Cold-Water Corals Trumping Ocean Acidification: How Is It Done?

The authors of an intriguing research report (McCullock et al., 2012), which was recently published in Geochimica et Cosmochimica Acta, write that "for cold-water corals, which are already living at low levels of carbonate saturation, the shoaling of the saturation horizon as carbonate saturation states decrease [in response, for example, to rising atmospheric CO₂ concentrations] has the potential to cause dramatic declines in rates of calcification, or the dissolution of the carbonate skeletons of those living at or close to the saturation horizon." But since these corals are indeed living there, they speculate that "they may have evolved adaptive strategies to counter the effects of low carbonate saturation states," one of which is to up-regulate their internal pH to a value that allows calcification to occur.

To further explore this suspected phenomenon, McCullock et al. extended the novel approach taken by Trotter et al. (2011), based on boron isotopic systematics, to determine the relationship between seawater pH and the internal (extracellular) pH_cf at the site of calcification for several azooxanthellate cold-water scleractinian corals, which were collected from a large range of depths and geographically disparate sites, including southeast Australia, Chile's Comau Fjord, the Marmara Sea, a number of sites in the Mediterranean Sea, the northeast Atlantic Ocean and the northwestern Hawaiian Islands.

This suite of "aragonitic cold-water coral species," as the eleven researchers describe them, "collectively show an overall trend of higher ΔpH [= pH_cf - seawater pH] values that is anti-correlated with seawater pH, with systematics generally consistent with biologically controlled pH up-regulation." And this result indicates that, "like symbiont-bearing tropical corals (Trotter et al., 2011), they have the ability to ameliorate or buffer external changes in seawater pH by up-regulating their pH_cf at the site of calcification."

In light of these several observations, McCullock et al. conclude that "cold-water corals are likely to be much more resilient to decreasing seawater pH from ocean acidification than previously realized," because, as they see it, "decreasing seawater pH alone will only marginally affect calcification rates since this process would be largely countered by pH_cf up-regulation in cold-water corals, together with enhanced calcification rates from warming of the deep oceans."

Once again, therefore, life appears to be well prepared for another environmental contingency.

Additional Reference
Trotter, J.A., Mpontagna, P., McCulloch, M.T., Silenzi, S., Reynaud, S., Mortimer, G., Martin, S., Ferrier-Pages, C., Gattuso, J.-P. and Rodolfo-Metalpa, R. 2011. Quantifying the pH 'vital effect' in the temperate zooxanthellate coral Cladocora caespitosa: validation of the boron seawater pH proxy. Earth and Planetary Science Letters 303: 163-173.