Diatoms are microscopic unicellular, eukaryotic organisms ranging in size from approximately 5 microns to 1000 microns and living wherever there is moisture. The word 'diatom' means 'cut in two' and is derived from the Greek: (dia) = 'through' + (temnein) = 'to cut', i.e., 'cut in half'.
Diatoms are box like in structure but not necessarily in shape. The basic shape of the entire diatom or frustule is constructed of two almost equal halves with the larger covering the smaller like a pill box or petri dish. Each half, intricately and variously sculptured, is called a valve and is surrounded by a marginal connective structure called the girdle.
The girdles, often furnished with rows of minute teeth, hold the valves together by their edges. The outline of the valves varies considerably and may be, circular, elliptical, lunate or polygonal whilst the valve surfaces may be flat, convex, concave, conical or undulating.
Valve structure is the characteristic that has been studied more than any other aspect of the diatom cell. While much classification is based on variations in the symmetry of the silicaceous frustule much also depends upon the intricacies of pore structure and the arrangement of wall features such as raphes, ocelli, portulae.
According to Werner (1977) about 12000 bona fide species have been described. Diatoms probably contribute about 20-25% of the world's net primary production & in freshwater systems, diatoms
yield from 20-50% of all the productivity of certain lakes (Siver 1977), thus
diatom productivity is as important as the boreal forest or the grasslands (Werner 1977).
N. Ingram Hendey. 1964. An Introductory Account of the Smaller Algae of British Coastal Waters
Siver, P.A. 1977. Comparison of attached diatom communities on
natural and artificial substrates. J. Phycol. 13:402-406.
Werner, D. ed. 1977. The Biology of Diatoms. Botanical Monographs.
Vol. 13. Univ. of California Press. 498 pp.
Barber & Haworth 1981. A guide to the morphology of the Diatom Frustule,
Round et al. 1990. The Diatom:Biology and Morphology of the Genera.
V Jones. 2007. Diatom Introduction