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Ocean acidification and rising seawater temperature are environmental stressors resulting from the continuous increase of the atmospheric CO 2 concentration due to anthropogenic activities. As a consequence, marine fish are expected to undergo conditions outside of their tolerance range, leading to physiological challenges with possible detrimental implications. Our research group has previously shown that exposure to elevated CO 2 modulated the immune system of the Atlantic halibut. To further investigate this finding, we analysed non-specific immune components in blood plasma of Atlantic halibut ( Hippoglossus hippoglossus ) juveniles acclimated to six different temperatures (5, 10, 12, 14, 16 and 18 °C), and to water pH of 8.0 (control) or 7.6 (predicted for year 2100) for three months. Plasma ions (K + , Na + , Ca ++ , Cl − ) and lactate concentrations were also measured. The analysis of plasma ions did not show any trends related to temperature or CO 2 exposure, and the majority of the experimental fish were able to maintain ionic balance. The results show that both innate immune components (lysozyme and alternative complement system) had increased activities in response to elevated CO 2 , representing a CO 2 -related impact on the halibut's immune system. The increased activity of lysozyme and complement system is possibly part of the acclimatization process, and might be protective. [ABSTRACT FROM AUTHOR]

CO 2 emissions from human activities are increasing, resulting in greater rates of change in the oceans, exceeding any other event in geological and historical records over the past 300 million years. Oceans are warming and pH levels are decreasing. Marine organisms will need to respond to multiple stressors and the potential consequences of global change-related effects in fish needs to be investigated. Fish are affected by many biotic and abiotic environmental variables, including temperature and CO 2 fluctuations, and it is therefore critical to investigate how these variables may affect physiological and biochemical processes. We investigated the effects of elevated CO 2 levels (pH of 8.0, which served as a control, or 7.6, which is predicted for the year 2100) combined with exposure to different temperatures (5, 10, 12, 14, 16, and 18 °C) in the Atlantic halibut (Hippoglossus hippoglossus) during a three month experiment. Since regulation of reactive oxygen species (ROS) is crucial for physiological processes the focus was on the antioxidant defense system and we assessed the effects on catalytic activities of antioxidant enzymes (SOD, CAT, GR, GST, GPx). In addition we also analyzed effects on cholinesterase enzymes (AChE and BChE), and CYP1A enzyme activities (EROD). The treatments resulted in oxidative stress, and damage was evident in the form of protein carbonyls which were consistently higher in the elevated CO 2 -treated fish at all temperatures. Analyses of antioxidant enzymes did not show the same results, suggesting that the exposure to elevated CO 2 increased ROS formation but not defences. The antioxidant defense system was insufficient, and the resulting oxidative damage could impact physiological function of the halibut on a cellular level. Unlabelled Image [ABSTRACT FROM AUTHOR]