The potential role of internal pH manipulation by active proton pumping in foraminifera during biomineralization

Abstract

Calcifying foraminifera are one of the major marine calcifiers in open oceans and they are affected by ocean acidification. However, their response to acidified seawater is not according to inorganic precipitation but a biological regulation takes place during biomineralization. Two widely accepted models exist to explain the biological control during calcification, both supporting internal pH regulation. To predict how these calcifying organisms will be affected by increased pCO2 levels and therefore, a lowered seawater pH, culture experiments - investigating calcification rates are necessary. Here we present results from a culture experiment investigating the response of two benthic symbiont-bearing calcifying foraminifera under a range of four pCO2 concentrations (400, 700, 1000 and 2200 ppm) projected for the future. The greatest change in total alkalinity caused by calcification was observed at a pCO2 concentration of 700 ppm, while it was significantly lower at 1000 and 2200 ppm, indicating less successful biomineralization under very high pCO2 concentrations. Calcification rates suggest species specific responses to ocean acidification with H. depressa performing worse under the 2200 ppm treatment than A. lessonii. The different responses of the foraminifera between varying pCO2 treatments and between the two different species suggest that at least some foraminifera will be able to cope with increasing atmospheric CO2 concentrations.