Computing the carbonate chemistry of the coral calcifying medium and its response to ocean acidification

Critical to determining vulnerability or resilience of reef corals to Ocean Acidification (OA) is a clearer understanding of the extent to which corals can control carbonate chemistry in their Extracellular Calcifying Medium (ECM) where the CaCO₃ skeleton is produced. Here, we employ a mathematical framework to calculate ECM aragonite saturation state (Ω_arag.(ECM)) and carbonate system ion concentration using measurements of calcification rate, seawater characteristics (temperature, salinity and pH) and ECM pH (pH_(ECM)). Our calculations of ECM carbonate chemistry at current-day seawater pH, indicate that Ω_arag.(ECM) ranges from ∼10 to 38 (mean 20.41), i.e. about 5 to 6-fold higher than seawater. Accordingly, Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) were calculated to be around 3 times higher in the ECM than in seawater. We also assessed the effects of acidification on ECM chemical properties of the coral Stylophora pistillata. At reduced seawater pH our calculations indicate that Ω_arag.(ECM) remains almost constant. DIC_(ECM) and TA_(ECM) gradually increase as seawater pH declines, reaching values about 5 to 6-fold higher than in seawater, respectively for DIC and TA. We propose that these ECM characteristics buffer the effect of acidification and explain why certain corals continue to produce CaCO₃ even when seawater chemistry is less favourable.