Increased activity of lysozyme and complement system in Atlantic halibut exposed to elevated CO₂ at six different...

Ocean acidification and rising seawater temperature are environmental stressors resulting from the continuous increase of the atmospheric CO₂ 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 CO₂ 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 CO₂ 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 CO₂, representing a CO₂-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.