Decoupling between the response of coral calcifying fluid pH and calcification to ocean acidification

Evaluating the factors responsible for differing species-specific sensitivities to declining seawater pH is central to understanding the mechanisms via which ocean acidification (OA) affects coral calcification. We report here the results of an experiment comparing the responses of the coral Acropora yongei and Pocillopora damicornis to differing pH levels (8.09, 7.81, and 7.63) over an 8-week period. Calcification of A. youngei was reduced by 35% at pH 7.63, while calcification of P. damicornis was unaffected. The pH in the calcifying fluid (pH_cf) was determined using δ¹¹B systematics, and for both species pH_cf declined slightly with seawater pH, with the decrease being more pronounced in P. damicornis. The dissolved inorganic carbon concentration at the site of calcification (DIC_cf) was estimated using geochemical proxies (B/Ca and δ¹¹B) and found to be double that of seawater DIC, and increased in both species as seawater pH decreased. As a consequence, the decline of the saturation state at the site of calcification (Ω_cf) with OA was partially moderated by the DIC_cf increase. These results highlight that while pH_cf, DIC_cf and Ω_cf are important in the mineralization process, some corals are able to maintain their calcification rates despite shifts in their calcifying fluid carbonate chemistry.