Ocean acidification (OA) can shift the ecological balance between interacting organisms. In this study, we have used a model-system to illustrate the interaction between a calcifying host-organism, the blue mussel Mytilus edulis, and a common bivalve bacterial-pathogen, Vibrio tubiashii, with organisms being exposed to a level of acidification projected to occur by the end of the 21st century. OA exposures of the mussels were carried out in relative long-term (4 months) and short-term (4 days) experiments. We found no effect of OA on the culturability of V. tubiashii, in broth or in seawater. OA inhibited mussel shell growth and impaired crystalline shell structures but did not appear to affect mussel immune parameters (i.e hemocyte counts and phagocytotic capacity). Despite no evident impact on host immunity or growth and virulence of the pathogen, V. tubiashii was clearly more successful in infecting mussels exposed to long-term OA compared to those maintained under ambient conditions. Moreover, OA exposed V. tubiashii increased their viability when exposed to hemocytes of OA treated mussel. Our findings suggest that even though host-organisms may have the capacity to cope with periods of OA, these conditions may alter the out-come of host-pathogen interactions, favoring the success of the latter
