Decrease in volume and density of foraminiferal shells with progressing ocean acidification (original) (raw)

Rapid increases in anthropogenic atmospheric CO 2 partial pressure have led to 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 by ocean acidification affects either foraminiferal shell volume or density, or both, has yet to be investigated. In this study, we cultured asexually reproducing specimens of Amphisorus kudakajimensis, a dinoflagellate endosymbiont-bearing large benthic foraminifera (LBF), under different pH conditions (pH 7.7-8.3, NBS scale). The results suggest 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. 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., Ca 2+ -ATPase and Ω) and (2) pH mainly affects calcification and temperature mainly affects photosynthesis. Our findings also suggest that, under the IPCC RCP8.5 scenario, 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. Following the Industrial Revolution, anthropogenic atmospheric CO 2 partial pressure (pCO 2 ) increased rapidly, and the pH of seawater decreased. By 2100, atmospheric CO 2 is predicted to increase to 420-1250 ppm 1 , with seawater pH decreasing to pH total 7.6 (pCO 2 = 1250 ppm; pH NBS ~ 7.7) in the tropical Pacific 2 . Total dissolved CO 2 concentration increases with pCO 2 , carbonate ions (CO 3 2-) decrease with decreasing pH, and the saturation state of calcium carbonate decreases. Calcifying organisms generally exhibit negative effects on survival, calcification, growth, and reproduction in response to ocean acidification 3 . Modern surface seawater is saturated with respect to calcium carbonate, including calcite, high-magnesian (high-Mg) calcite, and aragonite. However, it is expected that the seawater of the entire Southern Ocean south of 60° S and a part of the subarctic Pacific will become unsaturated with aragonite by 2100 (pCO 2 = 563 or 788 ppm 4 ). Foraminifera are one of the major carbonate producers in the ocean, accounting for 23-56% of the total CaCO 3 in the open ocean 5 . Foraminifera, both planktic and benthic, generally respond negatively to ocean acidification . The reduction in the calcification rate in foraminifera has generally been estimated from shell weight and size, using the parameter of size-normalized shell weight 6 . Although several studies on shell density have been reported , it remains unclear whether the change in shell weight in foraminifera of the same size reflects shell volume, shell density, or both, because it is difficult to measure their small shell volumes. The recently established micro X-ray computed tomography (microCT) technique enables direct and precise measurement of foraminiferal shell volume (i.e., density) with a resolution of less than 1 µm . Therefore, microCT measurements can provide more precise information about foraminifera shell dimensions.