Coccolithophores (also called coccolithophorids) are single-celled algae, protists and phytoplankton belonging to the division haptophytes. They are distinguished by special calcium carbonate plates (or scales) of uncertain function called coccoliths (calcareous nanoplankton), which are important microfossils. Coccolithophores are almost exclusively marine and are found in large numbers throughout the surface euphotic zone of the ocean. An example of a globally-significant coccolithophore is Emiliania huxleyi.
The arrangement of coccolithophores are as spherical cells about 15-100 micrometres across, enclosed by calcareous plates, coccoliths, about 2-25 micrometres across. Coccolithophores that are alive today have two brown pigment chloroplasts in their cell with the nucleus located between them. The pigments are home to the chlorophyll which carry out photosynthesis. In order to obtain the sunlight needed for photosynthesis, coccolithophores live near the surface of the ocean. Although they have the capability to swim, coccolithophores dominant mode of transport is drifting with ocean current and circulation patterns. However, they may maneuver individually to remain in the euphotic zone during times of extremely favorable conditions.
Due to their microscopic size and the broad distribution of many of their taxa, coccoliths have become very important as index fossils for solving various stratigraphic problems. Microfossils are sensitive indicators of changes in the temperature and salinity of the ocean and sea surface water. The quantitative analysis of calcareous nannoplankton assemblages is being employed to reveal such changes. They also produce alkenones, biomarkers of great utility in reconstructing ancient temperatures.
Coccolithophores have long been thought to respond to increased ocean acidity, caused by increasing CO2 levels, by becoming less calcified. Scientists were recently surprised to learn that in fact the opposite can happen in at least some circumstances, with the model species E. huxleyi becoming 40% heavier and more abundant in waters of higher CO2 concentration.
For more information (background, pictures, experiments and references): Coccolithophore
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