Plankton. Coloured scanning electron micrograph (SEM) of plankton containing mainly Ceratium dinoflagellates. A few diatoms are also present. Dinoflagellates are unicellular protozoans. About 90% are found in marine environments as plankton. They range in size from 2 millimetres to less than a hundredth of a milimetre across. These specimens were found off the coast of Scotland. Magnification: x200 when printed at 10 centimetres wide.