Various protective and metabolic functions for coccolithophore calcification have been proposed such as providing a means to supply CO 2 for photosynthesis. It has also been speculated that calcification helps to dissipate excess energy under high irradiance, thereby circumventing photoinhibition. To address these questions, cells of a calcifying strain of Emiliania huxleyi were grown at three irradiances (30, 300, and 800 μmol photons m-2 s-1) in combination with four calcium (Ca) concentrations (0.1, 1, 2.5, and 10 mmol L-1) leading to different degrees of calcification in the same strain. Growth rates (μ), particulate organic carbon (POC), and inorganic carbon (PIC) production as well as carbon isotope fractionation (εp) were determined. Photosynthetic O2 evolution and CO2 and HCO3- uptake rates were measured by membrane inlet mass spectrometry (MIMS). The application of this multimethod approach provides new information on the role of calcification in E. huxleyi. Noncalcifying cells showed POC production rates as high as calcifying ones. No differences in εp were observed under different Ca concentrations. MIMS measurements indicate that noncalcifying cells can photosynthesize as efficiently as, or even more efficiently than, calcifying ones and that both use HCO3- as the main carbon source. The ratio of photosynthetic HCO3- uptake relative to net fixation did not differ among cells acclimated to 10 mmol L-1 or to 0.1 mmol L-1 Ca. These results indicate that (1) calcification is not involved in photosynthetic carbon acquisition, and (2) calcification does not provide a means of energy dissipation under high irradiances. © 2007, by the American Society of Limnology and Oceanography, Inc.