Elevated environmental CO2 stimulates photosynthesis and induces differential oxidative responses in two Caribbean reef-building coral species
Reef-building corals maintain intracellular photosymbiosis with dinoflagellate algae, allowing them to use photosynthetic products as metabolic fuel. Carbon dioxide (CO2) is a key substrate for photosynthesis, but its rapid increase in the global ocean due to anthropogenic emissions (known as ocean acidification) may disrupt coral symbiosis. We examined the effects of elevated CO2 on photosynthetic O2 production, oxidative damage, and antioxidant systems in the Caribbean reef-building corals Porites astreoides and Orbicella franksi. Coral colonies collected at 3m depth in Bocas del Toro (Panama) were exposed to control (390 {micro}atm CO2, pH ~8.00) or elevated CO2 ({approx}1,060 {micro}atm CO2, pH {approx}7.70) conditions in outdoor flow-through aquaria for 10 days. Flowing seawater had higher O2O2 concentrations in elevated CO2 tanks, with the largest differences at higher irradiance. Microsensor measurements confirmed that acute and chronic exposure to elevated CO2 significantly stimulated photosynthetic O2 production in both species. Despite increased O2 production. neither species showed significant changes in lipid peroxidation, reduced glutathione, or superoxide dismutase, catalase, and glutathione-S-transferase activities. Protein oxidation was unchanged in O. franksi but significantly decreased in P. astreoides. These results are consistent with partial relief of carbon limitation in P. astreoides symbionts under elevated CO2, potentially improving photosynthetic efficiency and decreasing reactive oxygen species production. These findings support the resilience of P. astreoides to OA and suggest that elevated CO2 may increase daytime O2 levels on future reefs, with implications for the aerobic metabolism of reef organisms.