Diatoms (Bacillariophyceae) are a monophyletic group of photosynthetic unicellular organisms.
They are characterised by a unique cell wall of silica, the ‘frustule’. This is essentially in two parts - much like the dish and overlapping lid of a Petri dish - with the cell contained within.
The siliceous cell wall is inert and after the organism’s death these parts preserve exceptionally well. As a result diatoms have an extensive fossil record.
In the past diatoms were placed among the various algal groups. Currently they are in a heterogenous mix of photosynthetic and non-photosynthetic organisms collectively referred to as Stramenopiles - which include the giant kelps, water moulds and many other unicellular organisms.
Diatom species can occur singly or in colonies, and can be motile or sessile.
They occur mainly in freshwater and marine habitats, but also in soil.