Geochemical analyses (δ 11 B 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 CO 2 -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 (δ 11 B) of the coral skeleton. Furthermore, individual nubbins exhibited near constant δ 11 B 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 ∼8.4-8.6), with each nubbin having nearconstant 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.A tmospheric CO 2 has risen by more than 30% during the last century, causing a reduction in seawater pH of ∼0.1 units relative to preindustrial times, with a further reduction of 0.1-0.4 units predicted to occur by the end of this century (1). This process, commonly known as "ocean acidification," is expected to have severe impacts on calcifying marine organisms due to its effect on the thermodynamics of biomineralization (2). Our current understanding of the sensitivity of coral calcification to declining seawater pH has mainly been inferred from short-term laboratory-based studies that do not fully simulate real-world reef conditions, particularly the daily to seasonal variations in temperature, light, and pH (3-5). To address these shortcomings, we applied free ocean carbon enrichment (FOCE) technologies (6-9) to manipulate water chemistry in situ and thereby provide more realistic experimental conditions to investigate how future levels of acidification could affect marine organisms according to different representative concentration pathways (RCPs) (10). The FOCE system uses a flowthrough flume design that allows organisms to experience near natural conditions, in particular the daily and seasonal regimes of fluctuating temperature, light, and nutrients while maintaining offsets in flume water pH below ...

This study investigates the impact of extreme heat wave events on long‐lived massive corals (Porites spp.) from the central Saudi Arabian Red Sea using trace element (Sr/Ca, Li/Mg, Mg/Ca, U/Ca, B/Ca, and Li/Ca) records preserved in the coral skeleton for the period between 1992 and 2012. Prior to 1998, the trace element records show strong correlations with sea surface temperature. However, during the prolonged high temperature phase associated with the 1998 El Niño event, the seasonal trace element signals were disrupted, which also coincided with a reduction in extension rates. This disruption in normally highly correlated seasonal trace element ratios‐sea surface temperature relationships was unusually long, lasting for approximately 2 years in the inner‐shelf reef site and nearly 4 years in the outer‐shelf reef site. Although the seasonal signal of trace element ratios in both cores eventually stabilized, for the inner‐shelf core the amplitude and absolute values in most trace element ratios remained significantly different compared to pre‐1998 levels. This suggests that prolonged thermal stress can induce subtle but potentially long‐lasting physiological changes that affect the elemental composition of the coral's calcifying fluid. The lack of indication of stress in the core records during later bleaching events (2003, 2005, and 2010) suggests that some of these physiological changes could have induced increased thermal tolerance, particularly for inner‐shelf corals, lending support to the capacity for corals to acclimatize.