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

We show here that CO 2 partial pressure ( p CO 2 ) and temperature significantly interact on coral physiology. The effects of increased p CO 2 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 p CO 2 values of ca. 460 or 760 μatm for 5 weeks. The contents of chlorophyll c 2 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‐ p CO 2 ’ condition. The cell‐specific density was higher at ‘high p CO 2 ’ than at ‘normal p CO 2 ’ (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and p CO 2 , whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and p CO 2 were both elevated. Calcification under normal temperature did not change in response to an increased p CO 2 . This is not in agreement with numerous published papers that describe a negative relationship between marine calcification and CO 2 . The confounding effect of temperature has the potential to explain a large portion of the variability of the relationship between calcification and p CO 2 reported in the literature, and warrants a re‐evaluation of the projected decrease of marine calcification by the year 2100.