Photosynthesis and coccolith formation: Inorganic carbon sources and net inorganic reaction of deposition1

Abstract

The concept that the formation of CaCO 3 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 14 CO 2 and H 14 CO 3 − to buffered cultures; this method exploits the relatively long half‐time for the hydration or dehydration of dissolved CO 2 . In the second, shifts in pH and alkalinity resulting from carbon fixation by cells growing in liquid culture are assessed to indicate fluxes of CO 2 and HCO 3 − into cells and these values compared to measurements of 14 C 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, CO 2 is the substrate of photosynthesis and HCO 3 − is the form of carbon supplied to the calcification site. CO 2 resulting from carbonate deposition supplements the CO 2 from the medium that diffuses into cells as a source of carbon for photosynthesis.