Culturing foraminifera under controlled pCO₂ conditions shows that incorporation of certain elements (Zn, Ba) into foraminiferal shells is impacted by the inorganic carbonate system. Modeling the chemical speciation of these elements suggests that incorporation is determined by the availability of free ions. Furthermore, analyzing and comparing trends in element incorporation in hyaline and porcelaneous species may provide constrains on the differences between their calcification strategies.