Substrate control of community composition and functional adaptation in marine bacterioplankton

Abstract: A drop of sea-water is teeming with a million of bacteria, on which pelagic food-webs and biogeochemical cycles depend. These bacteria thrive on a wide range of dissolved organic carbon (DOC) compounds produced through biotic and abiotic processes. Molecular analyses have over the past decades shown that specific bacterial taxa differ in their capacity to exploit DOC, suggesting a tight link between bacterial community composition (BCC) and ocean biogeo-chemistry. Therefore, an understanding of how resource availability and mortality agents drive BCC and bacterial functional adaptation is a prerequisit for predictions of how marine ecosystems will respond to future global change.In this thesis, I have studied BCC and bacterial functionality in response to various controlling factors relevant in an environmental changes perspective. For instance, the extensive regional warming in Antarctica induces the proliferation of icebergs. By investigating the bacterioplankton in the surrounding of a drifting iceberg, hydrographical perturbations driven by the iceberg were found to affect BCC, functionality and the capacity of indigenous taxa to utilize specific DOC compounds. Furthermore, a study of community succession during DOC utilization assays demonstrated that bacterial assemblages adapt to the gradual exhaustion of available DOC through community compositional succession. In addition, the variation in substrate availability and temperature may also affect BCC in eutrophic systems.While substrate availability can have an important impact on BCC and bacterial functionality, it is also important to study the cascading effects of higher trophic levels on bacteria. During a mesocosm experiment, the presence of an invasive gelatinous top-predator was shown to have only limited effects on the structure and function of the bacterial community in the Baltic Sea due to nutrient limiting conditions and to the overall complexity of the food-web. However, this top-predator may have direct bottom-up impact on bacteria in its close surrounding.The results presented in this thesis show that the bacterioplankton is sensitive to the availability of substrates and that bacterial community composition responds to contemporary environmental conditions. These results contribute to our understanding of how ecosystem disturbances affect marine bacterioplankton; insights of relevance to biogeochemistry and food-webs in the oceans.