Diatom cell wall. Coloured scanning electron micrograph SEM of the pattern of small holes, called striae, in the cell wall of a diatom alga. This is a Coscinodiscus sp. diatom. A diatom is a planktonic unicellular alga that has silica in its cell wall frustule. Diatoms are photosynthetic, and they are found in both marine and freshwater environments.