COSTS AND BENEFITS OF CALCIFICATION IN COCCOLITHOPHORIDS

Calcification in coccolithophorids requires major intracellular fluxes of inorganic carbon and calcium. This paper summarises the major cell ular fluxes of substrates and products of calcification described in a simple four compartment model (cytosol, Golgi, coccolith vesicle and chloroplast). Measurements of the cytosolic and intra-coccolith vesicl e pH and electrical potentials across the plasma membrane and coccolit h vesicle membrane allow calculations of the proton electrochemical gr adients across these membranes and estimates of the free carbonate and calcium concentrations in the coccolith vesicle. Calcification may pr ovide a relatively low cost route for elevating the concentration of c arbon dioxide in the chloroplast. This may have benefits in terms of t he nutrient requirements for photosynthesis and growth, In particular, a close relationship appears to exist between calcification and the a vailability of phosphorus which may correlate with the occurrence of l arge scale blooms of Emiliania huxleyi in the North Atlantic.