Ocean acidification does not affect the physiology of the tropical coral Acropora digitifera during a 5-week experiment

The increase in atmospheric CO₂ concentration, which has resulted from the burning of fossil fuels, is being absorbed by the oceans and is causing ocean acidification. Ocean acidification involves the decrease of both the pH and the calcium carbonate saturation state. Ocean acidification is predicted to impact the physiology of marine organisms and reduce the calcification rates of corals. In the present study, we measured the rates of calcification, respiration, photosynthesis, and zooxanthellae density of the tropical coral Acropora digitifera under near-natural summertime temperature and sunlight for a 5-week period. We found that these key physiological parameters were not affected by both mid-CO₂ (pCO₂ = 744 ± 38, pH = 7.97 ± 0.02, Ω_arag = 2.6 ± 0.1) and high-CO₂ conditions (pCO₂ = 2,142 ± 205, pH = 7.56 ± 0.04, Ω_arag = 1.1 ± 0.2) throughout the 35 days experimental period. Additionally, there was no significant correlation between calcification rate and seawater aragonite saturation (Ω_arag). These results suggest that the impacts of ocean acidification on corals physiology may be more complex than have been previously proposed.