DIATOM

any member of the algal division or phylum Bacillariophyta (about 16,000 species) found floating in all the waters of the Earth. Diatoms may be either unicellular or colonial. The silicified cell wall forms a pillbox-like shell (frustule) composed of overlapping halves (epitheca and hypotheca) that contain intricate and delicate markings useful in testing the resolving power of microscope lenses. The beautiful symmetry and design of diatoms justify their title jewel of the sea. During reproduction, usually by cell division, the overlapping shell halves separate, and each secretes a smaller bottom half. Thus, individual diatoms formed from successive bottom halves show a progressive decrease in size with each division. In a few months there can be as much as a 60 percent decrease in size. Periodic spore formation serves to restore the diatom line to its original size. Diatoms are divided into two orders on the basis of symmetry and shape: the round, nonmotile Centrales have radial markings; the elongated Pennales, which move with a gliding motion, have pinnate (featherlike) markings. Food is stored as oil droplets, and the golden-brown pigment fucoxanthin masks the chlorophyll and carotenoid pigments that are also present. Diatoms, among the most important and prolific sea organisms, serve directly or indirectly as food for many animals. Diatomaceous earth, a substance composed of fossil diatoms, is used in filters, insulation, abrasives, paints, and varnishes.