Diatom. Coloured scanning electron micrograph (SEM) of an Actinoptychus.sp. diatom. 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. Through carbon fixation, diatoms remove carbon dioxide (CO2) from the atmosphere. The CO2 is converted to organic carbon in the form of sugar, and oxygen (O2) is released. We breathe the oxygen that diatoms release. Magnification: x2500 when printed at 10 centimetres wide.