Diatoms. Coloured scanning electron micrograph (SEM) of Paralia (top) and Stephanopyxis sp. diatoms. 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. The diatoms form an important part of the plankton at the base of the marine and freshwater food chains. Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatoms produce 20-30% of the air we breathe. We breathe the oxygen that diatoms release Radiolaria are single-celled protozoans that are found in marine plankton. This radiolarian has a silicate skeleton with pores through which pseudopodia (false feet) of protoplasm project. As it floats in ocean currents,.Magnification: x1250 when printed at 10 centimetres wide.