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{{for|a molecule of two atoms|Diatomic molecule}}{{short description|A class of microalgae, found in the oceans, waterways and soils of the world}}{{Taxobox| name = Diatoms| image = Diatoms through the microscope.jpg| image_caption = Light microscopy of a sampling of marine diatoms found living between crystals of annual sea ice in Antarctica, showing a multiplicity of sizes, shapes, and colors| domain = Eukaryota
SAR supergroup>SAR: a clade or supergroup that includes stramenopiles (heterokonts), alveolates, and Rhizaria.ARCHIBALD JM >TITLE=THE PUZZLE OF PLASTID EVOLUTION VOLUME=19 PAGES=R81–8 PMID=19174147 EDITOR1-FIRST=GERALDINE JOURNAL=PLOS ONE ISSUE=8 YEAR=2007 PMC=1949142 BIBCODE = 2007PLOSO...2..790B, HAMPL V, HUG L, LEIGH JW, ETAL >TITLE=PHYLOGENOMIC ANALYSES SUPPORT THE MONOPHYLY OF EXCAVATA AND RESOLVE RELATIONSHIPS AMONG EUKARYOTIC "SUPERGROUPS" VOLUME=106 PAGES=3859–64 PMID=19237557 PMC=2656170 JOURNAL=MOL. BIOL. EVOL. ISSUE=12 DATE=DECEMBER 2008 DOI=10.1093/MOLBEV/MSN206,weblink | superphylum = Heterokonta| phylum = Ochrophyta| classis = BacillariophyceaePierre Dangeard>Dangeard, 1933Dangeard, P. (1933). Traite d'Algologie. Paul Lechvalier and Fils, Paris, weblink.| subdivision_ranks = Orderssee text| synonyms =
  • Diatomea Dumortier, 1821Dumortier, B.-C. (1822). Commentationes botanicae. Observations botaniques, dédiées à la Société d'Horticulture de Tournay {{webarchive|url=https://web.archive.org/web/20151006091814weblink |date=6 October 2015 }} (disponible at Algaebase). pp. [i], [1]-116, [1, tabl., err.]. Tournay: Imprimerie de Ch. Casterman-Dieu, Rue de pont No. 10.
  • Diatomophyceae Rabenhorst, 1864Rabenhorst, L. Flora europaea algarum aquae dulcis et submarinae (1864–1868). Sectio I. Algas diatomaceas complectens, cum figuris generum omnium xylographice impressis (1864). pp. 1-359. Lipsiae [Leipzig]: Apud Eduardum Kummerum.
  • Bacillariae Haeckel, 1878Haeckel, E. (1878). Das Protistenreich.
  • Bacillariophyta Engler & Gilg, 1919Engler, A. & Gilg, E. (1919). Syllabus der Pflanzenfamilien: eine Ãœbersicht über das gesamte Pflanzensystem mit besonderer Berücksichtigung der Medizinal- und Nutzpflanzen, nebst einer Ãœbersicht über die Florenreiche und Florengebiete der Erde zum Gebrauch bei Vorlesungen und Studien über spezielle und medizinisch-pharmazeutische Botanik, 8th ed., Gebrüder Borntraeger Verlag, Berlin, 395 p.
}}Diatoms (diá-tom-os 'cut in half', from diá, 'through' or 'apart'; and the root of tém-n-ō, 'I cut'.) {{LSJ|dia/tomos|diá-tom-os}} "cut in half" (= dichó-tom-os) — {{LSJ|dia/|diá}} "through" or "apart" and the root of {{LSJ|te/mnw1|tém-n-ō}} "I cut". {{ablaut|e|o||verb root}} are a major group of algae,WEB, Definition of DIATOM,weblink www.merriam-webster.com, en, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms number in the trillions: they generate about 20 percent of the oxygen produced on the planet each year,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,Treguer, P.; Nelson, D. M.; Van Bennekom, A. J.; Demaster, D. J.; Leynaert, A.; Queguiner, B. (1995). "The Silica Balance in the World Ocean: A Reestimate". Science. 