature Geoscience|language=en|volume=5|issue=12|pages=886–891|doi=10.1038/ngeo1641|bibcode=2012NatGe...5..886B|issn=1752-0908|url-access=subscription}} When diatom cells are lysed in the upper ocean, their nutrients like, iron, zinc, and silicon, are brought to the lower ocean through a process called marine snow. Marine snow involves the downward transfer of particulate organic matter by vertical mixing of dissolved organic matter.{{Cite journal|last=Turner|first=Jefferson T.|date=January 2015|title=Zooplankton fecal pellets, marine snow, phytodetritus and the ocean's biological pump|url=http://dx.doi.org/10.1016/j.pocean.2014.08.005|journal=Progress in Oceanography|volume=130|pages=205–248|doi=10.1016/j.pocean.2014.08.005|bibcode=2015PrOce.130..205T|issn=0079-6611|url-access=subscription}} Availability of silicon appears crucial for diatom productivity, and as long as silicic acid is available for diatoms to utilize, the diatoms contribute other important nutrient concentrations in the deep ocean.{{Cite journal|last1=Yool|first1=Andrew|last2=Tyrrell|first2=Toby|date=2003|title=Role of diatoms in regulating the ocean's silicon cycle|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2002GB002018|journal=Global Biogeochemical Cycles|language=en|volume=17|issue=4|pages=n/a|doi=10.1029/2002GB002018|bibcode=2003GBioC..17.1103Y|s2cid=16849373 |issn=1944-9224|doi-access=free}}

In coastal zones, diatoms serve as the major phytoplanktonic organisms and greatly contribute to biogenic silica production. In the open ocean, however, diatoms have a reduced role in global annual silica production. Diatoms in North Atlantic and North Pacific subtropical gyres contribute only about 6% of global annual marine silica production, while the Southern Ocean produces about one-third of the global marine biogenic silica.{{cite journal |last1=Tréguer |first1=Paul J. |last2=De La Rocha |first2=Christina L. |title=The World Ocean Silica Cycle |journal=Annual Review of Marine Science |date=3 January 2013 |volume=5 |issue=1 |pages=477–501 |doi=10.1146/annurev-marine-121211-172346|pmid=22809182 }} The Southern Ocean is referred to as having a "biogeochemical divide", since only minuscule amounts of silicon is transported out of this region.{{cite journal |last1=Marinov |first1=I. |last2=Gnanadesikan |first2=A. |last3=Toggweiler |first3=J. R. |last4=Sarmiento |first4=J. L. |title=The Southern Ocean biogeochemical divide |journal=Nature |date=June 2006 |volume=441 |issue=7096 |pages=964–967 |doi=10.1038/nature04883|pmid=16791191 |bibcode=2006Natur.441..964M |s2cid=4428683 }}

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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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| footer = Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry

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| image2 = Pennate diatoms (3075304186).jpg

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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:Different shapes of diatoms.png

File:Structure of diatom frustules.png

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| image1 = Marine diatoms SEM2.jpg

| caption1 = Diatoms, major components of marine plankton, have silica skeletons called frustules. "The microscopic structures of diatoms help them manipulate light, leading to hopes they could be used in new technologies for light detection, computing or robotics.[https://horizon-magazine.eu/article/biodegradable-glitter-and-pollution-eating-microalgae-new-materials-inspired-nature.html Biodegradable glitter and pollution-eating microalgae: the new materials inspired by nature] Horizon, 28 May 2020.

| image2 = SEM images of pores in diatom frustules.webp

| caption2 = SEM images of pores in diatom frustulesAguirre, L. E., Ouyang, L., Elfwing, A., Hedblom, M., Wulff, A. and Inganäs, O. (2018) "Diatom frustules protect DNA from ultraviolet light". Scientific reports, 8(1): 1–6. {{doi|10.1038/s41598-018-21810-2}}. 50px Material 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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Diatom frustules have been accumulating for over 100 million years, leaving rich deposits of nano and microstructured silicon oxide in the form of diatomaceous earth around the globe. The evolutionary causes for the generation of nano and microstructured silica by photosynthetic algae are not yet clear. However, in 2018 it was shown that absorption of ultraviolet light by nanostructured silica protects the DNA in the algal cells, and this may be an evolutionary cause for the formation of the glass cages.De Tommasi, E., Congestri, R., Dardano, P., De Luca, A.C., Managò, S., Rea, I. and De Stefano, M. (2018) "UV-shielding and wavelength conversion by centric diatom nanopatterned frustules". Scientific Reports, 8(1): 1–14. {{doi|10.1038/s41598-018-34651-w}}. 50px Material 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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| image1 = Triparma laevis and shell.jpg

| caption1 = Triparma laevis and a drawing of its silicate shell, scale bar = 1 μm.

