The influence of seawater calcium ions on coral calcification mechanisms: Constraints from boron and carbon isotopes and B/Ca ratios in Pocillopora damicornis

In order to better understand the response of coral calcification to changes in seawater chemistry, fragments of the coral Pocillopora damicornis were grown in seawater with varying [Ca²⁺]. Using a combined trace element (B/Ca) and stable isotope (δ¹¹B, δ¹³C) approach, this paper explores the effect of seawater calcium concentrations ([Ca²⁺]_SW) on coral calcification mechanisms and describes the manipulation of the extracellular calcifying fluid (ECF) pH (pH_ECF), saturation state (Ω_ECF) and dissolved inorganic carbon (DIC) speciation of the ECF. In these experiments, increases in [Ca²⁺]_SW did not significantly influence calcification rates but caused the skeletal B/Ca ratio to significantly increase. To explain these results we propose a mechanism by which [CO₃ ²⁻]_ECF is influenced by [Ca²⁺]_SW. This mechanism suggests that the pH_ECF will be lowest in our highest [Ca²⁺] treatment and using this relationship we can place constraints on the pH_ECF. The difference in pH_ECF in our treatments is supported in part by a slight, but insignificant decrease in skeletal δ¹¹B and δ¹³C values. We propose a novel dual-proxy approach to estimate pH_ECF, where calcification occurs at a pH_ECF > 9. This work emphasizes the importance of multi-element proxy approaches in understanding pH up-regulation and CO₃ ²⁻ concentrations during calcification of corals.