Diatoms by the numbers
Diatoms are a lineage of unicellular photoautotrophs within the heterokont algae
(Goertzen and Theriot 2003; Graham and Wilcox 2000). Given sufficient moisture and
sunlight, diatoms can thrive in virtually any habitat, though they are principally found in
association with submerged surfaces (benthic) or suspended in the water column of lakes
and oceans (planktonic). The importance of marine planktonic diatoms in the
biogeochemical cycling of many elements, particulary carbon and silica, cannot be
overstated. As a whole, marine phytoplankton account for >46% of global net primary
production, despite representing only 0.2% of global primary producer biomass
(Behrenfeld and Falkowski 1997; Falkowski et al. 1998; Field et al. 1998). Diatoms alone
account for ≥40% of marine primary production, thus contributing 20–25% of global net
primary production (Nelson et al. 1995; Werner 1977), which is more than all the world’s
rainforests (Field et al. 1998). Diatoms play an integral role in the transfer of
photosynthetically fixed carbon to consumers. Diatom storage products include
chrysolaminaran and a suite of highly unsaturated fatty acids, which are an excellent food
source for invertebrate grazers (Anderson and Cummins 1979; Müller-Navarra et al.
2000). Nutrient flux from deep, nutrient-rich waters into the euphotic zones of areas of
coastal upwelling or high convective mixing promotes the diatom primary production
that sustains the world’s fisheries (Falkowski et al. 1998).
The diatom cell wall is composed of hydrated amorphous silica. Silica comprises
~25% (by weight) of the earth’s crust (Iler 1979). A silica molecule weathered from land
into the oceans passes through a diatom ~39 times before being buried in marine
sediments (Martin-Jezequel et al. 2000; Treguer et al. 1995). Diatoms control
biogeochemical cycling of silica in the sea, where the element is relatively scarce because
of their high demand for it (Martin-Jezequel et al. 2000; Nelson et al. 1995; Treguer et al.
1995).
No one can agree on the estimated number of diatom species. Diatomists
currently recognize 10,000–12,000 diatom species (Norton et al. 1996)—a small fraction
of the estimated 200,000 diatom species worldwide (Mann and Droop 1996). Some have
estimated that total diatom diversity might exceed 10,000,000 species (Norton et al.
1996).
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