Similar controls on calcification under ocean acidification across unrelated coral reef taxa

Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8–21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ωcf, pHcf, and DICcf). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa. We grew coral and coralline algal species under treatments where the seawater saturation state (Ω), pH, and dissolved inorganic carbon (DIC) were manipulated over long periods of time. Using geochemical tools, we show that calcifying fluid pH and DIC (pHcf and DICcf) are driven primarily by seawater pH and DIC and not Ω. We provide evidence that two unrelated coral reef taxa have the capacity to modulate their calcifying fluid pH, DIC and Ca2+ to achieve certain Ω thresholds necessary to precipitate calcium carbonate under a large range of carbonate chemistry conditions.