Pediastrum alga. Coloured scanning electron micrograph (SEM) of the disc-like colony of cells formed by the green alga, Pediastrum sp. Pediastrum is a type of freshwater colonial algae. The colony comprises a central cell surrounded by concentric rings of cells. The outermost cells each have two lobes projecting outwards, giving the colony a "spiky" appearance. Magnification: x3000 when printed at 10 centimetres wide.