Coccolithophore

Coccolithophores are single-celled algae belonging to the haptophytes. They are distinguished by special plates called coccoliths which are important microfossils. Coccolithophores are exclusively marine and are found in large numbers throughout the upper layers of the ocean.

See also What is a Coccolithophore? (http://earthobservatory.nasa.gov:81/Library/Coccolithophores/)