Diatom. Coloured scanning electron micrograph SEM of a diatom磗 Arachnoidiscus sp. cell wall, or frustule. The diatoms are a group of photosynthetic, single_celled algae containing about 10,000 species. They form an important part of the plankton at the base of the marine and freshwater food chains. The characteristic feature of diatoms is their intricately patterned, glass_like frustule. The frustule often has rows of tiny holes, known as striae.