| image2 = Triparma laevis exploded shell.jpg

| caption2 = Exploded drawing of the shell, D = dorsal plate, G = girdle plate, S = shield plate and V = ventral plate.

| footer = Triparma laevis belongs to the Bolidophyceae, a sister taxon to the diatoms.Booth, B.C. and Marchant, H.J. (1987) "Parmales, a new order of marine chrysophytes, with descriptions of three new genera and seven new species". Journal of Phycology, 23: 245–260. {{doi|10.1111/j.1529-8817.1987.tb04132.x}}.Kuwata, A., Yamada, K., Ichinomiya, M., Yoshikawa, S., Tragin, M., Vaulot, D. and Lopes dos Santos, A. (2018) "Bolidophyceae, a sister picoplanktonic group of diatoms – a review". Frontiers in Marine Science, 5: 370. {{doi|10.3389/fmars.2018.00370|doi-access=free}}. 50px Material 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=Ygty9HxhFK4&t=0s Diatoms: Tiny factories you can see from space]

| video2 = [https://www.youtube.com/watch?v=CYIC70MNRWM How diatoms build their beautiful shells] – Journey to the Microcosmos

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{{further|Radiolaria#Radiolarian shells}}

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Radiolarians are unicellular predatory protists encased in elaborate globular shells (or "capsules"), 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:Animation of radiolarian diversity.gif

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

File:Cladococcus abietinus.jpg| Cladococcus abietinus

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| image1 = Micro-CT model of radiolarian, Triplococcus acanthicus.png

| caption1 = X-ray microtomography of Triplococcus acanthicus. This is a microfossil from the Middle Ordovician with four nested spheres. The innermost sphere is highlighted red. Each segment is shown at the same scale.Kachovich, S., Sheng, J. and Aitchison, J. C., 2019. Adding a new dimension to investigations of early radiolarian evolution. Scientific reports, 9(1), pp. 1–10. {{doi|10.1038/s41598-019-42771-0}}. 50px Material 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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| header = Turing and radiolarian morphology

| footer = Computer simulations of Turing patterns on a sphere closely replicate some radiolarian shell patterns{{cite journal | last1 = Varea | first1 = C. | last2 = Aragon | first2 = J. L. | last3 = Barrio | first3 = R. A. | year = 1999 | title = Turing patterns on a sphere | journal = Physical Review E | volume = 60 | issue = 4| pages = 4588–4592 | doi = 10.1103/PhysRevE.60.4588 | pmid = 11970318 | bibcode = 1999PhRvE..60.4588V }}

| image1 = Spherical radiolarian.jpg

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| caption1 = Shell of a spherical radiolarian

| image2 = Radiolarians - Actinomma sol (33732012006).jpg

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| caption2 = Shell micrographs

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| video1 = [https://www.youtube.com/watch?v=5rxwn6vT9JE Radiolarian geometry]

| video2 = [https://www.youtube.com/watch?v=tl_onFMjJWA Ernst Haeckel's radiolarian engravings]

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{{see also|Marine biogenic calcification}}

{{further|coccoliths}}

Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell called a coccosphere. Coccospheres are covered with ornate circular plates or scales called coccoliths. The coccoliths are made from calcium carbonate. 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:Coccolithus pelagicus.jpg

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

File:Emiliania huxleyi coccolithophore (PLoS).png|The coccolithophore Emiliania huxleyi

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| footer = Coccolithophores build calcite skeletons important to the marine carbon cycleRost, 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}}.

| image1 = 9Calcidiscus leptoporus, diploid, SEM, showing coccoliths.tif

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| caption1 = Have plates called coccoliths

