Diatom chain. Light microscopy footage of a chain of marine 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. Diatoms form an important part of the plankton at the base of the marine and freshwater food chains. This sample was collected off the coast of Wales.