pH homeostasis during coral calcification in a free ocean CO ₂ enrichment (FOCE) experiment, Heron Island reef flat, Great Barrier Reef

Abstract

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 ...

Interactive Effects of Ocean Acidification and Warming on Growth, Fitness and Survival of the Cold-Water Coral Lophelia pertusa under Different Food Availabilities

“…Slower growth rates under ocean acidification are therefore presumably attributed to additional energetic costs that require ∼10% per 0.1 units decrease in seawater pH in order to maintain elevated pH values at the site of calcification (McCulloch et al, 2012b). pH homeostasis during calcification was also observed in several tropical shallow-water corals (Hönisch et al, 2004;Krief et al, 2010;Trotter et al, 2011;McCulloch et al, 2012a;Holocomb et al, 2014;Georgiou et al, 2015) but was found to be not as strong as in cold-water corals ( pH between seawater pH T and pH cf ) (McCulloch et al, 2012b), demonstrating the adaptation of these corals to live in environments with naturally lower aragonite saturation states.…”

The Dynamics of Coral-Algal Interactions in Space and Time on the Southern Great Barrier Reef

“…One site was selected on the reef crest (Reef Crest, RC), and within the lagoon, three sites were chosen [Reef Flat (RF), Shallow Lagoon (SL), Deep Lagoon (DL)] (Figure 1). The lagoonal study area is shallow and periodically isolated at low tide, resulting in extreme diel fluctuations in seawater conditions (Kinsey and Kinsey, 1967;Potts and Swart, 1984;Georgiou et al, 2015). …”

“…The southern, offshore reefs were the least affected GBR region from the cumulative footprint of the last three major coral bleaching events (Hughes et al, 2017), and since 1992, have experienced only one major damage-inducing cyclone in 2009 (Connell et al, 1997;Woolsey et al, 2012). Within Heron reef, specific geomorphological zones include a gamut of naturally variable benthic communities subject to distinct diel and seasonal changes in seawater conditions (Phinn et al, 2012;Georgiou et al, 2015). As such, Heron reef is an ideal ecosystem to explore the influence of natural environmental drivers on fluctuations in benthic cover and coral-algal interactions across a complete reef system.…”

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…Some coral species are able to maintain normal calcification rates across large natural acidification gradients (Fabricius et al, 2011;Shamberger et al, 2014;Barkley et al, 2015), indicative of adaptation or acclimation (Barkley et al, 2017). One potential mechanism to counteract acidification of surrounding seawater is pH homeostasis at the site of calcification (Georgiou et al, 2015;Barkley et al, 2017). Corals calcify from an isolated fluid located between the living tissue and the existing skeleton (Barnes, 1970;Cohen and McConnaughey, 2003;Venn et al, 2011;Tambutté et al, 2012).…”