Ocean acidification and host–pathogen interactions

Ocean acidification (OA), caused by anthropogenic CO₂ emissions is considered one of the major threats to marine ecosystems and seawater pH is predicted to decrease by 0.4 units by the end of the 21^st century. Many studies have shown the strong negative impacts of future OA, with calcifying organisms being particularly at risk due to their dependence on calcium carbonate. However, there is little evidence of the impact of OA on potentially associated heterotrophic bacteria. OA appears to influence organisms differently and as such the interactive processes between organisms, such as pathogens and hosts, are likely to vary. Asplund et al., (2014) set out to investigate the relationship between Mytilus edulis and Vibrio tubiashii in terms of pathogen growth, viability and virulence, the host growth, immune defence and stress response and the interaction between these organisms under OA conditions.

Adult bivalves are able to accumulate high numbers of pathogenic bacteria without being infected as the generally posses a number of defence mechanisms against these invasive microorganisms. In the arms-race between hosts and pathogens, the balance may however shift if the host is exposed to other stressors, such as OA, as the host may have to concentrate more resources on processes like shell formation or growth. OA appeared to have no distinct effect on the virulence of V. Tubiashii. M. Edulis immune responses, such as maeocyte numbers and phagocytotic capacity, appeared to be unaffected by OA either. However, OA had a negative effect on the average shell growth and impaired crystalline shell structures as expected. Despite no evident impact on host immunity or growth and virulence of the pathogen, V. tubiashii was more successful in infecting mussels exposed to long-term OA compared to those maintained under ambient conditions. The viability of OA exposed V. tubiashii decreased when exposed to haemocytes extracted from control mussels but interestingly the viability of OA exposed V. tubiashii increased when encountering haemocytes from OA exposed mussels.

This study highlights the importance of studying the impact of OA on the interaction between different organisms and that results from studies focusing on each organism’s response separately could be very misleading. As climate change does not only affect OA, but other factors such as temperature as well, it would be interesting to test the combined effects of OA and increased temperature on pathogen-hosts interactions. OA is also likely to change the bioavailability of metals ions and as this paper also mentioned, there are many metal ion dependant enzymes that affect virulence in vibrios. This means that it is possible that the virulence of the vibrios will increase putting further stress on the host increasing the chance of infection. Furthermore, this study used adults injected with an inoculated solution. This is of very little ecological significance as it is an unlikely scenario in any ecosystem. The authors also mention that adults are normally resistant and it is the juvenile stage that is the most susceptible to infection. The next step in the study of M.edulis -V. tubiashii should focus more on the juvenile stage as it is more sensitive to pathogens.

Asplund, M. E., Baden, S. P., Russ, S., Ellis, R. P., Gong, N., & Hernroth, B. E. (2014). Ocean acidification and host–pathogen interactions: blue mussels, Mytilus edulis, encountering Vibrio tubiashii. Environmental microbiology, 16(4), 1029-1039.

http://onlinelibrary.wiley.com.plymouth.idm.oclc.org/doi/10.1111/1462-2920.12307/epdf