Progressive ocean acidification will reduce foraminiferal shell volume and density: results from laboratory experiments using microCT analysis

The rapid increase in anthropogenic atmospheric CO₂ partial pressure have resulted in a decrease in the pH of seawater. Calcifying organisms generally respond negatively to ocean acidification. Foraminifera are one of the major carbonate producers in the ocean, however, whether calcification reduction due toocean acidification affects either foraminiferal shell volume or density, or both, has yet to be investigated. In the study, asexually reproducing specimens of Amphisorus kudakajimensis, a dinoflagellate endosymbiont-bearing large benthic foraminifera (LBF), were cultured under different pH conditions (pH 7.7–8.3, NBS scale) and determined their shell volume and density using micro X-ray computed tomography (microCT). The results showed that both the volume and density of the foraminiferal shells decreased with lowering pH. This suggests that changes in seawater pH would affect not only the quantity (i.e., shell volume) but also the quality (i.e., shell density) of foraminiferal calcification. Previous LBFs culturing study have reported that temperature affects only the shell volume, not the density. We proposed that pH and temperature affect these growth parameters differently because 1) they have differences in the contribution to the calcification process (e.g., Ca²⁺-ATPase and Ω) and 2) pH mainly affects calcification and temperature mainly affects photosynthesis. It implies that shell volume and density of LBFs can be considered reliable proxies for pH and temperature. Our findings also suggest that, under the IPCC RCP8.5 scenario, (pCO₂ = 1250 ppm and global mean temperature +3.7°C in 2100), both ocean acidification and warming will have a significant impact on reef foraminiferal carbonate production by the end of this century, even in the tropics.