Diatom algae. Coloured scanning electron micrograph (SEM) of diatoms (red). Diatoms are a distinct group of single-celled algae containing at least 10,000 species. These are pennate diatoms, meaning they are bilaterally symmetrical. Diatoms have mineralised cell walls (frustules) that are divided into two halves. The frustule contains silica and provides protection and support. This sample was obtained during a plankton trawl in the USA.