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| caption2 = Extinct fossil

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There are benefits for protists that carry protective shells. The diagram on the left below shows some benefits coccolithophore get from carrying coccoliths. In the diagram, (A) represents accelerated photosynthesis including carbon concentrating mechanisms (CCM) and enhanced light uptake via scattering of scarce photons for deep-dwelling species. (B) represents protection from photodamage including sunshade protection from ultraviolet light (UV) and photosynthetic active radiation (PAR) and energy dissipation under high-light conditions. (C) represents armour protection includes protection against viral/bacterial infections and grazing by selective and nonselective grazers.Monteiro, F.M., Bach, L.T., Brownlee, C., Bown, P., Rickaby, R.E., Poulton, A.J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S. and Gutowska, M.A. (2016) "Why marine phytoplankton calcify". Science Advances, 2(7): e1501822. {{doi|10.1126/sciadv.1501822}}. 50px Material 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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| caption1 = Benefits in coccolithophore calcification – see text above

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| caption2 = Energetic costs in coccolithophore calcification

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There are also costs for protists that carry protective shells. The diagram on the right above shows some of the energetic costs coccolithophore incur from carrying coccoliths. In the diagram, the energetic costs are reported in percentage of total photosynthetic budget. (A) represents transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca₂+ and HCO₃− (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca₂+ through the cytoplasm to the coccolith vesicle (CV) is the dominant cost associated with calcification. (B) represents metabolic processes include the synthesis of coccolith-associated polysaccharides (CAPs – gray rectangles) by the Golgi complex (white rectangles) that regulate the nucleation and geometry of CaCO₃ crystals. The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO₃ crystals. (C) Mechanical and structural processes account for the secretion of the completed coccoliths that are transported from their original position adjacent to the nucleus to the cell periphery, where they are transferred to the surface of the cell.

{{main|foraminifera test}}

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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, may be single-chambered or multi-chambered; multi-chambered forams add more chambers as they grow. The most famous of these are made of calcite, but tests may also be made of aragonite, agglutinated sediment particles, chiton, or (rarely) of silica. Most forams are benthic, but about 40 living 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. Some foraminifera lack tests altogether.{{Cite journal|last1=Pawlowski|first1=Jan|last2=Bolivar|first2=Ignacio|last3=Fahrni|first3=Jose F.|last4=Vargas|first4=Colomban De|last5=Bowser|first5=Samuel S.|date=November 1999|title=Molecular Evidence That Reticulomyxa Filosa Is A Freshwater Naked Foraminifer|url=https://onlinelibrary.wiley.com/doi/10.1111/j.1550-7408.1999.tb05137.x|journal=The Journal of Eukaryotic Microbiology|language=en|volume=46|issue=6|pages=612–617|doi=10.1111/j.1550-7408.1999.tb05137.x|pmid=10568034|s2cid=36497475|issn=1066-5234|url-access=subscription}}

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| footer = Foraminiferans are important unicellular zooplankton protists, often with calcite tests

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| caption1 = Empty Foraminiferan test, showing multiple chambers and pores

| image2 = G bulloides Brady 1884.jpg

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| caption2 = ...and in life, showing pseudopodia streaming from pores

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| video1 = [https://www.youtube.com/watch?v=JLSa8cGJixQ foraminiferans]

| video2 = [https://www.youtube.com/watch?v=q0WbN34Mh7k Foraminiferal networks and growth]

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File:Favulina hexagona.png

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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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File:Collection Penard MHNG Specimen 531-1-2 Pamphagus granulatus.tif]]

The cell body of many choanoflagellates is surrounded by a distinguishing extracellular matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. Many choanoflagellates build complex basket-shaped "houses", called lorica, from several silica strips cemented together. The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid attachment to the substrate. In planktonic organisms, there is speculation that the periplast increases drag, thereby counteracting the force generated by the flagellum and increasing feeding efficiency.{{cite journal | vauthors = Leadbeater BS, Thomsen H | year = 2000 | title = Order Choanoflagellida | journal = An Illustrated Guide to the Protozoa, Second Edition. Lawrence: Society of Protozoologists | volume = 451 | pages = 14–38}}{{cite journal | vauthors = Leadbeater BS, Kelly M | year = 2001 | title = Evolution of animals choanoflagellates and sponges | journal = Water and Atmosphere Online | volume = 9 | issue = 2 | pages = 9–11}}

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| video1 = [https://www.youtube.com/watch?v=9EutjfddS2c&t=9s&ab_channel=JourneytotheMicrocosmos Testate amoebas: blobby, modest shell dwellers {{space|24}} – Journey to the Microcosmos]

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File:Salpingoeca sp..jpg| Choanoflagellate

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File:Diatomaceous Earth BrightField.jpg is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms. This sample consists of a mixture of centric (radially symmetric) and pennate (bilaterally symmetric) diatoms. Click 3 times to fully enlarge.]]

