Protozoa and diatoms in periphyton. Polarised light micrograph of a group of Vorticella sp. protozoa (bell-shaped) with Cymbella sp. and Gomphonema sp. diatoms in a sample of periphyton. Periphyton is a complex mixture of algae, cyanobacteria, heterotrophic microbes and detritus that is attached to submerged surfaces in most aquatic ecosystems. Diatoms are single-celled photosynthetic algae, of which there are about 100,000 species. They have mineralised cell walls (frustules) that contain silica and provide protection and support. They form an important part of the plankton at the base of the marine and freshwater food chains. Magnification: x100 when printed 10 centimetres wide.