How does rising atmospheric CO2 affect marine organisms?

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Coral Resilience to Ocean Acidification and Global Warming
McCulloch, M., Falter, J., Trotter, J. and Montagna, P. 2012. Coral resilience to ocean acidification and global warming through pH up-regulation. Nature Climate Change 2: 623-627.

The authors write that "rapidly rising levels of atmospheric CO2 are not only causing ocean warming, but also lowering seawater pH [and] the carbonate saturation state of the oceans, on which many marine organisms depend to calcify their skeletons." And thus it is that these two phenomena have become the twin evils of the radical environmentalist crowd. But are they really as bad as they have been made out to be?

What was done
"Using boron isotope systematics (Trotter et al., 2011)," as they describe it, McCulloch et al. "show how scleractinian corals up-regulate pH at their site of calcification such that internal changes are approximately one-half of those in ambient seawater." And they explain how "this species-dependent pH-buffering capacity enables aragonitic corals to raise the saturation state of their calcifying medium, thereby increasing calcification rates at little additional energy cost."

What was learned
Based on their approach of combining internal pH regulation of the calcifying fluid (cf) with abiotic calcification, the four researchers' new IpHRAC model shows that "the enhanced kinetics of calcification owing to higher temperatures has the potential to counter the effects of ocean acidification." In fact, they report that their studies indicate an "enhanced pHcf by ~0.6-1.2 (and possibly up to 2) pH units above sea water during the daytime, when both net production and calcification are highest."

What it means
McCulloch et al. make a good case for their conclusion that their IpHRAC model indicates that "ocean acidification combined with rising ocean temperatures should have only minimal effects on coral calcification," which they say is "a direct outcome of their ability to up-regulate pH at the site of calcification." In fact, they say that the enhanced kinetics from increasing sea surface temperatures more than compensates for reduced calcification from declining seawater saturation state, "consistent with recent findings for long-lived Porites corals from Western Australia," citing Cooper et al. (2012).

Cooper, T.F., O'Leary, R. and Lough, J.M. 2012. Growth of Western Australian corals in the anthropocene. Science 335: 593-596.

Trotter, J.A., Montagna, P., McCulloch, M.T., Silenzi, S., Reynaud, S., Mortimer, G.E., Martin, S., Ferrier-Pagès, 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.

Reviewed 30 January 2013