Diatom alga. Coloured scanning electron micrograph (SEM) of a Bacteriastrum delicatulum 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. The long legs" of this diatom may aid floatation. Magnification: x200 at 6x7cm size."