{{main|Protists in the fossil record}}

{{see also|Microfossil|Pelagic sediment|Diatomaceous earth|Siliceous ooze}}

The shells or skeletons of many protists survive over geological time scales as microfossils. Microfossils are fossils that are generally between 0.001mm and 1 mm in size,{{cite web|last1=Drewes|first1=Charlie|title=Discovering Devonian Microfossils|url=http://www.eeob.iastate.edu/faculty/DrewesC/htdocs/microfossilsABLE.doc|publisher=Iowa State University|access-date=4 March 2017}} the study of which requires the use of light or electron microscopy. Fossils which can be studied by the naked eye or low-powered magnification, such as a hand lens, are referred to as macrofossils.

Microfossils are a common feature of the geological record, from the Precambrian to the Holocene. They are most common in marine sediments, but also occur in brackish water, fresh water and terrestrial sedimentary deposits. While every kingdom of life is represented in the microfossil record, the most abundant forms are protist skeletons or cysts from the Chrysophyta, Pyrrhophyta, Sarcodina, acritarchs and chitinozoans, together with pollen and spores from the vascular plants.

In 2017, fossilized microorganisms, or microfossils, were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an almost instantaneous emergence of life" (in a geological time-scale sense), after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago.{{cite journal |author=Dodd, Matthew S. |author2=Papineau, Dominic |author3=Grenne, Tor |author4=slack, John F. |author5=Rittner, Martin |author6=Pirajno, Franco |author7=O'Neil, Jonathan |author8=Little, Crispin T. S. |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates|journal=Nature |volume=543 |issue=7643 |pages=60–64 |date=2 March 2017 | doi=10.1038/nature21377|pmid=28252057 |bibcode=2017Natur.543...60D |url=http://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf |doi-access=free }}{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest |url=https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |date=1 March 2017 |work=The New York Times |access-date=2 March 2017 }}{{cite web |last=Ghosh |first=Pallab |title=Earliest evidence of life on Earth 'found |url=https://www.bbc.co.uk/news/science-environment-39117523 |publisher=BBC News |date=1 March 2017 |access-date=2 March 2017}}{{cite news |last1=Dunham |first1=Will |title=Canadian bacteria-like fossils called oldest evidence of life |url=http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |archive-url=https://web.archive.org/web/20170302114728/http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |url-status=dead |archive-date=2 March 2017 |date=1 March 2017 |publisher=Reuters |access-date=1 March 2017 }} Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.{{cite web |author=Staff |title=A timescale for the origin and evolution of all of life on Earth |url=https://phys.org/news/2018-08-timescale-evolution-life-earth.html |date=20 August 2018 |work=Phys.org |access-date=20 August 2018 }}{{cite journal |last1=Betts |first1=Holly C. |last2=Putick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C.J. |last6=Pisani |first6=Davide |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |date=20 August 2018 |journal=Nature |volume=2 |issue=10 |pages=1556–1562 |doi=10.1038/s41559-018-0644-x |pmid=30127539 |pmc=6152910 }}

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• Cytoskeleton
• Particulate inorganic matter

{{reflist}}

• Xu, K., Hutchins, D. and Gao, K. (2018) "Coccolith arrangement follows Eulerian mathematics in the coccolithophore Emiliania huxleyi". PeerJ, 6: e4608. {{doi|10.1126/science.aaa7378}}.
• [https://dash.harvard.edu/bitstream/handle/1/30403697/71123922.pdf?sequence=1&isAllowed=y Protistan Skeletons: A Geologic History of Evolution and Constraint]

Category:Protists

Category:Biomineralization

Category:Skeletal system