268 (5209): 375–9. Bibcode:1995Sci...268..375T. doi:10.1126/science.268.5209.375. {{PMID|17746543}} and contribute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half mile deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by east-to-west (easterly) transatlantic winds from the bed of a dried up lakeWEB,weblink King's College London - Lake Megachad, www.kcl.ac.uk, en-GB, 2018-05-05, once covering much of the African Sahara.JOURNAL, Bristow, C.S., Hudson-Edwards, K.A., Chappell, A., 2010, Fertilizing the Amazon and equatorial Atlantic with West African dust, Geophys. Res. Lett., 37, 14, L14807, 10.1029/2010GL043486, 2010GeoRL..3714807B, Diatoms are unicellular: they occur either as solitary cells or in colonies, which can take the shape of ribbons, fans, zigzags, or stars. Individual cells range in size from 2 to 200 micrometers. In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles approximately every 24 hours by asexual multiple fission; the maximum life span of individual cells is about six days.WEB,weblink Gas Guzzlers, Diatoms have two distinct shapes: a few (centric diatoms) are radially symmetric, while most (pennate diatoms) are broadly bilaterally symmetric. A unique feature of diatom anatomy is that they are surrounded by a cell wall made of silica (hydrated silicon dioxide), called a frustule.WEB, More on Diatoms, University of California Museum of Paleontology,weblink These frustules have structural coloration due to their photonic nanostructure, prompting them to be described as "jewels of the sea" and "living opals". Movement in diatoms primarily occurs passively as a result of both water currents and wind-induced water turbulence; however, male gametes of centric diatoms have flagella, permitting active movement for seeking female gametes. Similar to plants, diatoms convert light energy to chemical energy by photosynthesis, although this shared autotrophy evolved independently in both lineages. Unusually for autotrophic organisms, diatoms possess a urea cycle, a feature that they share with animals, although this cycle is used to different metabolic ends in diatoms. The family Rhopalodiaceae also possess a cyanobacterial endosymbiont called a spheroid body. This endosymbiont has lost its photosynthetic properties, but has kept its ability to perform nitrogen fixation, allowing the diatom to fix atmospheric nitrogen.Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle - NCBI
The study of diatoms is a branch of phycology. Diatoms are classified as eukaryotes, organisms with a membrane-bound cell nucleus, that separates them from the prokaryotes archaea and bacteria. Diatoms are a type of plankton called phytoplankton, the most common of the plankton types. Diatoms also grow attached to benthic substrates, floating debris, and on macrophytes. They comprise an integral component of the periphyton community.Wehr, J.D., Sheath, R.G., Kociolek, J.P.(Eds). 2015. ''Freshwater Algae of North America: Ecology and Classification''. San Diego: Academic Press. Another classification divides plankton into eight types based on size: in this scheme, diatoms are classed as microalgae. Several systems for classifying the individual diatom species exist. Fossil evidence suggests that diatoms originated during or before the early Jurassic period, which was about 150 to 200 million years ago.Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatomaceous earth (diatomite) is a collection of diatom shells found in the earth's crust. They are soft, silica-containing sedimentary rocks which are easily crumbled into a fine powder and typically have a particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, and as a dynamite stabilizer.

