Polarised light micrograph of Surirella sp. diatom and purple bacteria. 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. Purple bacteria are gram-negative proteobacteria that use photosynthesis to produce their own food. Magnification: 186x when printed 10 centimetres wide.