Bacillariophyceae

(botany) The diatoms, a class of algae in the division Chrysophyta.

A class of nonflagellate unicellular algae, commonly called diatoms, with boxlike silicified walls. Diatoms range in maximum dimension from 4 micrometers to more than 1 millimeter. The diatom wall or frustule (illus. a and b) comprises several interlocking, usually elaborately sculptured, lightly or heavily silicified pieces overlying a thin polysaccharide layer. The two largest pieces are the upper and lower valves, which fit together like the top and bottom of a petri dish or shoe box. Between the valves (along the side or girdle of the cell), several smaller pieces—hooplike girdle bands—are intercalated. Depending upon which dimension is larger, breadth or depth, a diatom tends to lie on the valve side or on the girdle side.

Diatoms. (a) Pinnularia, top view; (b) side view. (c) Arachnoidiscus ehenbergii.

Valves are honeycombed by perforate chambers arranged in patterns characteristic of individual species. The overall symmetry of the valves and details of their structure and ornamentation provide the basis for classification. More than 200 genera and 12,000 species of living diatoms have been described. Two main groups are recognized: those in which structural features of the valve are arranged with reference to a central pole (centric valve; illustration c) or to two or more poles (gonioid valve); and those in which the features are arranged with respect to a line, often symmetrically (pennate valve).

Most diatoms are photosynthetic. Many, however, are aux-otrophic, requiring an external source of certain vitamins. Diatoms are likely to occur wherever there is moisture. They are free-living or attached, solitary or colonial. Marine planktonic diatoms have been important primary producers for at least 100 million years, and over the millennia their frustules have accumulated on the ocean floor. From the abundant fossil record, it is known that centric diatoms evolved first (Cretaceous), followed by pennate diatoms (Paleocene). Uplifted deposits (diatomaceous earth) are mined at several locations. See also Algae; Chrysophyceae.