Diatom. Coloured scanning electron micrograph (SEM) of an unknown species of diatom. Diatoms are single-celled photosynthetic algae, of which there are about 100, 000 species. The characteristic feature of diatoms is their intricately patterned, glass-like cell wall, or frustule. The frustule shows rows of tiny holes, known as striae. The diatoms form an important part of the plankton at the base of the marine and freshwater food chains. Magnification: x2000 when printed at 10 centimetres wide.