Fosssil diatom. Light micrograph of a fossilised diatom. 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. Deposits of diatoms are known as diatomite, or diatomaceous earth. Magnification: x143 when printed at 10 centimetres across.