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Diatoms

Author: Michael Guiry, AlgaeBase

Diatoms are unicellular (single-celled) or colonial coccoid (spherical) algae Most diatoms are aquatic, occurring in the sea or in freshwater, but some occur on damp soil, trees, or rocks. A large fraction of marine and freshwater phytoplankton consists of diatoms. Photosynthesis by phytoplankton accounts for much of the Earth's primary productivity, the biological activity that captures the energy that is at the base of all food chains. In temperate ocean regions where upwelling brings nutrients to the surface, diatoms are extremely abundant and are responsible for the very high primary productivity in these areas.

Diatoms have silica cell walls and are composed of two units which fit together like the two parts of a Petri dish or pill box (the word "diatom" comes from the Greek for "cut in two"). When diatoms reproduce, the "lid" and slightly smaller "dish" separate. Each half then grows a new (slightly smaller) "dish". Thus, with each generation of this type of asexual reproduction, the resulting individuals are smaller. The "normal" size is eventually restored by a bout of sexual reproduction.

Dead diatoms accumulating in highly productive waters form diatom oozes. Geological deposits derived from such oozes (mainly from the Tertiary, millions of years ago) are now mined as diatomite or diatomaceous earth, which is used for water filtration, in toothpastes (as an abrasive), and in deodorants and decolouring agents. Diatoms in lungs are often used by forensic scientists as an indication of drowning and the types of diatoms can indicate where drowning took place.

It is estimated that the total annual primary production on earth is about 1.4 x 1014 kg of carbon, of which an estimated 20-25% can be attributed to marine planktonic diatoms and a further 15-20% to other planktonic algae.

Because the siliceous cell wall is so well preserved, the diatoms have an extensive fossil record, more extensive than any other group of algae, and can be used to determine whether deposits have a marine or freshwater origin. The oldest confirmed fossil diatoms date from the Lower Cretaceous (125 million years ago) and were marine. Diatoms have recently been shown to have a "urea cycle", previously thought to be unique to animals, but this seems to have evolved independently from the animal urea cycle.

There are two basic diatom morphologies: centric and pennate. Centric diatoms are radially symmetrical and pennate diatoms are bilaterally symmetrical. The centric form seems to have evolved first.

Diatoms belong to the Kingdom Chromista, Phylum Heterokontophyta (the stramenopiles). There are about 750 genera of living diatoms in three classes and currently about 12,000 described species, making the diatoms by far the most species-rich group of algae. There are varying opinions as to the actual numbers of diatom species, with some authors speaking of 20,000, some of 200,000, and some of in excess of 1,000,000 species. To date, at least 62,000 names have been applied to diatoms, but many of these may turn out to be synonyms, i.e., duplicate names mistakenly given to already named species. EOL includes about 8,000 species of diatoms.

Useful Diatom Resources

  • Diatoms of the United States website: http://westerndiatoms.colorado.edu/
  • PhycoKey: http://cfb.unh.edu/phycokey/phycokey.htm