Diatom. Coloured scanning electron micrograph (SEM) of part of the cell wall (frustule) of the diatom Coscinodiscus sp. 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, which often has rows of tiny holes, known as striae (shown here).