Paper Reviewed
Barkley, H.C., Cohen, A.L., McCorkle, D.C. and Golbuu, Y. 2017. Mechanisms and thresholds for pH tolerance in Palau corals. Journal of Experimental and Marine Biology and Ecology 489: 7-14.

In light of the many negative predictions of coral reef responses to ocean acidification (OA), Barkley et al. (2017) introduce their work by saying that "finding coral reef communities that may be resilient to ocean acidification and elucidating mechanisms for acidification tolerance are increasingly important conservation priorities."

One such region where incredible natural resilience to OA is found is that of the coral reefs surrounding Palau's Rock Islands in the western Pacific Ocean. There, according to Barkley et al., "calcification rates of two reef-building coral genera (Porites and Favia) do not change across Palau's steep Ωar gradient (Ωar = 3.7 to 2.3)," corresponding to a pH range of 7.84 to 8.04. In an effort to discern whether or not the apparent resilience of Palau's corals to OA results from local adaptation to reduced seawater pH or other environmental factors that allow these corals to thrive despite extreme pH conditions, the team of four scientists conducted two analyses: (1) a laboratory CO2 manipulation experiment and (2) a field-based reciprocal transplant study.

For the laboratory experiment, Barkley et al. collected Porites samples from two reefs (one high-Ωar barrier reef and one low-Ωar Rock Island reef) and grew them at three pH/Ωar levels (7.98/3.0, 7.83/2.3 and 7.60/1.5) in a controlled environment for eight weeks, varying only the seawater pH/Ωar levels. In the field-based study, they examined the response of Porites corals to pH/Ωar in the context of other environmental variability that exists in the real world of nature by transplanting corals in the field between the two reefs over a period of 17 months.

Results of the laboratory experiment revealed, in the words of Barkley et al., that "regardless of reef of origin, corals in the CO2 manipulation experiment showed no calcification sensitivity to Ωar after eight weeks in experimental conditions." Or, stated more plainly, there was no difference in calcification rates in the corals despite reef origin, with corals maintaining calcification rates over a pH/Ωar range of 7.98/3.0 to 7.60/1.5. Commenting on this finding, the scientists say it is "suggestive of adaptation" and indicates "that corals in Palau are not living close to their Ωar threshold and are tolerant to acidification levels far below those to which they are currently exposed."

With respect to the transplant study, Barkley et al. report that "corals transplanted between reefs at different ambient Ωar levels showed significant declines in calcification rates and high mortality, while corals returned back to their reef of origin were alive after 17 months in the field." Given as much, they conclude that "interpreted within the framework of the experimental result, the failure of pH/Ωar-tolerant corals to successfully transplant between different reef sites hints at local adaptation to other (non-pH) environmental factors such as light, temperature, and/or flow that co-vary with Ωar across Palau's natural acidification gradient." And that is great news for those who are concerned about the potential negative effects of OA.