The effect of ocean acidification on tropical coral calcification:insights from calcification fluid DIC chemistry

• Published in: Chemical Geology
• Main Authors: Allison, Nicola, Cole, Catherine, Hintz, Chris, Hintz, Ken, Rae, James, Finch, Adrian
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: B/Ca; Dissolved inorganic carbon; Coral; Calcification; Calcification fluid; δ11B; Ocean acidification
• Online Access: https://risweb.st-andrews.ac.uk/portal/en/researchoutput/the-effect-of-ocean-acidification-on-tropical-coral-calcification(c92cb7cd-c2b4-4fb3-8a6f-5e1099532479).html; https://doi.org/10.1016/j.chemgeo.2018.09.004; https://research-repository.st-andrews.ac.uk/bitstream/10023/18421/1/B_exp_1_paper_for_PURE_inc_figures.pdf

Ocean acidification typically reduces calcification in tropical marine corals but the mechanism for this process is not understood. We use skeletal boron geochemistry (B/Ca and δ 11 B) to reconstruct the calcification fluid DIC of corals cultured over both high and low seawater pCO 2 (180, 400 and 750 μatm). We observe strong positive correlations between calcification fluid pH and concentrations of the DIC species potentially implicated in aragonite precipitation (be they CO 3 2- , HCO 3 - or HCO 3 - +CO 3 2- ). Similarly, with the exception of one outlier, the fluid concentrations of precipitating DIC species are strongly positively correlated with coral calcification rate. Corals cultured at high seawater pCO 2 usually have low calcification fluid pH and low concentrations of precipitating DIC, suggesting that a reduction in DIC substrate at the calcification site is responsible for decreased calcification. The outlier coral maintained high pHCF and DICCF at high seawater pCO 2 but exhibited a reduced calcification rate indicating that the coral has a limited energy budget to support proton extrusion from the calcification fluid and meet other calcification demands. We find no evidence that increasing seawater pCO 2 enhances diffusion of CO 2 into the calcification site. Instead the overlying [CO 2 ] available to diffuse into the calcification site appears broadly comparable between seawater pCO 2 treatments, implying that metabolic activity (respiration and photosynthesis) generates a similar [CO 2 ] in the vicinity of the calcification site regardless of seawater pCO 2 .