When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Anthropogenic pressures on aquatic environments are leading to rapid environmental change. One of the main environmental stressors in marine environments is change in salinity. It is a major factor that controls the distribution and diversity of organisms. As microbes are essential to maintaining ecosystem services and biogeochemical cycling, it is important to understand the diversity, distribution, ecology, and metabolism of bacteria living in these rapidly changing environments. In this thesis, bacterial adaptation and evolution is investigated across salinity gradients spatially and temporally. A targeted approach, using the Methylophilaceae as a model family to monitor change, allowed us to explore and increase our understanding niche differentiation, genome streamlining, and ecological adaptations.
We first studied the diversity and distribution of the Methylophilaceae family across a spatial salinity gradient where we detected the OM43/LD28 clade from freshwater to brackish-marine waters in the Saint Lawrence Estuary and Bedford Basin. These distinct populations were related to specific salinity concentrations, which suggests different ecological roles in response to environmental gradients. This led us to also investigate the adaptation and evolution of Methylophilaceae in response to changing environmental conditions over time and space in the increasingly freshening Arctic Ocean. We found evidence for endemic Arctic Ocean taxa as well as a novel clade, BS01, which was adapted to the unique lower salinity surface waters of the Arctic Ocean. We also discovered that over time, the Methylophilaceae family showed an increase in specific taxa associated to the years experiencing the highest amount of freshening.
The work in this thesis has increased our understanding of the Methylophilaceae family and their niche differentiation to distinct environmental conditions. Overall, this work has larger implications for understanding how climate change is affecting northern aquatic ecosystems and how microbes are evolving and adapting to this change.