Coccoliths. Scanning electron micrograph (SEM) of coccoliths from a coccolithophore. Coccolithophore algal organisms are calcareous phytoplankton and are surrounded by a skeleton (coccosphere) of calcium carbonate plates (coccoliths, shown here). When the organism dies, these 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. Magnification: x1000 when printed at 10 centimetres wide.