Diatom. Coloured scanning electron micrograph (SEM) of a diatom (Coscinodiscus sp.). Diatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silica (hydrated silicon dioxide), called a frustule. Magnification: x600 when printed at 10 centimetres wide.