BACILLARIOPHYCEAE

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). 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 Röttger, 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 box-like cell wall composed of silica, called the frustule. The chloroplasts contain chlorophylls a, c1, and c2 with the major carotenoid being the golden-brown 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 (SiO2) 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 (Figs. 17.17, 17.46) 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.