Structure

File:Diatom Anatomy.svg|thumb|left|300px| Diagrammatic representation of a diatom.1) Nucleus; holds the genetic material2) Nucleolus; Location of the chromosomes3) Golgi complex; modifies proteins and sends them out of the cell4) Cell Wall; Outer membrane of the cell5) Pyrenoid; center of carbon fixation6) Chromatophore; pigment carrying membrane structure7) Vacuoles; vesicle of a cell that contains fluid bound by a membrane8) Cytoplasmic strands; hold the nucleus9) Mitochondria; create ATP (energy) for the cell10) Valves/Striae; allow nutrients in, and waste out, of the cell]](File:20110123 185042 Diatom.jpg|thumb|255px|Light microscopy of a living diatom.  Numbered graduations are 10 micrometres apart)(File:Surirella spiralis - SEM MUSE - sharpened.jpeg|thumb|300px|right|{{center|Scanning electron microscope image of the diatom Surirella spiralis}})Diatoms are 2 to 200 micrometers in length. Their yellowish-brown chloroplasts, the site of photosynthesis, are typical of heterokonts, having four membranes and containing pigments such as the carotenoid fucoxanthin. Individuals usually lack flagella, but they are present in male gametes of the centric diatoms and have the usual heterokont structure, except they lack the hairs (mastigonemes) characteristic in other groups.Diatoms are often referred as "jewels of the sea" or "living opals" due to their photonic crystal properties.JOURNAL, 10.1038/nnano.2007.152, 18654305, Biomimetics of photonic nanostructures, Nature Nanotechnology, 2, 6, 347–53, 2007, Parker, Andrew R., Townley, Helen E., 2007NatNa...2..347P, The biological function of this structural coloration is not clear, but it is speculated that it may be related to communication, camouflage, thermal exchange and/or UV protection.JOURNAL, 10.1016/j.tibtech.2008.11.003, 19167770, The Glass Menagerie: Diatoms for novel applications in nanotechnology, Trends in Biotechnology, 27, 2, 116–27, 2009, Gordon, Richard, Losic, Dusan, Tiffany, Mary Ann, Nagy, Stephen S., Sterrenburg, Frithjof A.S., Diatoms build intricate hard but porous cell walls called frustules composed primarily of silica.{{rp|25–30}} This siliceous wallWEB, Glass in Nature,weblink The Corning Museum of Glass, 19 February 2013, can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations; all of which can be used to delineate genera and species.The cell itself consists of two halves, each containing an essentially flat plate, or valve and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical. Their distinguishing feature is a hard mineral shell or frustule composed of opal (hydrated, polymerized silicic acid).

Behaviour

Most diatoms are nonmotile, as their relatively dense cell walls cause them to readily sink. Planktonic forms in open water usually rely on turbulent mixing of the upper layers of the oceanic waters by the wind to keep them suspended in sunlit surface waters. The only mechanism for regulating buoyancy is an ionic pump.JOURNAL, Anderson, Lars W. J., Sweeney, Beatrice M., Diel changes in sedimentation characteristics of Ditylum brightwelli: Changes in cellular lipid and effects of respiratory inhibitors and ion-transport modifiers1, Limnology and Oceanography, 1 May 1977, 22, 3, 539–552, 10.4319/lo.1977.22.3.0539, en, 1939-5590, 1977LimOc..22..539A, Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures; mucilage pads, stalks or tubes; amorphous masses of mucilage; or by threads of chitin (polysaccharide), which are secreted through strutted processes of the cell.

Biochemistry

Energy source

Diatoms are mainly photosynthetic; however a few are obligate heterotrophs and can live in the absence of light provided an appropriate organic carbon source is available.{{citation needed|date=December 2018}}

