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 ofMontastraea cavernosaandDuncanopsammia 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. ForM. cavernosain 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. ForD. axifugain 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)forM. cavernosabut was forD. 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.