Pediastrum alga. Coloured scanning electron micrograph (SEM) of a close up of the centre 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 (not seen) each have two lobes projecting outwards, giving the colony a 'spiky' appearance.