Tabellaria diatoms. Coloured scanning electron micrograph (SEM) of a Tabellaria sp. diatom colony. 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 frustules (siliceous cell walls) of Tabellaria sp. diatoms are attached at the corners giving the colonies a zigzag shape (seen here). Magnification: x1200, when printed 10 centimetres wide.