Background
The authors write that "physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO2 for calcification, not seawater carbonate," but that "a number of investigators are attributing observed negative effects of experimental seawater acidification by CO2 or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state." Thus, they state there is "a discrepancy between the physiological and geochemical views of coral biomineralization." In addition, they report that "not all calcifying organisms respond negatively to decreased pH or saturation state," and they say that "together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification."

What was done
In an effort to shed more light on this conundrum, Jury et al. performed incubations with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries, where, as they describe it, "carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries."

What was learned
The three scientists report that among-treatment differences "were highly significant," and that "the corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH." They found, for example, that "corals calcified at normal or elevated rates under low pH (7.6-7.8) when the sea water bicarbonate concentrations were above 1800 µM," and, conversely, that "corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered."

What it means
Jury et al. conclude that "coral responses to ocean acidification are more diverse than currently thought," and they question "the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification," stating that "if we truly wish to decipher the response of coral calcification to ocean acidification, a firmer grasp of the biological component of biomineralization is paramount." Hopefully, their contribution to the subject will mark the beginning of the scientific journey that will ultimately lead to the better understanding that all of us are seeking.