Interacting effects of CO2 partial pressure and temperature on photosynthesis and calcification in a scleractinian coral

We show here that CO₂ partial pressure (pCO₂) and temperature significantly interact on coral physiology. The effects of increased pCO₂ and temperature on photosynthesis, respiration and calcification rates were investigated in the scleractinian coral Stylophora pistillata. Cuttings were exposed to temperatures of 25°C or 28°C and to pCO₂ values of ca. 460 or 760 μatm for 5 weeks. The contents of chlorophyll c₂ and protein remained constant throughout the experiment, while the chlorophyll a content was significantly affected by temperature, and was higher under the ‘high‐temperature–high‐pCO₂’ condition. The cell‐specific density was higher at ‘high pCO₂’ than at ‘normal pCO₂’ (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and pCO₂, whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and pCO₂ were both elevated. Calcification under normal temperature did not change in response to an increased pCO₂. This is not in agreement with numerous published papers that describe a negative relationship between marine calcification and CO₂. The confounding effect of temperature has the potential to explain a large portion of the variability of the relationship between calcification and pCO₂ reported in the literature, and warrants a re‐evaluation of the projected decrease of marine calcification by the year 2100.