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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 A microscopic alga, belonging to the division Bacillariophyta, in which the cell wall (frustule) is composed of silica and consists of two halves, one of which overlaps the other like the lid of a box. Frustules are often delicately ornamented. Most diatoms are unicellular, but some are colonial or filamentous. Most are photosynthetic, but some species lack chlorophyll and live heterotrophically among decaying marine algae. Pennate (i.e. bilaterally symmetrical) diatoms occur in both freshwater and marine habitats; centric diatoms (i.e. radially symmetrical) occur predominantly as part of the marine plankton. There are more than 10 000 species. See diatomaceous earth; diatomite; and diatom ooze.

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diatom

di·a·tom / ˈdīəˌtäm/ • n. Biol. a single-celled alga (class Bacillariophyceae) that has a cell wall of silica. Many kinds are planktonic. DERIVATIVES: di·a·to·ma·ceous / ˌdīətəˈmāshəs/ adj.

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diatom

diatom Any of a group of tiny microscopic single-celled algae (phylum Bacillariophyta) characterized by a shell-like cell wall made of silica. Diatoms live in nearly all bodies of salt and freshwater, and even soil and tree bark.

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diatom

diatom See BACILLARIOPHYCEAE.

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diatom

diatom See BACILLARIOPHYTA.

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diatom

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