Pathogenic challenge reveals immune trade-off in mussels exposed to reduced seawater pH and increased temperature

• Mussels tolerant to seawater pH's that are projected to occur by 2300 due to ocean acidification.
• Exposure to pH 6.50 reduced mussel immune response, yet in the absence of a pathogen.
• Subsequent pathogenic challenge led to a reversal of immune suppression at pH 6.50.
• Study highlights the importance of undertaking multiple stressor exposures.
• Shows a need to consider physiological trade-offs and measure responses functionally

Ocean acidification (OA) and warming pose a considerable threat to marine ecosystems. Previous studies show that these environmental co-stressors significantly impact upon a number of key physiological functions, including calcification, metabolism and growth, in many marine organisms. Yet despite the importance of the immune system, to date only a handful of studies have investigated the impact of reduced seawater pH on an organism's immune response. Furthermore, whilst temperature has received far greater attention with respect to host defence, there is a dearth of information concerning the possible synergism of these two stressors on immune defence. Here we show that a 90 day exposure to reduced seawater pH led to a reduction in the antibacterial activity of cell-free haemolymph in the blue mussel Mytilus edulis, whilst temperature led to an increase in this immune parameter. However in contrast to previous research, following this initial 90 day exposure, mussels in the current study were then exposed to the pathogenic bacterium, Vibrio tubiashii. Crucially, whilst reduced seawater pH initially appeared to impair immunological functioning, as has been interpreted previously, mussels demonstrated the ability to restore haemolymph bactericidal activity when required. This indicated that the initial reduction in antibacterial activity was in fact a reversible physiological trade-off, rather than an irreversible impairment of immune function. By demonstrating this plasticity, the current study illustrates the need to measure organism responses within a realistic natural context (i.e. measuring the immune response of an organism in the presence of a pathogen). Failure to do so may result in a misleading interpretation of the ecological relevance of experimental data, and thus the sensitivity of different species in a rapidly changing environment.