Diatom Triceratium formosum, phase contrast photomicrograph
There are more than 200 genera of living diatoms, and it is estimated that there are approximately 100,000 extant species. 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 (, golden algae, kelp) and heterotrophs (, 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.
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Photo credit: © Scenics & Science / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: contrast, diatom, ecology, formosum, heterokonts, micrograph, microscope, phase, photomicrograph, plankton, triceratium
