diatom

diatom (dī`ətŏm', -tōm'), unicellular organism of the kingdom Protista, characterized by a silica shell of often intricate and beautiful sculpturing. Most diatoms exist singly, although some join to form colonies. They are usually yellowish or brownish, and are found in fresh- and saltwater, in moist soil, and on the moist surface of plants. They carry chlorophylls a and c and the carotenoid fucoxanthin contained in plastids. They reproduce asexually by cell division. Some 40,000 species (5,600 living species) are either bilaterally or radially symmetrical. For the most part they lack flagella. Although most diatoms are autotrophic, some heterotrophic or symbiotic species can be found in particular habitats. The living matter of each diatom is enclosed in a shell of silica that it secretes. These shells are marked by minute pores or depressions that allow the living organism access to its environment. As the principal constituent of plankton (see marine biology), diatoms are an important food source for fish and other aquatic animals, e.g., the baleen whales.

When aquatic diatoms die they drop to the bottom, and the shells, not being subject to decay, collect in the ooze and eventually form the material known as diatomaceous earth (sometimes called kieselguhr). When it occurs in a more compact form as a soft, chalky, light-weight rock, it is called diatomite. Deposits of diatomaceous material, formed underwater in past geologic time and now exposed above water, are found in all parts of the world. Diatomite is much used as an insulating material against both heat and sound, in making dynamite and other explosives, and for filters, abrasives, and similar products. Most of the earth's limestone has been deposited by diatoms, and much petroleum is of diatom origin.

Diatoms are classified in the phylum (division) Chrysophyta, class Bacillariophyceae.

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diatom

Diatom (highly magnified)

(credit: Eric Grave—Photo Researchers)

Any member of the algal division or phylum Bacillariophyta (about 16,000 species), tiny planktonic (see plankton), unicellular or colonial algae found floating in all the waters of the earth. The intricate and delicate markings of the silicified cell wall are useful in testing the resolving power of microscope lenses. The beautiful symmetry and design of diatoms justify their title “jewels of the sea.” Among the most important and prolific sea organisms, diatoms serve directly or indirectly as food for many animals. Diatomaceous earth, composed of fossil diatoms, is used in filters, insulation, abrasives, paints, and varnishes, and as an insecticide.