Silica metabolism

Diatom cells are contained within a unique silica cell wall known as a frustule made up of two valves called thecae, that typically overlap one another.WEB, Diatoms,weblink 13 February 2016, The biogenic silica composing the cell wall is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is then extruded to the cell exterior and added to the wall. In most species, when a diatom divides to produce two daughter cells, each cell keeps one of the two halves and grows a smaller half within it. As a result, after each division cycle, the average size of diatom cells in the population gets smaller. Once such cells reach a certain minimum size, rather than simply divide, they reverse this decline by forming an auxospore. This expands in size to give rise to a much larger cell, which then returns to size-diminishing divisions.{{citation needed|date=January 2016}} Auxospore production is almost always linked to meiosis and sexual reproduction.The exact mechanism of transferring silica absorbed by the diatom to the cell wall is unknown. Much of the sequencing of diatom genes comes from the search for the mechanism of silica uptake and deposition in nano-scale patterns in the frustule. The most success in this area has come from two species, Thalassiosira pseudonana, which has become the model species, as the whole genome was sequenced and methods for genetic control were established, and Cylindrotheca fusiformis, in which the important silica deposition proteins silaffins were first discovered.JOURNAL, Thamatrakoln, K., Alverson, A.J., Hildebrand, M., COMPARATIVE SEQUENCE ANALYSIS OF DIATOM SILICON TRANSPORTERS: TOWARD A MECHANISTIC MODEL OF SILICON TRANSPORT, Journal of Phycology, 2006, 42, 4, 822–834, 10.1111/j.1529-8817.2006.00233.x, Silaffins, sets of polycationic peptides, were found in C. fusiformis cell walls and can generate intricate silica structures. These structures demonstrated pores of sizes characteristic to diatom patterns. When T. pseudonana underwent genome analysis it was found that it encoded a urea cycle, including a higher number of polyamines than most genomes, as well as three distinct silica transport genes.JOURNAL, Kröger, Nils, Deutzmann, Rainer, Manfred, Sumper, Polycationic Peptides from Diatom Biosilica That Direct Silica Nanosphere Formation., Science, November 1999, 286, 5442, 1129–1132, 10.1126/science.286.5442.1129, In a phylogenetic study on silica transport genes from 8 diverse groups of diatoms, silica transport was found to generally group with species. This study also found structural differences between the silica transporters of pennate (bilateral symmetry) and centric (radial symmetry) diatoms. The sequences compared in this study were used to create a diverse background in order to identify residues that differentiate function in the silica deposition process. Additionally, the same study found that a number of the regions were conserved within species, likely the base structure of silica transport.These silica transport proteins are unique to diatoms, with no homologs found in other species, such as sponges or rice. The divergence of these silica transport genes is also indicative of the structure of the protein evolving from two repeated units composed of five membrane bound segments, which indicates either gene duplication or dimerization. The silica deposition that takes place from the membrane bound vesicle in diatoms has been hypothesized to be a result of the activity of silaffins and long chain polyamines. This Silica Deposition Vesicle (SDV) has been characterized as an acidic compartment fused with Golgi-derived vesicles.BOOK, Kroger, Nils, Handbook of Biomineralization: Biological Aspects and Structure Formation, 2007, Wiley-VCH Verlag GmbH, Weinheim, Germany, chapter 3, These two protein structures have been shown to create sheets of patterned silica in-vivo with irregular pores on the scale of diatom frustules. One hypothesis as to how these proteins work to create complex structure is that residues are conserved within the SDV's, which is unfortunately difficult to identify or observe due to the limited number of diverse sequences available. Though the exact mechanism of the highly uniform deposition of silica is as yet unknown, the Thalassiosira pseudonana genes linked to silaffins are being looked to as targets for genetic control of nanoscale silica deposition.

Urea cycle

A feature of diatoms is the urea cycle, which links them evolutionarily to animals. This was discovered in research carried out by Andrew Allen, Chris Bowler and colleagues. Their findings, published in 2011, that diatoms have a functioning urea cycle was highly significant, since prior to this, the urea cycle was thought to have originated with the metazoans which appeared several hundreds of millions of years after the diatoms. Their study showed that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionally linked in such a way that animals and plants are not.JOURNAL, 10.1038/nature10074, 21562560, Science Daily, May 12, 2011,weblink Evolution and metabolic significance of the urea cycle in photosynthetic diatoms, Nature, 473, 7346, 203–7, 2011, Allen, Andrew E., Dupont, Christopher L., Oborník, Miroslav, Horák, Aleš, Nunes-Nesi, Adriano, McCrow, John P., Zheng, Hong, Johnson, Daniel A., Hu, Hanhua, Fernie, Alisdair R., Bowler, Chris, 2011Natur.473..203A,

Pigments

Major pigments of diatoms are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.

Storage products

Storage products are chrysolaminarin and lipids.

