Astrid Gärdes

• Diatom blooms are frequently terminated by aggregation of algae and subsequent sedimentation in form of marine snow. Transparent exopolymer particles (TEP) are produced by phytoplankton or bacteria and play an important role in aggregate formation. This study was aimed at analyzing the interactions between bacteria and phytoplankton and their influences on TEP formation, aggregation dynamics, and sedimentation of phytoplankton. For this, an in vitro model system consisting of the diatom Thalassiosira weissflogii and the marine Gammaproteobacterium Marinobacter adhaerens sp. nov. HP15 was established and was analyzed in rolling tank experiments. From the results it could be concluded, that interaction of M. adhaerens with T. weissflogii increased aggregate formation and particle sinking. Detailed investigation of this interaction revealed differences in the response of T. weissflogii, towards bacteria, namely diatom growth and exopolymer production. A careful interpretation of these results suggested a close and specific interaction of HP15 with the diatom in a mutualistic relationship. The type of interaction shifted to commensalism during nutrient stress conditions. Depending on the nature of M. adhaerens - T. weissflogii interactions the extent and quality of algal exudation in form of TEP dramatically changed and consequently influenced aggregate formation. Another part of this thesis focused on the chemotaxonomic and genomic characterization of M. adhaerens as the bacterial counterpart of the established model system. In this context the bacterial genome sequence was determined and a genetic system for molecular work has been established. A site-directed deletion mutation of the flagellin-encoding gene fliC of HP15 was generated and confirmed the suitability of M. adhaerens as a genetically accessible model organism. With this bilateral model system being generated, a species-specific interaction can now be intensively and mechanistically studied in depth.