The Influence of Seawater pH on Respiration, Calcification, and Mg/Ca in the Cultured Foraminifer Globigerina bulloides

Planktic foraminifera are excellent recorders of water column chemistry and play a key role in planktic ecosystems and open ocean inorganic carbon flux. Perturbations to the oceanic inorganic carbon system, due to global or regional decreases in seawater pH, have the potential to profoundly affect foraminifera and their contribution to the biological carbon pump. Previous culture and field studies have documented a decrease in calcification in planktic foraminifera when exposed to low pH and low concentrations of carbonate ion. Incorporating physiological as well as trace metal analyses in the study of these planktic organisms will improve (a) our understanding of how the role of foraminifera in planktic ecosystems will change in the future as well as (b) our interpretation of shell geochemical proxies to reconstruct paleoenvironment conditions. We utilize a novel combination of metrics, including respirometry and fluorescent labeling of calcite, to better understand the impact of pH on foraminiferal metabolism and the relationship between oxygen consumption and calcification. The planktic foraminifer, Globigerina bulloides, was cultured across a range of treatments from 7.5 < pH > 8.2. Incorporation of fluorescent calcein dye into shells along with measurements of oxygen consumption showed that both oxygen consumption and calcification responded to changes in environmental pH. Shells with new chambers grown in culture were additionally analyzed for trace element content by Laser-Ablation Inductively Coupled Plasma Mass Spectrometry. Both calcification and the incidence of spine repair decreased under lowered pH. By contrast, the response of oxygen consumption to decreasing pH was parabolic, declining at pH levels above and below pH ~ 8. Our results suggest that both performance and calcification are likely to be impacted by future changes in global and regional pH, and past variability in pH may influence how individual foraminifera shells can be interpreted as proxies.