Diatom alga. Coloured scanning electron micrograph (SEM) of a colonial marine diatom. Diatoms are a distinct group of single-celled algae containing at least 10,000 species. They form a large part of the plankton in marine and freshwater food chains. This is a pennate diatom, meaning that it is bilaterally symmetrical. Each individual is joined to two others, forming a chain. Bacteria (pink) are also visible on the surface of the diatoms.