Polarized light micrograph of Fragilaria 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. They form an important part of the plankton at the base of the marine and freshwater food chains.Magnification: x186 when printed at 10 cm wide.