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The concept that the formation of CaCO3 coccoliths functions as a photosynthetic adaptation for the use of bicarbonate is evaluated in the coccolithophorids Coccolithus huxleyi and Cricosphaera carterae by two new methods. In the first, carbon fixation is measured at 10‐s intervals in the first 2 min after addition of 14CO2 and H14CO3− to buffered cultures; this method exploits the relatively long half‐time for the hydration or dehydration of dissolved CO2. In the second, shifts in pH and alkalinity resulting from carbon fixation by cells growing in liquid culture are assessed to indicate fluxes of CO2 and HCO3− into cells and these values compared to measurements of 14C incorporation in photosynthesis and carbonate deposition. The data are interpreted in terms of one of several net inorganic reactions of deposition considered. In this reaction, CO2 is the substrate of photosynthesis and HCO3− is the form of carbon supplied to the calcification site. CO2 resulting from carbonate deposition supplements the CO2 from the medium that diffuses into cells as a source of carbon for photosynthesis.

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