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Diatoms〔 "cut in half" (= (''dichó-tom-os'' )) — "through" or "apart" and the root of "I cut". 〕 are a major group of algae, and are among the most common types of phytoplankton. Diatoms are unicellular, although they can form colonies in the shape of filaments or ribbons (e.g. ''Fragilaria''), fans (e.g. ''Meridion''), zigzags (e.g. ''Tabellaria''), or stars (e.g. ''Asterionella''). The first diatom formally described in scientific literature, the colonial ''Bacillaria paradoxa'', was found in 1783 by Danish naturalist Otto Friedrich Müller. Diatoms are producers within the food chain. A unique feature of diatom cells is that they are enclosed within a cell wall made of silica (hydrated silicon dioxide) called a frustule.〔(【引用サイトリンク】 website=University of California Museum of Paleontology )〕 These frustules show a wide diversity in form, but are usually almost bilaterally symmetrical, hence the group name. The symmetry is not perfect since one of the valves is slightly larger than the other, allowing one valve to fit inside the edge of the other. Fossil evidence suggests that they originated during, or before, the early Jurassic period. Only male gametes of centric diatoms are capable of movement by means of flagella. Diatom communities are a popular tool for monitoring environmental conditions, past and present, and are commonly used in studies of water quality. ==General biology== There are more than 200 genera of living diatoms, and it is estimated that there are approximately 100,000 extant species.〔Canter-Lund, H. and Lund, J.W.G. (1995). ''Freshwater Algae: Their microscopic world explained'', Biopress Limited. ISBN 0-948737-25-5.〕 Diatoms are a widespread group and can be found in the oceans, in freshwater, in soils and on damp surfaces. Most live pelagically in open water, although some live as surface films at the water-sediment interface (benthic), or even under damp atmospheric conditions. They are especially important in oceans, where they are estimated to contribute up to 45% of the total oceanic primary production. Spatial distribution of marine phytoplankton species is restricted both horizontally and vertically. Though usually microscopic, some species of diatoms can reach up to 2 millimetres in length. Diatoms belong to a large group called the heterokonts, including both autotrophs (''e.g.'', golden algae, kelp) and heterotrophs (''e.g.'', water moulds). Their yellowish-brown chloroplasts 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. Most diatoms are non-motile, 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 by the wind to keep them suspended in sunlit surface waters. Some species actively regulate their buoyancy with intracellular lipids to counter sinking. 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 who 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. Diatom cells are contained within a unique silica cell wall comprising two separate valves (or shells). The biogenic silica that the cell wall is composed of is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is then extruded to the cell exterior and added to the wall. Diatom cell walls are also called frustules or "tests", and their two valves typically overlap one over the other like the two halves of a petri dish. 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. Auxospore production is almost always linked to meiosis and sexual reproduction. Decomposition and decay of diatoms leads to organic and inorganic (in the form of silicates) sediment, the inorganic component of which can lead to a method of analyzing past marine environments by corings of ocean floors or bay muds, since the inorganic matter is embedded in deposition of clays and silts and forms a permanent geological record of such marine strata. (See siliceous ooze). The study of diatoms is a branch of phycology, and phycologists specializing in diatoms are called diatomists. ==Classification== The classification of heterokonts is still unsettled, and they may be treated as a division (or phylum), kingdom, or something in-between. Accordingly, groups like the diatoms may be ranked anywhere from class (usually called Diatomophyceae or Bacillariophyceae) to division (usually called Bacillariophyta), with corresponding changes in the ranks of their subgroups. Diatoms are traditionally divided into two orders: * centric diatoms (Centrales), which are radially symmetrical * pennate diatoms (Pennales), which are bilaterally symmetrical. The former are paraphyletic to the latter. Due to the difference noted in pennate diatoms, of the presence or absence of a raphe (a longitudinal groove in the valve),〔O.E.D. 2nd edition 2005〕 a more recent classification by Round, Crawford & Mann (1990)〔 divides the diatoms (as Bacillarophyta) into three classes and a number of orders: *Class Coscinodiscophyceae Round & R.M.Crawford: centric diatoms * * Anaulales Round & R.M.Crawford * * Arachnoidiscales Round * * Asterolamprales Round * * Aulacoseirales R.M.Crawford * * Biddulphiales * * Chaetocerotales Round & R.M.Crawfor * * 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 * Class Fragilariophyceae F.E.Round: pennate diatoms without a raphe (araphids) * * 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 * Class Bacillariophyceae Haeckel, 1878, ''emend.'' D.G.Mann: pennate diatoms with a raphe (raphids) * * 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 It is probable there will be further revisions as understanding of their relationships increases. Medlin & Kaczmarska (2004) propose the following classification for the diatoms: * Bacillaryophyta * * Coscinodiscophytina * * * Coscinodiscophyceae ('radial centrics') * * Bacillariophytina * * * Mediophyceae ('polar centrics') * * * Bacillariophyceae (pennate diatoms) Diatoms generally range in size from 2 to 200μm,〔 and build intricate hard but porous cell walls (called frustules or tests) composed primarily of silica.〔 This siliceous wall〔(【引用サイトリンク】url=http://www.cmog.org/article/glass-nature )〕 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. 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. Major pigments of diatoms are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.〔 Diatoms are mainly photosynthetic. A few, however, are obligate heterotrophs, while others can live heterotrophically in the absence of light, provided an appropriate organic carbon source is available. Storage products are chrysolaminarin and lipids.〔 Hoek ''et al.'' (1995) also provide a comprehensive coverage of diatom taxonomy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diatom」の詳細全文を読む スポンサード リンク
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