In this work, the effect of human-induced ocean acidification on the coccolithophore Emiliania huxleyi was examined. To this end, cells of the highly calcifying strain RCC 1216 were acclimated to present-day and expected future CO2 partial pressures (pCO2; 380 and 950 µatm). The ecophysiological responses were assessed by measuring growth rates as well as cellular quotas and production rates of organic and inorganic carbon. Applying the 14C disequilibrium technique, the photosynthetic CO2 and HCO3- uptake behavior of the differently treated cells was determined. This technique is usually carried out at the relatively high pH of 8.5, often being very different from acclimation pH. To assess possible effects of the assay pH on the carbon uptake behavior, the method was modified and applied over a range of ecologically relevant pH values (pH 7.9, 8.1, 8.3, 8.5 and 8.7). The reliability of the extended method was judged by performing model runs. Elevated pCO2 highly stimulated organic carbon production, but reduced the calcification of E. huxleyi. Growth rates, however, remained constant. While these results are in line with previous findings, the modification of the 14C disequilibrium method provided new insights into modes of carbon acquisition. It was shown that the carbon uptake behavior of E. huxleyi strongly depends on the assay pH. At low pH, cells preferentially used CO2 for carbon uptake (pH 7.9 > 90% CO2 usage), whereas at high pH primarily HCO3 - was taken up (pH 8.7 ~25% CO2 usage). In contrast, acclimation had no effect on the carbon uptake behavior. Hence, E. huxleyi mainly uses CO2 as carbon source at typical pH values of the ocean, which contradicts previous assumption of predominant HCO3- usage. The sensitivity study indicated that the modified 14C disequilibrium technique provides reliable results. In conclusion, the higher biomass production at elevated pCO2 could, in fact, be explained by the higher relative CO2 usage determined under low pH. With this extension of the 14C disequilibrium technique, new aspects about the mode of carbon acquisition were revealed, which should be considered for future process studies on ocean acidification effects.