Impact of dissolved CO(2) on calcification in two large, benthic foraminiferal species

Rising atmospheric CO(2) shifts the marine inorganic carbonate system and decreases seawater pH, a process often abbreviated to ‘ocean acidification’. Since acidification decreases the saturation state for crystalline calcium carbonate (e.g., calcite and aragonite), rising dissolved CO(2) levels will either increase the energy demand for calcification or reduce the total amount of CaCO(3) precipitated. Here we report growth of two large benthic photosymbiont-bearing foraminifera, Heterostegina depressa and Amphistegina lessonii, cultured at four different ocean acidification scenarios (400, 700, 1000 and 2200 ppm atmospheric pCO(2)). Using the alkalinity anomaly technique, we calculated the amount of calcium carbonate precipitated during the incubation and found that both species produced the most carbonate at intermediate CO(2) levels. The chamber addition rates for each of the conditions were also determined and matched the changes in alkalinity. These results were complemented by micro-CT scanning of selected specimens to visualize the effect of CO(2) on growth. The increased chamber addition rates at elevated CO(2) concentrations suggest that both foraminifera species can take advantage of the increased availability of the inorganic carbon, despite a lower saturation state. This adds to the growing number of reports showing the variable response of foraminifera to elevated CO(2) concentrations, which is likely a consequence of differences in calcification mechanisms.