Reef-building corals are among the most sensitive organisms to changes in ocean chemistry, making them both susceptible to the negative consequences of ocean acidification and powerful proxies for past ocean conditions. However, most coral records of seawater pH are derived from living colonies, and as such they do not exceed the past ~400 years. Therefore, not much is known about the carbonate chemistry of ancient corals or whether there are differences in how modern and fossil corals calcify, leaving the longer-term effects of ocean acidification on coral growth unknown.
Xuefei Chen at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China, and colleagues, measured boron (δ11B and B/Ca) geochemical proxies for pH and dissolved inorganic carbon (DIC) in both modern and fossil Porites spp. collected from east Hainan Island, northern South China Sea. As corals form their skeletons from an internal calcifying fluid (CF), δ11B- and B/Ca-derived measurements are interpreted as the pH and carbonate chemistry of the CF (pHCF and DICCF), unless translated to seawater values. Stark differences in boron isotope measurements between modern and fossil colonies were observed (a mean difference of ~1.67‰ for δ11B), exceeding the intercolony difference. The results purport that ancient corals maintained a higher pH (+ 0.12 units) at their site of calcification relative to modern corals, but exhibited almost equivalent DIC at the site of calcification, meaning that over time, the ability of corals to up-regulate their CF pH has decreased. Therefore, while modern corals are able to concentrate carbon for calcification similarly to ancient corals, ocean acidification has decreased the ability of modern corals to up-regulate their pHCF, potentially indicating a loss of resilience to ocean acidification on coral growth.
Although these results come from only one location and one coral genus, they highlight the high level of control coral can have over their calcification while also underscoring the impact of ocean acidification and warming on coral growth. Future research should test more species and locations to determine whether the impacts of ocean acidification are consistent or specific to this coral species.
Ulrich R., in press. Corals losing control. Nature Reviews Earth & Environment. Article (subscription required).
