A coccolithophore is a unicellular, eukaryotic phytoplankton (alga). Coccolithophores belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae. Coccolithophores are distinguished by special calcium carbonate plates of uncertain function called coccoliths, which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths, so not every member of Prymnesiophyceae is a coccolithophore. Coccolithophores are almost exclusively marine, are photosynthetic, and exist in large numbers throughout the sunlight zone of the ocean.