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Insight into the response of reef corals and other major marine calcifiers to ocean acidification is limited by a lack of knowledge about how seawater pH and carbonate chemistry impact the physiological processes that drive biomineralization. Ocean acidification is proposed to reduce calcification rates in corals by causing declines in internal pH at the calcifying tissue-skeleton interface where biomineralization takes place. Here, we performed an in vivo study on how partial-pressure CO 2 -driven seawater acidification impacts intracellular pH in coral calcifying cells and extracellular pH in the fluid at the tissue-skeleton interface [subcalicoblastic medium (SCM)] in the coral Stylophora pistillata. We also measured calcification in corals grown under the same conditions of seawater acidification by measuring lateral growth of colonies and growth of aragonite crystals under the calcifying tissue. Our findings confirm that seawater acidification decreases pH of the SCM, but this decrease is gradual relative to the surrounding seawater, leading to an increasing pH gradient between the SCM and seawater. Reductions in calcification rate, both at the level of crystals and whole colonies, were only observed in our lowest pH treatment when pH was significantly depressed in the calcifying cells in addition to the SCM. Overall, our findings suggest that reef corals may mitigate the effects of seawater acidification by regulating pH in the SCM, but they also highlight the role of calcifying cell pH homeostasis in determining the response of reef corals to changes in external seawater pH and carbonate chemistry.he impacts of ocean acidification on marine calcifying organisms are predicted to be varied and in many cases deleterious (1-3). Though several studies on marine calcifiers have investigated how rates of calcification respond to ocean acidification scenarios (4), comparatively few studies tackle how ocean acidification impacts the physiological mechanisms that drive calcification itself. A mechanistic understanding of calcification responses to shifts in external seawater pH and carbonate chemistry is critical to predicting how corals and other major marine calcifiers respond and potentially acclimate to ocean acidification (5).The calcium carbonate (aragonite) skeletons of scleractinian corals make up a large component of shallow and deepwater reefs at both tropical and temperate latitudes. Corals form their skeletons by producing aragonite crystals in a fluid-filled medium called the subcalicoblastic medium (SCM) underlying the calcifying tissue [calicoblastic epithelium (CE)] (Fig. S1). The calicoblastic epithelium promotes calcification by exerting biological control over the SCM in a number of ways (reviewed in refs. 6 and 7). One important process is that the CE elevates pH in the SCM (pH SCM ) relative to the exterior seawater pH (8), possibly by the removal of protons via a by a Ca 2+ ATPase (9-12). This increase in pH SCM favors the conversion of bicarbonate to carbonate, elevating the saturation state ...

The threat posed to coral reefs by changes in seawater pH and carbonate chemistry (ocean acidification) raises the need for a better mechanistic understanding of physiological processes linked to coral calcification. Current models of coral calcification argue that corals elevate extracellular pH under their calcifying tissue relative to seawater to promote skeleton formation, but pH measurements taken from the calcifying tissue of living, intact corals have not been achieved to date. We performed live tissue imaging of the reef coral Stylophora pistillata to determine extracellular pH under the calcifying tissue and intracellular pH in calicoblastic cells. We worked with actively calcifying corals under flowing seawater and show that extracellular pH (pHe) under the calicoblastic epithelium is elevated by ∼0.5 and ∼0.2 pH units relative to the surrounding seawater in light and dark conditions respectively. By contrast, the intracellular pH (pHi) of the calicoblastic epithelium remains stable in the light and dark. Estimates of aragonite saturation states derived from our data indicate the elevation in subcalicoblastic pHe favour calcification and may thus be a critical step in the calcification process. However, the observed close association of the calicoblastic epithelium with the underlying crystals suggests that the calicoblastic cells influence the growth of the coral skeleton by other processes in addition to pHe modification. The procedure used in the current study provides a novel, tangible approach for future investigations into these processes and the impact of environmental change on the cellular mechanisms underpinning coral calcification.

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