Description: The coccolithophore Emiliania huxleyi is one of the major producers of biogenic calcite in the ocean. lt has been studied extensively with regard to biogeochemical processes and the global carbon cycle. However, the process of calcification in this microalga is still not completely understood. In this Master's thesis, the limiting parameters of the marine carbonate system for calcification in E. huxleyi were investigated. In two different approaches, cultures of E. huxleyi were grown under carbonate chemistry conditions that suppressed calcification (low DIC and low pH) resulting in non-calcifying cells. Subsequently, the carbonate chemistry conditions were altered either by external manipulation or by the physiological processes of the cells themselves, eventually reaching favorable conditions for calcification. The carbonate chemistry conditions and the degree of calcification were examined. The results of this thesis confirm bicarbonate (HC03-) as main substrate for calcification in E. huxleyi, but they also suggest a supportive role for carbon dioxide (C02). Further, calcification is not regulated by the substrates or any other carbonate chemistry parameter alone, but it depends on an interaction between HC03- and pHf. Protons were found to have an inhibiting effect on the calcification process which can be overcome with a certain concentration of HC03-. This threshold substrate concentration increases with an increasing concentration of protons, i.e. decreasing pHf. Furthermore, f. huxleyi does not lose the ability to calcify when the cells are cultured noncalcifying for many generations. Recalcification will start instantaneously as soon as favorable carbonate chemistry conditions are reached. These findings imply that carbonate chemistry parameters should not be considered individually when assessing their effects on biological and biogeochemical processes, because complex interactions may occur.