Resilience of cold-water scleractinian corals to ocean acidification: Boron isotopic systematics of pH and saturation state up-regulation

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: McCulloch, Malcolm, Trotter, Julie, Montagna, Paolo, Falter, Jim, Dunbar, Robert, Freiwald, André, Försterra, Günter, López Correa, Matthias, Maier, Cornelia, Rüggeberg, Andres, Taviani, Marco
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2012
• Subjects: Lophelia pertusa; Ocean acidification
• Online Access: https://oceanrep.geomar.de/id/eprint/20104/; https://oceanrep.geomar.de/id/eprint/20104/1/McCulloch.pdf; https://doi.org/10.1016/j.gca.2012.03.027

The boron isotope systematics has been determined for azooxanthellate scleractinian corals from a wide range of both deep-sea and shallow-water environments. The aragonitic coral species, Caryophyllia smithii, Desmophyllum dianthus, Enallopsammia rostrata, Lophelia pertusa, and Madrepora oculata, are all found to have relatively high δ11B compositions ranging from 23.2‰ to 28.7‰. These values lie substantially above the pH-dependent inorganic seawater borate equilibrium curve, indicative of strong up-regulation of pH of the internal calcifying fluid (pHcf), being elevated by ∼0.6–0.8 units (ΔpH) relative to ambient seawater. In contrast, the deep-sea calcitic coral Corallium sp. has a significantly lower δ11B composition of 15.5‰, with a corresponding lower ΔpH value of ∼0.3 units, reflecting the importance of mineralogical control on biological pH up-regulation. The solitary coral D. dianthus was sampled over a wide range of seawater pHT and shows an approximate linear correlation with ΔpHDesmo = 6.43 − 0.71pHT (r2 = 0.79). An improved correlation is however found with the closely related parameter of seawater aragonite saturation state, where ΔpHDesmo = 1.09 − 0.14Ωarag (r2 = 0.95), indicating the important control that carbonate saturation state has on calcification. The ability to up-regulate internal pHcf, and consequently Ωcf, of the calcifying fluid is therefore a process present in both azooxanthellate and zooxanthellate aragonitic corals, and is attributed to the action of Ca2+-ATPase in modulating the proton gradient between seawater and the site of calcification. These findings also show that the boron isotopic compositions (δ11Bcarb) of aragonitic corals are highly systematic and consistent with direct uptake of the borate species within the biologically controlled extracellular calcifying medium. We also show that the relatively strong up-regulation of pH and consequent elevation of the internal carbonate saturation state (Ωcf ∼8.5 to ∼13) at the site of calcification by cold-water corals, ...