Diatom. Coloured scanning electron micrograph (SEM) of a Bacteriastrum varians diatom. This is a marine planktonic unicellular alga. It has a mineralised cell wall (frustule) divided into two halves. The frustule contains silica and provides protection and support. It is a colonial diatom. The long branching legs are used as an aid to floatation. Magnification unknown.