Combining Multiple Geochemical Approaches (B/Ca, δ11B, δ13C, δ18O, ∆47) to Study the Biocalcification Responses of Scleractinian Corals to Seawater Temperature and pH.

Ilian Antoine DeCorte, University of California - Los Angeles, Atmospheric and Oceanic Sciences, Los Angeles, CA, United States, Maxence Guillermic, University of California Los Angeles, Earth, Planetary and Space Sciences, Los Angeles, United States; IUEM Institut Universitaire Européen de la Mer, Plouzané, France, Colleen Bove, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, Sambuddha Misra, Indian Institute of Science, Bangalore, Centre for Earth Sciences, Bangalore, India, Mervyn Greaves, University of Cambridge, Godwin Laboratory for Palaeoclimate Research, Cambridge, United Kingdom, Karl Castillo, University of North Carolina at Chapel Hill, Department of Marine Sciences, Chapel Hill, NC, United States, Justin B Ries, Northeastern University, Department of Marine and Environmental Sciences, Boston, MA, United States and Robert Eagle, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States
Scleractinian corals are known to elevate pH and aragonite saturation state (ΩA) of a semi-isolated extracellular calcification fluid , a process that may facilitate calcification. However, the exact composition and nature of this calcifying fluid, and its importance to coral calcification and coral calcification responses to ocean acidification, are debated. Here we examine the skeletal geochemistry of four tropical stony coral species, Porites astreoides, Psuedodiploria strigosa, Undaria tenuifolia, and Siderastrea siderea, which have been previously cultured under a range of controlled pCO2 and temperature conditions (Bove et al., Proc. Royal. Soc. B., 2019). We use B/Ca and boron isotope (δ11B) analyses of the coral skeleton as proxies of calcification fluid dissolved inorganic carbon (DIC) concentration and pH, respectively. Skeletal samples were also analyzed for stable carbon and oxygen isotopes (δ13C, δ18O) and carbonate clumped isotopes (∆47)— parameters that may exhibit disequilibrium isotope effects compared to seawater, thereby reflecting physiological processes in the coral. Notably, all 4 species exhibited calcifying fluid pH and ΩA that was elevated relative to their surrounding seawater, and which declined with decreasing seawater pH (increasing pCO2). However, Siderastrea siderea was the only species investigated that exhibited a significant correlation between calcifying fluid pH (and ΩA) and net calcification rate across seawater pH treatments. In the case of S. siderea, a significant change in both δ18O and ∆47 across pCO2 treatments was also observed, consistent with the assertion that equilibration of the DIC-H2O pool within the calcifying fluid of this species may be outpaced by the rate of calcification and/or photosynthesis, thereby yielding the apparent disequilibrium isotope effects. The other 3 species investigated exhibited a weak correlation or decoupling of calcifying fluid carbonate chemistry and net calcification rate, suggesting for these coral species that other factors (e.g., dissolution of aragonite skeleton and/or coral symbiont health) have a greater influence on net calcification response to CO2-induced ocean acidification than calcifying fluid chemistry.