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 (O 2 ) 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 O 2 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 O 2 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 O 2 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.