The effect of carbonate chemistry on calcification and photosynthesis in the hermatypic coral Acropora eurystoma

The rise in atmospheric CO₂ has caused significant decrease in sea surface pH and carbonate ion (CO₃^2-) concentration. This decrease has a negative effect on calcification in hermatypic corals and other calcifying organisms. We report the results of three laboratory experiments designed specifically to separate the effects of the different carbonate chemistry parameters (pH, CO₃^2-, CO₂ [aq], total alkalinity [A_T], and total inorganic carbon [C _T]) on the calcification, photosynthesis, and respiration of the hermatypic coral Acropora eurystoma. The carbonate system was varied to change pH (7.9-8.5), without changing C_T; C_T was changed keeping the pH constant, and C_T was changed keeping the pCO₂ constant. In all of these experiments, calcification (both light and dark) was positively correlated with CO₃^2- concentration, suggesting that the corals are not sensitive to pH or C_T but to the CO ₃^2- concentration. A decrease of ∼30% in the CO ₃^2- concentration (which is equivalent to a decrease of about 0.2 pH units in seawater) caused a calcification decrease of about 50%. These results suggest that calcification in today's ocean (pCO₂ = 370 ppm) is lower by ∼20% compared with preindustrial time (pCO₂ = 280 ppm). An additional decrease of ∼35% is expected if atmospheric CO ₂ concentration doubles (pCO₂ = 560 ppm). In all of these experiments, photosynthesis and respiration did not show any significant response to changes in the carbonate chemistry of seawater. Based on this observation, we propose a mechanism by which the photosynthesis of symbionts is enhanced by coral calcification at high pH when CO₂(aq) is low. Overall it seems that photosynthesis and calcification support each other mainly through internal pH regulation, which provides CO₃^2- ions for calcification and CO₂(aq) for photosynthesis.