Diatoms. Coloured scanning electron micrograph (SEM) of two Coscinodiscus sp. (top and bottom) diatoms and an Actinoptychus sp. (centre) diatom. 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 cell wall, or frustule. The frustule often has rows of tiny holes, known as striae. Magnification: x460 when printed at 10 centimetres wide.