Diatom. Light micrograph of a centric (circular) marine diatom (Aulacodiscus sp.) with an intri- cately patterned cell wall. Diatoms are single- celled algae that exist in huge numbers in marine and freshwater environments. They account for a large proportion of the microscopic plankton that forms the basis of marine food chains. Their ornate, silica-rich cell walls consist of two overlapping valves called frustules, which clip together like the lid and body of a pillbox. These persist long after diatoms die, building up on the sea bed to form sedimentary deposits of chalky rock. Magnification x500 at 6x7cm size.