Life cycle

File:Centric diatom life-cycle.jpg|thumb|200px|Sexual reproduction of a centric diatom (oogamyoogamyFile:Diatom pennate life cycle.jpg|thumb|200px|Sexual reproduction of a pinnate diatom (morphological isogamy, physiological anisogamyanisogamy

Reproduction and cell size

Reproduction among these organisms is asexual by binary fission, during which the diatom divides into two parts, producing two "new" diatoms with identical genes. Each new organism receives one of the two frustules - one larger, the other smaller - possessed by the parent, which is now called the epitheca; and is used to construct a second, smaller frustule, the hypotheca. The diatom that received the larger frustule becomes the same size as its parent, but the diatom that received the smaller frustule remains smaller than its parent. This causes the average cell size of this diatom population to decrease. It has been observed, however, that certain taxa have the ability to divide without causing a reduction in cell size.BOOK, G. Drebes, Dietrich Werner, The Biology of Diatoms,weblink 2013-11-14, Volume 13 of Botanical Monographs, 1 January 1977, University of California Press, 978-0-520-03400-6, 250–283, Chapter 9: Sexuality, Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and auxospore formation must occur.

Cell division

Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form the zygote. The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore. A new diatom cell of maximum size, the initial cell, forms within the auxospore thus beginning a new generation. Resting spores may also be formed as a response to unfavourable environmental conditions with germination occurring when conditions improve.

Sperm motility

Diatoms are mostly non-motile; however, sperm found in some species can be flagellated, though motility is usually limited to a gliding motion. In centric diatoms, the small male gametes have one flagellum while the female gametes are large and non-motile (oogamous). Conversely, in pennate diatoms both gametes lack flagella (isoogamous). Certain araphid species, that is pennate diatoms without a raphe (seam), have been documented as anisogamous and are, therefore, considered to represent a transitional stage between centric and raphid pennate diatoms, diatoms with a raphe.

Degradation by microbes

Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by using hydrolytic enzymes to break down the organic algal material.JOURNAL, 10.1038/17351, {{INIST, 1755031, |year=1999 |last1=Azam |first1=Farooq |title= Accelerated dissolution of diatom silica by marine bacterial assemblages |journal=Nature |volume=397 |issue=6719 |pages=508–12 |last2=Bidle |first2=Kay D. |bibcode= 1999Natur.397..508B }}JOURNAL, 10.1371/journal.pone.0059977, 23560063, 3613400, The Structure of Microbial Community and Degradation of Diatoms in the Deep Near-Bottom Layer of Lake Baikal, PLoS ONE, 8, 4, e59977, 2013, Zakharova, Yulia R., Galachyants, Yuri P., Kurilkina, Maria I., Likhoshvay, Alexander V., Petrova, Darya P., Shishlyannikov, Sergey M., Ravin, Nikolai V., Mardanov, Andrey V., Beletsky, Alexey V., Likhoshway, Yelena V., 2013PLoSO...859977Z,

Classification

Structural

File:Diatomeas-Haeckel.jpg|thumb|right|Selections from Ernst Haeckel's 1904 Kunstformen der NaturKunstformen der NaturDiatoms are divided into two groups that are distinguished by the shape of the frustule: the centric diatoms and the pennate diatoms.Pennate diatoms are bilaterally symmetric. Each one of their valves have openings that are slits along the raphes and their shells are typically elongated parallel to these raphes. They generate cell movement through cytoplasm that streams along the raphes, always moving along solid surfaces.Centric diatoms are radially symmetric. They are composed of upper and lower valves - epitheca and hypotheca - each consisting of a valve and a girdle band that can easily slide underneath each other and expand to increase cell content over the diatoms progression. The cytoplasm of the centric diatom is located along the inner surface of the shell and provides a hollow lining around the large vacuole located in the center of the cell. This large, central vacuole is filled by a fluid known as "cell sap" which is similar to seawater but varies with specific ion content. The cytoplasmic layer is home to several organelles, like the chloroplasts and mitochondria. Before the centric diatom begins to expand, its nucleus is at the center of one of the valves and begins to move towards the center of the cytoplasmic layer before division is complete. Centric diatoms have a variety of shapes and sizes, depending on from which axis the shell extends, and if spines are present.

