The Bacillariophyceae or the diatoms probably evolved from a scaly member of the Chrysophyceae (similar to the organisms in the Parmales) or Bolidophyceae (Guillou et al., 1999 ; Medlin, 2016) around the Devonian–Carboniferous transition (360 Ma)(Brown, 2010). The diatoms are unicellular, sometimes colonial, algae found in almost every aquatic habitat as free-living photosynthetic autotrophs, colorless heterotrophs, or photosynthetic symbiotes (Schmaljohann and Rottger, 1978). They may occur as plankton or periphyton, with most brownish-green films on substrates such as rocks or aquatic plants being composed of attached diatoms. The cells are surrounded by a rigid two-part boxlike cell wall composed of silica, called the frustule. The chloroplasts contain chlorophylls a, c 1, and c 2 with the major carotenoid being the goldenbrown fucoxanthin, which gives the cells their characteristic color.

In discussing diatoms and silica, there is often confusion over terminology in regard to silicon. Silicon is the element. Silica is a short convenient designation for silicon dioxide (SiO 2) in all of its crystalline, amorphous, and hydrated or hydroxylated forms. Silicate is any of the ionized forms of monosilicic acid [Si(OH) 4] (Iler, 1979).

Cell Structure

The two-part frustule surrounds protoplasm that has a more or less central nucleus suspended in a system of protoplasmic threads. The chloroplasts occupy most of the cell usually as two parietal plastids although sometimes as numerous discoid plastids. The storage product, chrysolaminarin, occurs in vesicles in the protoplasm.

Cell Wall

The characteristic feature of the Bacillariophyceae is their ability to secrete an external wall composed of silica, the frustule. It is constructed of two almost equal halves, the smaller fitting into the larger like a Petri dish (Figs. 17.1, 17.8, 17.9, 17.34). The outer of the two half-walls is the epitheca and the inner the hypotheca. Each theca is composed of two parts, the valve, a more or less flattened plate, and the connecting band, attached to the edge of the valve. The two connecting bands, one attached to each valve, are called the girdle (von Stosch, 1975). Sometimes the connecting bands themselves are called girdle bands (Fig. 17.34).