Responses of coral gastrovascular cavity pH during light and dark incubations to reduced seawater pH suggest species-specific responses to the effects of ocean acidification on calcification.

Coral polyps have a fluid-filled internal compartment, the gastrovascular cavity (GVC). Respiration and photosynthesis cause large daily excursions in GVC oxygen concentration (O₂) and pH, but few studies have examined how this correlates with calcification rates. We hypothesized that GVC chemistry can mediate and ameliorate the effects of decreasing seawater pH (pH_SW) on coral calcification. Microelectrodes were used to monitor O₂ and pH within the GVC of Montastraea cavernosa and Duncanopsammia axifuga (pH only) in both the light and the dark, and three pH_SW levels (8.2, 7.9, and 7.6). At pH_SW 8.2, GVC O₂ ranged from ca. 0 to over 400% saturation in the dark and light, respectively, with transitions from low to high (and vice versa) within minutes of turning the light on or off. For all three pH_SW treatments and both species, pH_GVC was always significantly above and below pH_SW in the light and dark, respectively. For M. cavernosa in the light, pH_GVC reached levels of pH 8.4–8.7 with no difference among pH_SW treatments tested; in the dark, pH_GVC dropped below pH_SW and even below pH 7.0 in some trials at pH_SW 7.6. For D. axifuga in both the light and the dark, pH_GVC decreased linearly as pH_SW decreased. Calcification rates were measured in the light concurrent with measurements of GVC O₂ and pH_GVC. For both species, calcification rates were similar at pH_SW 8.2 and 7.9 but were significantly lower at pH_SW 7.6. Thus, for both species, calcification was protected from seawater acidification by intrinsic coral physiology at pH_SW 7.9 but not 7.6. Calcification was not correlated with pH_GVC for M. cavernosa but was for D. axifuga. These results highlight the diverse responses of corals to changes in pH_SW, their varying abilities to control pH_GVC, and consequently their susceptibility to ocean acidification. [ABSTRACT FROM AUTHOR]