Diatoms and desmids. Polarised light micrograph of Fragilaria sp. diatoms (long, bottom) and Staurastrum sp. desmids (top). 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. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Magnification: x400 when printed 10 centimetres wide.