Calcification and inorganic carbon acquisition in coccolithophores

A number of species of coccolithophorid phytoplankton precipitate calcite inside intracellular vesicles (coccolith vesicles). They can form vast blooms under certain conditions, and account for major fluxes of inorganic carbon (Ci) to the ocean floor. The functions of calcification have been debated for many years, and a role in carbon acquisition has been proposed by several workers. The precipitation of calcite from HCO3^- involves the production of protons that can potentially be used to facilitate the use of external HCO3^- as a photosynthetic substrate. For this function to be feasible, certain criteria must be met. HCO3^- (rather than CO3^2-) should be the external substrate for calcification, photosynthesis should be facilitated by HCO3^- in calcifying cells when CO2 availability is limiting, and the transport of Ci and Ca²⁺ to the site of calcification should be energetically and kinetically feasible. Considerable evidence exists for HCO3^- as the substrate for calcification in coccolithophores. However, evidence for a direct role for calcification in supply of Ci for photosynthesis is less clear. The environmental factors that regulate calcification are still uncertain but appear to be related as much to the availability of nutrients as CO2. Transport of Ci to the intracellular site of calcification and removal of H⁺ from the coccolith vesicle appear to present few energetic or kinetic constraints. However, the large sustained transcellular fluxes of Ca²⁺ required for calcification probably occur via a pathway that does not involve diffusion across the cytoplasm.