Paper Reviewed
Georgiou, L., Falter, J., Trotter, J., Kline, D.I., Holcomb, M., Dove, S.G., Hoegh-Guldberg, O. and McCulloch, M. 2015. pH homeostasis during coral calcification in a free ocean CO2 enrichment (FOCE) experiment, Heron Island reef flat, Great Barrier Reef. Proceedings of the National Academy of Science USA 112: 13,219-13,224.

The free-ocean carbon enrichment (FOCE) system of this study was, in the words of Georgiou et al. (2015), "constructed on the Heron Island reef flat with four submerged flumes (two controls and two treatments) oriented parallel to the shore, each flume open to the environment at both ends and on the bottom," into which colonies of P. cylindrica coral collected from the Heron Island reef flat were first acclimatized for four weeks, after which the pH values of the FOCE treatment flumes were progressively offset relative to the ambient reef flat water by -0.05 pH unit in July, by -0.15 in August, and by -0.25 in September, during which time "both the controls and treatments were subject to strong natural diel and seasonal forcing independent of the FOCE experiment," which thus allowed them to "simultaneously examine the response of each coral's internal chemistry to both high levels of natural variability in ambient pH together with systemic shifts in pH expected to occur over this century."

As for what they learned from this unique experiment, the eight Australian researchers report that (1,2) "analyses of skeletal growth showed no systematic differences between low-pH FOCE treatments and present-day controls for calcification rates or the pH of the calcifying fluid," that (3) "individual nubbins exhibited near constant δ¹¹B [boron] compositions along their primary apical growth axes regardless of the season or treatment," and that (4) "under the highly dynamic conditions of the Heron Island reef flat, P. cylindrica up-regulated the pH of its calcifying fluid [cf], with each nubbin having near-constant pHcf values independent of the large natural seasonal fluctuations of the reef flat waters or the superimposed FOCE treatments."

In light of these extremely positive findings, Georgiou et al. collectively concluded that "this newly discovered phenomenon of pH homeostasis during calcification indicates that coral living in highly dynamic environments exert strong physiological controls on the carbonate chemistry of their calcifying fluid, implying a high degree of resilience to ocean acidification within the investigated ranges."