Linnaean

Relationship to other organisms

(File:Diatomeas w.jpg|thumb|300px|right|{{center|Light microscopy of several species of living freshwater diatoms}})Diatoms belong to a large group called the heterokonts, which include both autotrophs such as golden algae and kelp; and heterotrophs such as water moulds. The classification of heterokonts is still unsettled: they may be designated a division, phylum, kingdom, or something intermediate to those. Consequently, diatoms are ranked anywhere from a class, usually called Diatomophyceae or Bacillariophyceae, to a division, usually called Bacillariophyta, with corresponding changes in the ranks of their subgroups.

Genera and species

More than 200 genera of living diatoms are known, with an estimated 100,000 extant species.BOOK, Grethe R. Hasle, Erik E. Syvertsen, Karen A. Steidinger, Karl Tangen, Carmelo R. Tomas, Identifying Marine Diatoms and Dinoflagellates,weblink 2013-11-13, 1996-01-25, Academic Press, 978-0-08-053441-1, 5–385, Marine Diatoms, BOOK, Frank Eric Round, R. M. Crawford, D. G. Mann, The Diatoms: Biology & Morphology of the Genera,weblink 2013-11-13, 1990, Cambridge University Press, 978-0-521-36318-1, {{page needed|date=March 2017}}Canter-Lund, H. and Lund, J.W.G. (1995). Freshwater Algae: Their microscopic world explained, Biopress Limited. {{ISBN|0-948737-25-5}}.{{page needed|date=March 2017}}JOURNAL, 10.1007/BF00940439, 23675282, The species concept in diatoms: Evidence for morphologically distinct, sympatric gamodemes in four epipelic species, Plant Systematics and Evolution, 164, 1/4, 215–37, 2005, Mann, David G.,

Classes and orders

Based on the fact that pennate diatoms either do or do not have a longitudinal groove in the valve, called a raphe,O.E.D. 2nd edition 2005 a 1990 classification by Round, Crawford & Mann divides the diatoms (as Bacillarophyta) into three classes, centric (22 orders); pennate without a raphe (12 orders); and pennate with a raphe (11 orders), as follows:WEB, Bacillariophyceae, algaebase, M. D. Guiry,weblink 2013-11-11, {{Div col}}
* Anaulales Round & R.M.Crawford * Arachnoidiscales Round * Asterolamprales Round * Aulacoseirales R.M.Crawford * Biddulphiales * Chaetocerotales Round & R.M.Crawford * Chrysanthemodiscales Round * Corethrales Round & R.M.Crawford * Coscinodiscales Round * Cymatosirales Round & R.M.Crawford * Ethmodiscales Round * Hemiaulales Round & R.M.Crawford * Leptocylindrales Round & R.M.Crawford * Lithodesmiales * Melosirales R.M.Crawford * Orthoseirales R.M.Crawford * Paraliales R.M.Crawford * Rhizosoleniales * Stictocyclales Round * Stictodiscales Round & R.M.Crawford * Thalassiosirales * Triceratiales Round & R.M.Crawford
{{div col end}} {{Div col}}
* Ardissoneales F.E.Round * Climacospheniales Round * Cyclophorales Round & R.M.Crawford * Fragilariales P.C.Silva * Licmophorales Round * Protoraphidales Round * Rhabdonematales Round & R.M.Crawford * Rhaphoneidales Round * Striatellales F.E.Round * Tabellariales Round * Thalassionematales Round * Toxariales Round
{{div col end}}
  • Class Bacillariophyceae: pennate diatoms with a raphe (raphids) Haeckel, 1878, emend. D.G.Mann
{{Div col}}
* Achnanthales P.C.Silva * Bacillariales Hendey * Cymbellales D.G.Mann * Dictyoneidales D.G.Mann * Eunotiales P.C.Silva * Lyrellales D.G.Mann * Mastogloiales D.G.Mann * Naviculales Bessey * Rhopalodiales D.G.Mann * Surirellales D.G.Mann * Thalassiophysales D.G.Mann{{div col end}}

