Diatoms on cladophora algae. Polarised light micrograph of a section through Cocconeis sp. diatoms (small, oval) on Cladophora sp. green algae (green). Diatoms are planktonic unicellular algae. They have a mineralised cell wall (frustule) divided into two halves. The frustule contains silica and provides protection and support. Diatoms form an important part of the plankton at the base of the marine and freshwater food chains. They also form an important part of the fossil record. Magnification: x140 when printed 10 centimetres wide.