Unravelling adaptive evolution in response to changing salinities in a tripartite species interaction

Over a long time, marine organism have adapted to their biotic and abiotic environment. Currently, anthropogenic induced climate change is rapidly altering the environment with unpredictable consequences for marine ecosystems. To predict how organisms will cope with those changes in a future ocean, research mainly focused on exposing individual species to elevated water temperatures and ocean acidification scenarios. In the Baltic Sea, a decrease in salinity due to increased rainfall is predicted to be an additional stressor for marine life. However, the impact of low salinity levels on marine organisms has been ignored. By studying the interaction between a filamentous phage and an opportunistic bacterium (Vibrio alginolyticus) as well as the interaction between V. alginolyticus and the pipefish Syngnathus typhle, this thesis provides empirical data on the ecological and coevolutionary consequences of altered salinity levels on species interactions. Filamentous phages can infect and integrate in the genome of Vibrio bacteria. Whether filamentous phages are detrimental or beneficial for the bacterium depends not only on the costs they are causing for the bacterium, but also on the additional genes they are carrying and the environment. The genes introduced by the phage can provide the bacterium with additional properties which help the bacterium to infect marine animals. The results of chapter I, show that filamentous phages predominantly infect Vibrio bacteria within one clade. Infections and thus potential transfer of genes across bacterial clades occur at lower frequencies. In chapter II, I showed that reduced salinity levels made the bacteria more susceptible for phage infections, which may result in an increased transfer of genes between bacteria and facilitate the spread of virulence and antibiotic resistant genes in the future Baltic Sea. In chapter III, I used an evolution experiment to find out that Vibrio bacteria can quickly become resistant against filamentous phages and that resistance evolution is delayed a low salinity level. At low salinity levels, phage-infected bacteria persisted longer in the populations compared to coevolving populations at high salinity levels suggesting that phage-infected bacteria are more competitive in predicted salinity conditions. In chapter IV, I investigated how the pipefish S. typhle copes with reduced salinity levels and the associated shift in the microbial community. By taking advantage of the natural salinity gradient of the Baltic Sea, I investigated the role of genetic adaptation, transgenerational plasticity, developmental plasticity and their interactions in responding towards a shift in biotic and abiotic conditions. Pipefish males collected at high salinity and exposed to low salinity levels in the lab were infected by a fungus in the brood pouch, which points out that rapid salinity changes can impair the immune system resulting in a higher susceptibility to ambient pathogens. Gene expression analysis, survival measurements of juveniles and resistance to a fungus infection suggest that pipefish collected at low salinity are locally adapted but retain the phenotypic plasticity to cope with ancestral salinity levels and associated pathogens. This PhD thesis provides evidence that a rapid salinity reduction in the Baltic Sea has negative effects on individual organisms ranging from microbes to teleost fish. The negative impact of an altered environment can be amplified by an additional organism. On evolutionary time scales changing environmental conditions in combination with threatening biotic species can be overcome via resistance evolution and local adaptation to salinity conditions. The results suggest that conclusions drawn from single species studies fail to capture fundamental interactions between organisms which constrains our ability to predict the future of any species in a rapidly changing ocean.