Cladistic

An alternate classification for the diatoms based on clades is as follows:JOURNAL, 10.2216/i0031-8884-43-3-245.1, Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision, Phycologia, 43, 3, 245–70, 2004, Medlin, Linda K., Kaczmarska, Irena,weblink
  • Bacillaryophyta


* Coscinodiscophytina
* Coscinodiscophyceae ('radial centrics')
* Bacillariophytina
* Mediophyceae ('polar centrics') * Bacillariophyceae (pennate diatoms)
{{clear}}

Other

Another systematic approach to classification was proposed in 1995, the Hoek, Mann and Jahns system.BOOK, Christiaan Van Den Hoek, D. G. Mann, H. M. Jahns, Algae: An Introduction to Phycology,weblink 2013-11-13, 1995, Cambridge University Press, 978-0-521-31687-3, {{page needed|date=March 2017}}{{clear}}

Evolution and fossil record

Origin

Heterokont chloroplasts appear to derive from those of red algae, rather than directly from prokaryotes as occurred in plants. This suggests they had a more recent origin than many other algae. However, fossil evidence is scant, and only with the evolution of the diatoms themselves do the heterokonts make a serious impression on the fossil record.

Earliest fossils

The earliest known fossil diatoms date from the early Jurassic (~185 Ma ago),JOURNAL, 10.1006/mpev.1996.0088, 8975694, Evolution of the Diatoms (Bacillariophyta), Molecular Phylogenetics and Evolution, 6, 3, 391–407, 1996, Kooistra, Wiebe H.C.F., Medlin, Linda K., although the molecular clock and sedimentaryJOURNAL, 10.1038/35023143, 10984049, 2000, Schieber, Jürgen, Diagenetic origin of quartz silt in mudstones and implications for silica cycling, Nature, 406, 6799, 981–5, Krinsley, Dave, Riciputi, Lee, evidence suggests an earlier origin. It has been suggested that their origin may be related to the end-Permian mass extinction (~250 Ma), after which many marine niches were opened.JOURNAL, 10013/epic.12689, Medlin, L. K., Kooistra, W. H. C. F., Gersonde, R., Sims, P. A., Wellbrock, U., 1997, Is the origin of the diatoms related to the end-Permian mass extinction?, Nova Hedwigia, 65, 1–11, The gap between this event and the time that fossil diatoms first appear may indicate a period when diatoms were unsilicified and their evolution was cryptic.JOURNAL, 10.1111/j.1469-8137.2004.01022.x, 1514475, The evolution of silicification in diatoms: Inescapable sinking and sinking as escape?, New Phytologist, 162, 1, 45–61, 2004, Raven, J. A., Waite, A. M., Since the advent of silicification, diatoms have made a significant impression on the fossil record, with major fossil deposits found as far back as the early Cretaceous, and with some rocks such as diatomaceous earth, being composed almost entirely of them.

Relation to silicon cycle

{{Further|Silica cycle}}Although diatoms may have existed since the Triassic, the timing of their ascendancy and "take-over" of the silicon cycle occurred more recently. Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle.BOOK, R. Siever, Stephen Henry Schneider, Penelope J. Boston, Scientists on Gaia,weblink 2013-11-14, January 1993, MIT Press, 978-0-262-69160-4, 287–295, Silica in the oceans: biological-geological interplay, JOURNAL, 10.1086/320794, Secular Distribution of Biogenic Silica through the Phanerozoic: Comparison of Silica‐Replaced Fossils and Bedded Cherts at the Series Level, The Journal of Geology, 109, 4, 509–22, 2001, Kidder, David L., Erwin, Douglas H., 2001JG....109..509K, JOURNAL, 10.1130/0091-7613(2003)0312.0.CO;2, {{INIST, 14692468, |year=2003 |volume=31 |issue=4 |pages=319–22 |title=Paleozoic and Mesozoic silica-rich seawater: Evidence from hematitic chert (jasper) deposits |journal=Geology |last1=Grenne |first1=Tor |last2=Slack |first2=John F. |bibcode=2003Geo....31..319G }} Subsequently, the cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter-feeders primarily on the continental shelves.JOURNAL, 10.1016/S0012-8252(00)00024-6, Radiolarian palaeoecology and radiolarites: Is the present the key to the past?, Earth-Science Reviews, 52, 1, 83–120, 2000, Racki, G, Cordey, Fabrice, 2000ESRv...52...83R, Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms.However, the precise timing of the "take-over" remains unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the displacement of siliceous sponges from the shelves,JOURNAL, 10.1038/44560, {{INIST, 1990263, |year=1999 |last1=Maldonado |first1=Manuel |title=Decline in Mesozoic reef-building sponges explained by silicon limitation |journal=Nature |volume=401 |issue=6755 |pages=785–8 |last2=Carmona |first2=M. Carmen |last3=Uriz |first3=María J. |last4=Cruzado |first4=Antonio |bibcode=1999Natur.401..785M }} suggests that this takeover began in the Cretaceous (146 Ma to 66 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (66 Ma to present).JOURNAL, 10.1130/0091-7613(1975)32.0.CO;2, 1975, 3, 4, 175–7, Silica, diatoms, and Cenozoic radiolarian evolution, Geology, Harper, Howard E., Knoll, Andrew H., 1975Geo.....3..175H,

