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 - PNAS Vol. 112; no. 43; pp. 13219 - 13224
• Main Authors: Georgiou, Lucy, Falter, James, Trotter, Julie, Kline, David I., Holcomb, Michael, Dove, Sophie G., Hoegh-Guldberg, Ove, McCulloch, Malcolm
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 27.10.2015; National Acad Sciences
• Subjects: Biological Sciences; Boron - analysis; Calcification, Physiologic - physiology; Carbon Dioxide - chemistry; Coral Reefs; Homeostasis - physiology; Hydrogen-Ion Concentration; Models, Biological; Pacific Ocean; Queensland; Pacific Ocean; Queensland; ocean acidification; pH up-regulation; coral resilience; Heron Island; FOCE
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1505586112

Geochemical analyses (δ11B and Sr/Ca) are reported for the coralPorites cylindricagrown 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 (ΔpH = ∼−0.05 to −0.25 units below ambient) and present day controls (ΔpH = 0) for calcification rates or the pH of the calcifying fluid (pH cf ); the latter was derived from boron isotopic compositions (δ11B) of the coral skeleton. Furthermore, individual nubbins exhibited near constant δ11B 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. cylindricaup-regulated the pH of its calcifying fluid (pH cf ∼8.4–8.6), with each nubbin having near-constant pH cf values independent of the large natural seasonal fluctuations of the reef flat waters (pH ∼7.7 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.