Diatom. Coloured scanning electron micrograph (SEM) of a Bacteriastrum varians diatom. They are marine planktonic unicellular algae. They have mineralised cell walls (frustules) divided into two halves. The frustules contain silica and provide protection and support. They are colonial, and their long branching legs are used as an aid to floatation. 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: x1300 when printed 10 centimetres wide.