Positive and negative responses of coral calcification to elevated pCO2: case studies of two coral species and the implications of their responses

Abstract

Fragments of 2 coral species (Acropora nasuta and Pocillopora damicornis), collected from the South China Sea, were incubated for 94 d under controlled conditions of pCO(2) = 389, 700, and 1214 mu atm. Our incubation experiments showed that the net calcification rate of A. nasuta responded negatively to elevated pCO(2) in both short and mid-term incubations. In contrast, the net calcification rate of P. damicornis increased under elevated pCO(2) during the first 17 d, but then returned to background rates. Based on previous models, our modified models showed that the different responses of these 2 coral species depended on the dissolved inorganic carbon (DIC) and pH levels in coral calcifying fluid. In the studied models, the positive responses of coral calcification to higher pCO(2) could be explained by either low DIC due to a higher photosynthesis rate or a stronger H+ pump. If DIC in calcifying fluid is greatly reduced by photosynthesis, the de creased DIC in calcifying fluid may be benefited by enhanced external seawater CO2, which will partially offset the dissolution due to ocean acidification. Therefore, in addition to our experimental results, our model provides a theoretical basis showing how coral may respond negatively and positively to ocean acidification.