| Summary: | The oceans are an extremely diverse environment due to the numerous physicochemical gradients occurring throughout the water column. Microorganisms have adapted to live in virtually all available niches. Therefore, an important focus in marine microbiology is the investigation of changes in marine microbial community structures in response to changes in physicochemical characteristics. In the past decades, with the advent of new technologies, major advances have been made in our understanding of microbial communities, but we have only scratched the surface of the extent and the complexity of the microbial realm. The work presented here investigates microbial communities in an area that has so far been little studied, the Nordic Seas. These waters, situated between Norway and Greenland, are a complex entanglement of water masses and host hydrothermal activities along the Arctic Mid-Ocean Ridge. They are therefore a suitable area for studying the influences of hydrothermal activities and dispersal processes on microbial communities. The work uses a holistic approach to investigate these influences on individual prokaryotic species and on microbial communities in hydrothermal deposits, hydrothermal plumes and the water column. In this prospect, interdisciplinary methods were used such as culture in- and dependent microbiology techniques, multivariate analysis, chemical modeling, and oceanographic tools. The influence of the hydrothermal environment was visible on a newly isolated Bacteroidetes species by, for example, the ability to grow under various oxygen concentrations and build biofilms. Also, the influence of hydrothermal fluid chemistry was visible within hydrothermal chimneys, where different functional groups inhabited different sections of the chimney in response to change in chemical energy landscapes. However, the specific chemistry of the hydrothermal plume did not have a visible influence on microbial community structure. Instead, the pelagic communities seemed to be subject to the dispersal influence ...
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