The effect of carbonic anhydrase on foraminiferal Mg/Ca

Marine biogenic calcium carbonate production plays a role in the exchange of CO₂ between ocean and atmosphere. The effect of increased CO₂ on calcification and on the resulting chemistry of shells and skeletons, however, is only partly understood. Foraminifera are among the main marine CaCO₃ producers and the controls on element partitioning and isotope fractionation is the subject of many recent investigations. The enzyme carbonic anhydrase (CA) was, for example, shown to be vital for CaCO₃ deposition in benthic foraminifera and indicates their ability to manipulate their intracellular inorganic carbon chemistry. Here, we tested whether CA affects the partitioning of Na, Mg and Sr in the perforate, large benthic, symbiont-bearing foraminifer Amphistegina lessonii by addition of the inhibitor acetazolamide (AZ). The effect of dissolved CO₂ on the effect of CA on element partitioning was also determined using a culturing setup with controlled atmospheric carbon dioxide levels (400-1,600 ppm). Results show that inhibition by AZ reduces calcification greatly and that CO₂ has a small, but positive effect on the amount of calcite formed during the incubations. Furthermore, the inhibition of CA activity has a positive effect on element partitioning, most notably Mg. This may be explained by a (n indirect) coupling of inorganic carbon uptake and inward calcium ion pumping.