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The diatoms live in glass houses (frustules), and are often dominant both in the oceans and estuaries (along with dinoflagellates) and in freshwater lakes and streams. On a global scale they have been estimated to produce ~ 50% of all photosynthesis. In freshwater the availability of dissolved silica is a major determinant of diatom abundance. Large pulses of diatoms in the vernal period of temperate lakes usually depletes silica concentration below ending the pulse (Lund et al. 1963) found the concentration triggering declined to be 0.5 mg L^-1 for Asterionella formosa in Lake Windermere, English Lake District. Autumnal mixing (equivalent to upwelling of silica-rich hypolimnetic water) supports a second annual pulse of diatoms.

The diplontic life cycle of diatoms is almost unique among the algae, the only other case being Vaucheria in class Tribophyceae (Mann 1993) -- they are diploid except for meiotic production of gametes. Sexual reproduction of diatoms has been reviewed (Ibid).

With the exception of zoogametes in a few centrics, diatoms lack flagella. Some pennate diatoms (monoraphes and biraphes) have a rapid “gliding motility” on solid surfaces, excreting a mucilaginous propellent from a raphe (elongate “slit” in the raphe, exposing the cell membrane to the solid surface). A few centric diatoms have a “shuffling motility” on the same basis, involving the labiates process (rimoportula) rather than a raphe, but much slower than the pennates (Medlin et al. 1986).

Because of their silica frustules diatoms are more dense than most photosynthetic plankton and most are meroplankton requiring water currents to remain in suspension. A few are euplankton, decreasing their specific gravity by accumulating less silica in their frustules and carrying less-dense lipid droplets.

The diatoms, thought to have had flagella and lost them through time, are also grouped with other classes such as goldens and browns that have in common two different types of flagella in the motile stages, into the "stramenopiles" ("straw hairs") and/or "heterokonts" ("differing poles", i.e. flagella). Included are both photosynthetic and heterotrophic forms. They are also grouped into the Ochrophytes ("yellow plants", an antiquated misnomer).

Date of origin of diatoms is unclear but undisputed fossils date to the Upper Cretaceous (100 - 65 million years ago), with some evidence of fossils from the Lower Crecaceous Period (143 - 100 mya). Likely they are monophyletic based on the universality of two valves and "girdle bands" (Round and Crawford 1981). It is also argued that the date of origin could have been much earlier, as long as 2 billion years ago (2 Ga) -- late Proterozoic through late Jurrasic, ~ 2000 - 143 mya (Ibid.)

Perhaps the classic early key to diatoms, at least freshwater forms, is Bacillariophyta (Diatomeae) (Hustedt 1930).

An excellent online image-based key is being developed (since 2009) at the University of Colorado at Boulder, by Sarah A. Spaulding, US Geological Survey Institute of Arctic and Alpine Research.

Dispersal and Diversity:

The concept that microbes (both procaryotes and eucaryotes) are easily dispersed and therefore ‘everything is everywhere’ (Baas-Becking 1934, Finlay and Clarke 1999, Finlay 2002) , given that a compatible habitat exists globally, has been challenged with the use of diatom databases in both the north and south hemispheres of earth (Vyverman et al. 2007).  The results indicate asymmetry between north and south hemispheres, and suggest that mainly historical factors and isolation of lakes (similar to prediction of the ‘island effect’ in ecology) play a major role in constraining dispersion – approximately 70% role, while local environmental factors including temperature and lake chemistry played a much lesser role (< 30%) in determining the relative richness of genera and species of diatoms in various lakes and lake regions.

More on morphology

References:

Baas-Becking, L.G.M.  1934.  Geobiologie of Inleiding tot de Milieukunde.  Van Stockum and Zoon, The Hague, The Netherlands.

Finley, BJ., and K.J. Clarke  1999.  Ubiquitous dispersal of microbial species.  Nature 400:828.

Hustedt, F. 1930. Bacillariophyta (Diatomeae). In: Die Susswasser-Flora Mitteleuropas. Heft 10. Jena. Verlag von Gustav Fischer.

Lund, J.W.G., F.J. Mackereth, and C.H. Mortimer 1963. changes in depth and time of certain chemical and physical conditions and of the standing crops of Asterionella formosa Hass. in the North Basin of Windermere in 1947. Phil. Trans. R. Soc. Lond. Ser. B 731:255-290.

Mann, D.G. Patterns of sexual reproduction in diatoms. Hydrobiologia 269-270:11-20.

Round, F.E., and R.M. Crawford 1981. The lines of evolution of the diatoms. I. Origin. Proc. Royal Soc. London. Series B, Biological Sciences 211(1183):237-260.

Vyverman, W., E. Verleyen, K. Sabbe, K. Vanyoutte, M. Steerken, D.A. Hodgson, D.G. Mann, S. Juggins, B. Ben de Vijver, V. Jones, R. Flower, D. Roberts, V.A. Chepurnov, C. Kilroy, P. Vanormelingen, and A. De Wever  2007.  Historical processes constrain patterns in global diatom diversity.  Ecology 88(8):1924-1931.