PH homeostasis during coral calcification in a free ocean CO2 enrichment (FOCE) experiment, Heron Island reef flat, Great Barrier Reef

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Georgiou, Lucy, Falter, James, Trotter, Julie, Kline, David I., Holcomb, Michael, Dove, Sophie G., Hoegh-Guldberg, Ove, McCulloch, Malcolm
• Format: Article in Journal/Newspaper
• Language: English
• Published: National Academy of Sciences 2015
• Subjects: Coral resilience; FOCE; Heron Island; Ocean acidification; PH up-regulation; 1000 General
• Online Access: https://espace.library.uq.edu.au/view/UQ:373501

Geochemical analyses (delta B-11 and Sr/Ca) are reported for the coral Porites cylindrica grown within a free ocean carbon enrichment (FOCE) experiment, conducted on the Heron Island reef flat (Great Barrier Reef) for a 6-mo period from June to early December 2010. The FOCE experiment was designed to simulate the effects of CO2-driven acidification predicted to occur by the end of this century (scenario RCP4.5) while simultaneously maintaining the exposure of corals to natural variations in their environment under in situ conditions. Analyses of skeletal growth (measured from extension rates and skeletal density) showed no systematic differences between low-pH FOCE treatments (Delta pH = similar to-0.05 to -0.25 units below ambient) and present day controls (Delta pH = 0) for calcification rates or the pH of the calcifying fluid (pH(cf)); the latter was derived from boron isotopic compositions (delta B-11) of the coral skeleton. Furthermore, individual nubbins exhibited near constant delta B-11 compositions along their primary apical growth axes (+/- 0.02 pH(cf) units) regardless of the season or treatment. Thus, under the highly dynamic conditions of the Heron Island reef flat, P. cylindrica up-regulated the pH of its calcifying fluid (pH(cf) similar to 8.4-8.6), with each nubbin having near-constant pHcf values independent of the large natural seasonal fluctuations of the reef flat waters (pH similar to 7.7 to similar to 8.3) or the superimposed FOCE treatments. 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.