| Summary: | 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 p CO 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.
|
|---|