Few things would be better than getting rid of CO2 while producing useful materials. Coccolithophores use CO2 in their photorespiration, in addition to using CO2 to produce coccoliths – small platelets made of calcite. Ca is a central cation in this process, and the goal of this project is to investigate to what extent other divalent cations can partially substitute for Ca and become part of the growing coccolith. The long term goal is to enable algae to harvest cations and produce materials for us, such as cathode materials for batteries. The experiments was performed by controlling the seawater composition with respect to cations like Ca2+, Mg2+, Fe2+ and Mn2+ for growth of two coccolithophore algae. The resulting biomineralized materials, the coccoliths, were analyzed by SEM and XRD. It appears that coccolithophores are capable of substituting Ca with Mg, up to a certain, low, level, but we have not found significant signs of inclusion of Mn or Fe. Uptake of Mg has been reported in other calcifying species. Nevertheless, the results from this thesis will need further studies to conclude.