Diatom alga. Coloured scanning electron micrograph (SEM) of a diatom (orange). Diatoms are a distinct group of single-celled algae containing at least 10,000 species. This is a pennate diatom, which means that it is bilaterally symmetrical. Diatoms have mineralised cell walls (frustules) that are divided into two halves. The frustule contains silica and provides protection and support. Several bacteria (blue) are also seen.