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

Abstract

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 CaCO 3 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 CaCO 3 even when seawater chemistry is less favourable.

ATP Supply May Contribute to Light-Enhanced Calcification in Corals More Than Abiotic Mechanisms

“…The pH values in the ECM are in the range of measured values for the same species (Raybaud et al, 2017). Our model qualitatively agrees with the observation that the conditions to favor calcification are attained in the ECM more by upregulation of total alkalinity than by upregulation of dissolved inorganic carbon (Cai et al, 2016).…”

“…However, we notice that our estimate of the parameter s, diffusion coefficient between seawater and coelenteron, of 0.013 cm/s, is in the range of the clearance rates measured for C. caespitosa in Tremblay et al (2011) (∼0.008: 0.02 cm/s, inferred from the slopes of the regression lines in Figure 6). The model compartments' behavior under alternating light and dark conditions and the direction of the mass fluxes agrees with several experimental observations and modeling results: The ECM pH is always higher than external pH and higher in the light than in the dark (Figure 4D), in agreement with experimental observations (Al-Horani et al, 2003;Raybaud et al, 2017) and modeling results Merico, 2012, 2015;Nakamura et al, 2013); also, the value of the ECM pH in the light (∼8.7 at pH 8.06) is in the range of the values calculated for Cladocora in Raybaud et al (2017) (ca. 8.6-8.7 for a seawater pH of 8.2).…”

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…This result is somewhat surprising because the average annual calcification 5264 T. M. DeCarlo et al: Raman proxy for aragonite saturation state rate of Porites living at 25 • C is typically 1.3 g cm −2 yr −1 (Lough, 2008), a factor of 6.5 faster than expected on the basis of the inorganic calcifying fluid Ar systematics. Since coral calyces have complex 3-D shape such that their skeletal surface area is actually much greater than the planar area of the colony surface (Barnes, 1970), this implies that calcifying surface area is an important factor in determining coral calcification rates and it complicates the use of bulk calcification rates for estimating Ar (Raybaud et al, 2017). Multiple studies using micro-CT have shown increases in porosity and surface-area : volume ratios in corals cultured under elevated CO 2 levels (Tambutté et al, 2015;Foster et al, 2016).…”

“…Observing this fluid has proved difficult though, due to its small size and isolation beneath the living polyp (Clode and Marshall, 2002). Estimates of fluid carbonate chemistry have so far been derived from micro-electrodes, pH-sensitive dyes, boron isotopes, B/Ca, U/Ca, and bulk calcification rates (AlHorani et al, 2003;Trotter et al, 2011;Venn et al, 2011;DeCarlo et al, 2015;Cai et al, 2016;Holcomb et al, 2016;Raybaud et al, 2017). However, these approaches do not always agree (Ries, 2011;Holcomb et al, 2014), and they have so far focused on calcifying fluid carbonate chemistry without considering the effect of [Ca 2+ ] on Ar .…”

Response to Referee Comment #1

“…Observing this fluid has proved difficult though, due to its small size and isolation beneath the living polyp (Clode and Marshall, 2002). Estimates of fluid carbonate chemistry have so far been derived from micro-electrodes, pH-sensitive dyes, boron isotopes, B/Ca, U/Ca, and bulk calcification rates (Al-Horani et al, 2003;Trotter et al, 2011;Venn et al, 2011;DeCarlo et al, 2015;Cai et al, 2016;Holcomb et al, 2016;Raybaud et al, 2017). However, these approaches do not always agree (Ries, 2011;Holcomb et al, 2014), and they have so far focused on calcifying fluid carbonate chemistry without considering the effect of [Ca 2+ ] on Ar .…”