Planktonic diatom alga. Coloured scanning electron micrograph (SEM) of a Coscinodiscus sp. diatom alga (orange). 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. The cell wall of a diatom is often intricately patterned, and has rows of tiny holes, known as striae.