Laboratory culture experiments were conducted to determine effects of seawater carbonate ion concentration ([CO32-]), and thereby calcite saturation state (Ω), on Mg and Sr incorporation into calcite of two species of shallow-water benthic foraminifera: Ammonia tepida and Heterostegina depressa. Impact on Mg and Sr incorporation by increased seawater [CO32-] and thereby higher Ω is absent in either species. Comparison to results from a similar culturing experiment, in which Ω was varied as a function of [Ca2+], reveals that saturation state affects incorporation of Mg and Sr through calcium-rather than carbonate availability. The similarity in response by both species is surprising since the average Mg/Ca ratio is ~70 times higher in H. depressa than in A. tepida. Furthermore, these results suggest that the ions involved in biomineralization (i.e. Ca2+ and DIC) are processed by separate cellular transport mechanisms. The similar response of Mg and Sr incorporation in this study suggests that only differences in the Ca2+ transport mechanism affect divalent cation partitioning. © 2011 Elsevier B.V.