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The diatoms are one of the largest and ecologically most significant groups of organisms on Earth. They are also one of the easiest to recognize, because of their unique cell structure, silicified cell wall and life cycle. They occur almost everywhere that is adequately lit (because most species need light for photosynthesis) and wet - in oceans, lakes and rivers; marshes, fens and bogs; damp moss and rock faces; even on the feathers of some diving birds. Some have been captured by other organisms and live as endosymbionts, e.g. in dinoflagellates and foraminifera. Because of their abundance in marine plankton, especially in nutrient-rich areas of the world's oceans, diatoms probably account for as much as 20% of global photosynthetic fixation of carbon (~ 20 Pg carbon fixed per year: Mann 1999), which is more than all the world's tropical rainforests.
Hydrosera. © David G. Mann. This image comes from the Professor Frank Round Image Archive at the Royal Botanic Garden Edinburgh
Diatom cells have regular geometrical shapes. In a mathematical sense, they are always 'closed generalized cylinders' and they are usually straight ('right') but the cross section of the cylinder can vary from circular to elliptical to spicular to complex lobed shapes like the Hydrosera cell shown above. The shape is maintained faithfully, whatever the environmental conditions, because the cell wall contains a large proportion of hard, brittle silica, which is partially hydrated [(SiO2)m.nH2O] and non-crystalline. Basically, diatoms live in glass boxes. The silica shell of the diatom is called the 'frustule' and is made of two halves, each in turn composed of several different pieces. Hydrosera frustules, like those of all other diatoms, are perforated by many small holes, which allow water, dissolved material and solids (gases, inorganic nutrients, and organic substrates and secretions) to pass in or out.
Left: Living diatoms and other algae from a freshwater loch in Scotland. Right: False-colour picture of a subfossil assemblage from a muddy deposit a few metres below the surface of a mire in SW Scotland. © 2008 David G. Mann
The silica of the diatom cell wall is resistant to decay, although it will begin to dissolve once its organic coating has been stripped off. Once incorporated into silica-rich sediments, however, frustules may survive for hundreds to millions of years and can be used to monitor changes in freshwater or marine environments. The left-hand picture above shows a spread of living diatoms and other algae from a freshwater loch in Scotland. Each cell contains one to several brownish chloroplasts. Shown in the right-hand (false-colour) picture is a subfossil assemblage from a muddy deposit a few metres below the surface of a mire in SW Scotland. Here, all the cells are empty - only the cell walls remain; indeed, in many cases the cell walls have fallen apart into their component pieces. But it is still possible to identify them, because the walls retain their shape and pattern. Consequently, if the ecologies of the species are known, then the fossil assemblage can be used to estimate what conditions were like when it was formed. In the assemblage illustrated there are both planktonic species (the circular Cyclotella valves) and benthic species, which have become mixed together after death.
Life cycle series of Navicula reinhardtii valves. © 2008 David G. Mann
Because of the construction of the silica frustule and the way in which cells divide, average cell size declines during the life cycles of most diatoms. The shape often changes too, as in the series of Navicula reinhardtii valves shown. It can take a long time for cells to decline to their smallest size - often several years in nature - but sooner or later there is an abrupt restitution of size, taking a few days, involving formation of a special cell, called an auxospore. This behaviour is unique.
Variation in shape and size during the life cycle causes major problems for people trying to identify diatom species and also for taxonomists, if only a few dead specimens are available for study. If diatoms 'miss' the chance to form auxospores (for example, if suitable mates are not available, or if environmental conditions are unsuitable), the cells continue to divide, getting smaller and smaller until they die.