Light micrograph (fluorescence microscopy) of a colonial diatom Tabellaria fenestrata var. asterionelloides. The chloroplasts of the diatom fluoresce in red when illuminated by ultraviolet light. Diatoms are 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 exist in huge numbers in marine and freshwater environments. They form an important part of the plankton at the base of the marine and freshwater food chains.. Diatoms are 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 exist in huge numbers in marine and freshwater environments. They form an important part of the plankton at the base of the marine and freshwater food chains. Magnification: x450 when printed at 10 centimetres wide at its longest edge.