Diatom. Coloured scanning electron micrograph (SEM) of the Navicula sp. 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: x400 when printed 10 cm wide.