Relation to grasslands

The expansion of grassland biomes and the evolutionary radiation of grasses during the Miocene is believed to have increased the flux of soluble silicon to the oceans, and it has been argued that this promoted the diatoms during the Cenozoic era.JOURNAL, 10.1126/science.1095964, 15256663, The Evolution of Modern Eukaryotic Phytoplankton, Science, 305, 5682, 354–60, 2004, Falkowski, P. G., Katz, Miriam E., Knoll, Andrew H., Quigg, Antonietta, Raven, John A., Schofield, Oscar, Taylor, F. J. R., 2004Sci...305..354F, 10.1.1.598.7930, JOURNAL, 10.2110/palo.2003.p03-108, 27670327, Impact of Grassland Radiation on the Nonmarine Silica Cycle and Miocene Diatomite, PALAIOS, 20, 2, 198–206, 2005, Kidder, D. L., Gierlowski-Kordesch, E. H., 2005Palai..20..198K, Recent work suggests that diatom success is decoupled from the evolution of grasses, although both diatom and grassland diversity increased strongly from the middle Miocene.JOURNAL, 10.1371/journal.pone.0084857, 24465441, 3898954, Cenozoic Planktonic Marine Diatom Diversity and Correlation to Climate Change, PLoS ONE, 9, 1, e84857, 2014, Lazarus, David, Barron, John, Renaudie, Johan, Diver, Patrick, Türke, Andreas, 2014PLoSO...984857L,

Relation to climate

Diatom diversity over the Cenozoic has been very sensitive to global temperature, particularly to the equator-pole temperature gradient. Warmer oceans, particularly warmer polar regions, have in the past been shown to have had substantially lower diatom diversity. Future warm oceans with enhanced polar warming, as projected in global-warming scenarios,IPCC Core Writing Team, 2007. Climate Change 2007: Synthesis Report. 104. could thus in theory result in a significant loss of diatom diversity, although from current knowledge it is impossible to say if this would occur rapidly or only over many tens of thousands of years.

Method of investigation

The fossil record of diatoms has largely been established through the recovery of their siliceous frustules in marine and non-marine sediments. Although diatoms have both a marine and non-marine stratigraphic record, diatom biostratigraphy, which is based on time-constrained evolutionary originations and extinctions of unique taxa, is only well developed and widely applicable in marine systems. The duration of diatom species ranges have been documented through the study of ocean cores and rock sequences exposed on land.Scherer, R.P., Gladenkov, A.Yu., and Barron, J.A. (2007). Methods and applications of Cenozoic marine diatom biostratigraphy. "Paleontological Society Papers" 13, 61–83 Where diatom biozones are well established and calibrated to the geomagnetic polarity time scale (e.g., Southern Ocean, North Pacific, eastern equatorial Pacific), diatom-based age estimates may be resolved to within


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