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In this era of global climate change, ocean acidification is becoming a serious threat to the marine ecosystem. Despite this, it remains almost unknown how fish will respond to the co-occurrence of ocean acidification with other conventional environmental perturbations typically salinity fluctuation and high ammonia threat. Therefore, the present work evaluated the interactive effects of elevated pCO(2), salinity reduction and high environmental ammonia (HEA) on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to seawater (32 ppt), to brackish water (10 ppt) and to hyposaline water (2.5 ppt). Following acclimation to different salinities for at least two weeks, fish were exposed to CO2-induced water acidification representing present-day (control pCO(2), 400 mu atm, LoCO(2)) and future (high pCO(2), 1000 mu atm, HiCO(2) ) sea-surface CO2 level for 3, 7 and 21 days. At the end of each exposure period, fish were challenged with HEA for 6 h (1.18 mM representing 50% of 96 h LC50). Results show that, in response to the individual HiCO(2) exposure, fish within each salinity compensated for blood acidosis. Fish subjected to HiCO(2) were able to maintain ammonia excretion rate (J(amm)) within control levels, suggesting that HiCO(2) exposure alone had no impact on J(mmm) at any of the salinities. For 32 and 10 ppt fish, up-regulated expression of Na+/K+.ATPase was evident in all exposure groups (HEA, HiCO(2) and HEA/HiCO(2) co-exposed), whereas Na+/K+/2Cl(-) co-transporter was up-regulated mainly in HiCO(2) group. Plasma glucose and lactate content were augmented in all exposure conditions for all salinity regimes. During HEA and HEA/HiCO(2), J(amm) was inhibited at different time points for all salinities, which resulted in a significant build-up of ammonia in plasma and muscle. Branchial expressions of Rhesus glycoproteins (Rhcg isoforms and Rhbg) were upregulated in response to HiCO(2) as well as HEA at 10 ppt, with a more moderate response in 32 ppt groups. Overall, our findings denote that the adverse effect of single exposures of ocean acidification or HEA is exacerbated when present together, and suggests that fish are more vulnerable to these environmental threats at low salinities.