Interacting effects of CO 2 partial pressure and temperature on photosynthesis and calcification in a scleractinian coral STE ´ PHANIE REYNAUD *, NICOLAS LECLERCQ *, SAMANTHA ROMAINE-LIOUD * { , CHRISTINE FERRIER-PAGE ` S *, JEAN JAUBERT * w andJEAN-PIERREGATTUSO z *Centre Scientifique de Monaco, Avenue Saint-Martin, MC-98000 Monaco, Principality of Monaco, wThe Cousteau Society, 710 Settlers Landing Road, Hampton, VA 23669-4035, USA, zLaboratoire d’Oce ´anographie, UMR 7093 CNRS-UPMC, BP 28, F-06234 Villefranche-sur-mer Cedex, France Abstract We show here that CO 2 partial pressure (pCO 2 ) and temperature significantly interact on coral physiology. The effects of increased pCO 2 and temperature on photosynthesis, respiration and calcification rates were investigated in the scleractinian coral Stylophora pistillata. Cuttings were exposed to temperatures of 25 1C or 28 1C and to pCO 2 values of ca. 460 or 760 latm 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-pCO 2 ’ condition. The cell-specific density was higher at ‘high pCO 2 ’ than at ‘normal pCO 2 ’ (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and pCO 2 , whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and pCO 2 were both elevated. Calcification under normal temperature did not change in response to an increased pCO 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 pCO 2 reported in the literature, and warrants a re- evaluation of the projected decrease of marine calcification by the year 2100. Keywords: calcification, coral, global change, photosynthesis, pCO 2 , temperature Received 21 December 2002; revised version received and accepted 15 May 2003 Introduction The response of ecosystems to global environmental change, and its retroaction on climate and human societies, is one of the major challenges facing science. The partial pressure of CO 2 (pCO 2 ) increases in the atmosphere due to anthropogenic inputs of carbon dioxide. It has increased by 32% between 1880 and 2000 (280 vs. 370 matm; Houghton et al., 2002). This has important consequences on the Earth’s climate, includ- ing air temperature, which has risen by 0.6 1C between 1880 and 2000. These past and predicted changes of atmospheric pCO 2 and temperature have prompted several studies of the response of terrestrial organisms and ecosystems. For example, the free-air CO 2 enrich- ment (FACE) and free-air temperature increase (FATI) experiments have provided considerable information on the response of terrestrial plants and communities to climatic changes. However, interactions between en- vironmental parameters must be investigated because they vary in combination. Studies of interactions between CO 2 enrichment and climate have begun at the community level in terrestrial ecosystems quite some time ago, for example by combining the FACE and FATI approaches (Nijs et al., 1996). A model demonstrates that the increase in global net ecosystem production since 1861 will decline as the CO 2 fertiliza- tion effect becomes saturated and is diminished by changes in climatic factors (Cao & Woodward, 1998). Recent experiments in terrestrial community highlight the need for a multifactor experimental approach in Correspondence: Ste ´phanie Reynaud, tel. +377 97 77 08 73, fax 1 377 92 16 79 81, e-mail: sreynaud@centrescientifique.mc { Deceased. Global Change Biology (2003) 9, 1660–1668, doi: 10.1046/j.1529-8817.2003.00678.x 1660 r 2003 Blackwell Publishing Ltd