Mechanisms of calcification and its relation to photosynthesis and respiration in the coral Seriatopora hystrix at a volcanic carbon dioxide seep

Ocean acidification due to rising partial pressure of carbon dioxide (pCO₂) in the atmosphere is predicted to profoundly affect marine ecosystems. Studies on coral reef communities at volcanic CO₂ seeps in Papua New Guinea (PNG) show reductions in coral diversity and structural complexity where mean pH is reduced by 0.3 units. For example, the abundance of the scleractinian coral Seriatopora hystrix is significantly reduced at seep sites in PNG. To assess the physiological mechanisms for these community shifts in response to ocean acidification, we collected branches of S. hystrix at a seep (pCO₂= 803, pH_Total = 7.8) and a control site (pCO₂ = 323, pH_Total = 8.1) in PNG. We determined rates of oxygen production, oxygen consumption and calcification of live coral branches in light and dark incubation experiments. While net photosynthesis and dark respiration rates in the corals remained similar at high and low pCO₂, their rates of light and dark calcification considerably decreased at high pCO₂. In order to investigate the mechanism of calcification under acidified and ambient conditions and its coupling to photosynthesis and respiration, we further studied Ca²⁺, pH and O₂ dynamics with microsensors. The results of these analyses will allow us to determine whether limited capacity for physiological acclimatization rather than lower recruitment success have led to reduced densities of sensitive corals such as S. hystrix at high pCO₂ sites.