Diatoms

(Bacillariophyta), a division of algae comprising about 20,000 species.

Diatoms are microscopic (0.75-1500 microns), unicellular, and solitary or colonial algae. Among the colonial forms are species that live in slimy pipes, forming brown clusters that reach 20 cm in height. Diatom cells have hard siliceous shells (frustules) consisting of two overlapping halves or valves. The upper valve is called the epitheca; the lower, the hypotheca. The shell walls have pores through which there is an exchange of matter with the external environment. Many diatoms with slitlike openings (so-called raphes) along each half of the shell are apparently capable of moving along the substrate by depositing slime. The cells contain one nucleus with one or more nucleoli and one or more yellow-brown chromatophores (the brown pigments beta-carotene and xanthrophyll are present along with chlorophyll a). The products of assimilation are oil and volutin.

Diatoms multiply by division, each daughter cell receiving a half of the parent shell, the other half regenerates while the older half grips the new one with its edges. Because of the manner of division and the fact that the hard shells, impregnated with silicon, are nearly or completely incapable of further growth, diatoms become gradually smaller as they multiply. To form an auxospore (growth spore), cellular matter leaves the membrane and grows considerably, starting a new, larger generation. Auxospores can also be formed sexually, by conjugation of the cellular matter of two cells. Some diatoms reproduce through zoospores, the sexual process involves the participation of flagellate gametes (isogametes, heterogametes, or oogametes). Dormant spores are found in still other genera. Diatoms are diploid; only the gametes are haploid.

Diatoms are divided into three classes, according to valve structure: Centrophyceae, Mediatophyceae, and Pennatophyceae. Classes Centrophyceae and Pennatophyceae are most numerous. In Centrophyceae the valves of the shell have a radial structure and are always without a raphe. Most species of this group are planktonnic. The shells of the Pennatophyceae are usually bilaterally symmetrical, although some are asymmetrical. Many species have raphes and are included among the benthos. Class Mediatophyceae includes forms that are transitional between the other two; most of them are known only as fossils, although isolated marine genera can still be found. Diatoms are the most widespread group of algae found in nature, living in fresh and salt waters—particularly among marine plankton, where they serve as food for animals. They are also found in the bottom silt of bodies of water, on aquatic plants and underwater objects, on damp ground, rocks, and in moss. Numerous fossilized diatoms are known, dating as far back as the Jurrasic period; at times they form large deposits of diatomaceous earth (so-called diatomite or tripoli), which is important commercially. Because of their delicate and regular structure, the shells of diatoms are used to test the resolving power of optical microscopes.