Elevated environmental CO 2 stimulates photosynthesis and induces differential oxidative responses in two Caribbean reef-building coral species

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Abstract
Reef-building corals maintain intracellular photosymbiosis with dinoflagellate algae, allowing them to use photosynthetic products as metabolic fuel. Carbon dioxide (CO 2 ) 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 CO 2 on photosynthetic O 2 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 μatm CO 2 , pH ∼8.00) or elevated CO 2 (∼1,060 μatm CO 2 , pH ∼7.70) conditions in outdoor flow-through aquaria for 10 days. Flowing seawater had higher O 2 concentrations in elevated CO 2 tanks, with the largest differences at higher irradiance. Microsensor measurements confirmed that acute and chronic exposure to elevated CO 2 significantly stimulated photosynthetic O 2 production in both species. Despite increased O 2 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 CO 2 , potentially improving photosynthetic efficiency and decreasing reactive oxygen species production. These findings support the resilience of P. astreoides to OA and suggest that elevated CO 2 may increase daytime O 2 levels on future reefs, with implications for the aerobic metabolism of reef organisms.
Funding Information
  • Alfred P. Sloan Foundation (BR2013-103)
  • UC San Diego

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