Coccolithophore

Coccolithophores are single-celled marine algae that surround themselves with tiny plates of calcium carbonate called coccoliths. They are a major group of phytoplankton, and their remains form much of the chalk and limestone found on land and in ocean sediments.

Each cell is only a few micrometers wide, and it makes plates of calcite inside the cell and then moves them to the surface, where they interlock into a sphere. They are photosynthetic, and the best-known species, Emiliania huxleyi, forms enormous blooms of milky turquoise water that can be seen from space. Blooms often end when they are infected by giant viruses, which have been studied as part of virus ecology.

Photosynthesis removes CO₂ and sends organic matter to the deep, as in the biological pump, while making calcium carbonate releases CO₂ in the surface water, so the net effect on climate is complex. When cells die, coccoliths sink and are buried above the carbonate compensation depth, a long-term store of carbon in the carbon cycle. Some species release dimethyl sulfide, a gas that helps to seed clouds.

The White Cliffs of Dover are made of chalk built from trillions of coccoliths in the Cretaceous sea. Today the effect of ocean acidification on their plates is under study, with mixed results between strains. Together with foraminifera and diatoms, they form a large share of the microscopic life of the sea, and they are included among the algae.