Coralline algae elevate pH at the site of calcification under ocean acidification

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species‐specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pH_cf) using δ¹¹B as a proxy. Declines in δ¹¹B for all three species are consistent with shifts in δ¹¹B expected if B(OH)₄^? was incorporated during precipitation. In particular, the δ¹¹B ratio in Amphiroa anceps was too low to allow for reasonable pH_cf values if B(OH)₃ rather than B(OH)₄^? was directly incorporated from the calcifying fluid. This points towards δ¹¹B being a reliable proxy for pH_cf for coralline algal calcite and that if B(OH)₃ is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)₄^?. We thus show that pH_cf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO₂, as did their pH_cf. Neogoniolithon sp. had the highest pH_cf, and most constant calcification rates, with the decrease in pH_cf being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pH_cf under ocean acidification is physiologically important and should be included in future models involving calcification.