Coccolithophore

Coccolithophores are a group of single-celled marine algae in the Haptophyta. They are characterized by calcium carbonate plates called coccoliths that cover the cell or are shed into the surrounding water, forming a coccosphere. They are primarily photosynthetic plankton living in the photic zone of oceans and seas.

Most species are tiny, typically 2 to 20 micrometers in diameter. They possess two flagella for movement and a haptonema, a filament used for sensing. Coccoliths are produced inside Golgi-derived vesicles and secreted to the cell surface; assembled coccoliths form a calcareous covering that can be complete or patchy, and may overlie the cell in various patterns.

Coccolithophores contribute to marine primary production and to the biogeochemical cycling of carbon. Photosynthesis removes CO2 from seawater, while calcification converts carbonate and calcium into CaCO3, releasing CO2 to seawater. As a result, the carbonate tests they produce can contribute to long-term carbon storage when they sink as sediment. Their blooms can be extensive and are detectable from space as bright, white patches; Emiliania huxleyi is among the best-known bloom-forming species. Bloom dynamics are influenced by light, nutrients, temperature, and ocean conditions and can be affected by viral infections, such as Emiliania huxleyi virus (EhV).

Coccolithophores have a rich fossil record dating to the Jurassic and are a major component of Cretaceous and Cenozoic sediments. Their fossil coccoliths, from genera such as Coccolithus, Discoaster, and others, form extensive chalk and limestone deposits and are widely used in biostratigraphy and paleoclimate reconstructions. The White Cliffs of Dover are a well-known example of chalk derived partly from coccolithophore tests.