Coccolithophore. Coloured scanning electron micrograph (SEM) of Rhabdosphaera claviger. This small algal organism is a type of calcareous phytoplankton and is surrounded by a skeleton (coccosphere) of calcium carbonate plates (coccoliths). When the organism dies, the plates separate and sink to the ocean floor. Individual plates have been found in vast numbers and can make up the major component of a particular rock, such as the chalk of England. R. claviger is found in marine environments world wide. Magnification: x15000 when printed at 10 centimetres wide.