Description: Climate change with increasing temperature and ocean acidification (OA) poses risks for marine ecosystems. According to Portner and Farrell [1], synergistic effects of elevated temperature and CO(2)-induced OA on energy metabolism will narrow the thermal tolerance window of marine ectothermal animals. To test this hypothesis, we investigated the effect of an acute temperature rise on energy metabolism of the oyster, Crassostrea gigas chronically exposed to elevated CO(2) levels (partial pressure of CO(2) in the seawater similar to 0.15 kPa, seawater pH similar to 7.7). Within one month of incubation at elevated PCO(2) and 15 degrees C hemolymph pH fell (pH(e) = 7.1 +/- 0.2 (CO(2)-group) vs. 7.6 +/- 0.1 (control)) and P(e)CO(2) values in hemolymph increased (0.5 +/- 0.2 kPa (CO(2)-group) vs. 0.2 +/- 0.04 kPa (control)). Slightly but significantly elevated bicarbonate concentrations in the hemolymph of CO(2)-incubated oysters ([HCO(3)(-)](e) = 1.8 +/- 0.3 mM (CO(2)-group) vs. 1.3 +/- 0.1 mM (control)) indicate only minimal regulation of extracellular acid-base status. At the acclimation temperature of 15 degrees C the OA-induced decrease in pHe did not lead to metabolic depression in oysters as standard metabolism rates (SMR) of CO(2)-exposed oysters were similar to controls. Upon acute warming SMR rose in both groups, but displayed a stronger increase in the CO(2)-incubated group. Investigation in isolated gill cells revealed a similar temperature-dependence of respiration between groups. Furthermore, the fraction of cellular energy demand for ion regulation via Na(+)/K(+)-ATPase was not affected by chronic hypercapnia or temperature. Metabolic profiling using (1)H-NMR spectroscopy revealed substantial changes in some tissues following OA exposure at 15 degrees C. In mantle tissue alanine and ATP levels decreased significantly whereas an increase in succinate levels was observed in gill tissue. These findings suggest shifts in metabolic pathways following OA-exposure. Our study confirms that OA affects energy metabolism in oysters and suggests that climate change may affect populations of sessile coastal invertebrates such as mollusks.