Shalin Seebah

• Transparent exopolymeric particles (TEP) and marine snow aggregates are vital components of the oceanic carbon cycle, leading to a substantial fraction of organic carbon sinking to depth or being provided for further recycling. Diatom-associated bacteria have been recently shown to directly impact TEP production and aggregate formation. However, very little is known about the molecular components that govern this interaction or the dynamics of TEP production and marine aggregate formation under changing environmental conditions. By combining molecular techniques and ecological experiments, we use an interdisciplinary approach to unveil hitherto unknown processes of diatom-bacteria interactions. As part of a collaborative and concerted effort, we initially established a genetically accessible bilateral model system consisting of the diatom Thalassiosira weissflogii and the marine gammaproteobacterium Marinobacter adhaerens HP15. Herein, we taxonomically established M. adhaerens HP15 as a novel member of the Marinobacter genus, revealed its genome sequence and established a genetic system to allow for the precise manipulation of this bacterium at the molecular level. In a second part, we used the established genetic toolbox to investigate the role of M. adhaerens HP15s motility during its interaction with the diatom. By generating M. adhaerens HP15 flagellum- and MSHA type IV pilus-deficient mutants, we demonstrate that a fullyfunctional flagellum is a pre-requisite for the bacterial attachment to both abiotic and diatom surfaces. We further show that the MSHA type-IV pilus is important for attachment, albeit to a lesser extent. Although both cellular appendages were shown to be crucial for attachment to diatom surfaces, this type of attachment was demonstrated to not be essential for inducing the formation of diatom-borne transparent exopolymeric particles (TEP). In the final part of this work, how TEP production dynamics and aggregate formation might be impacted in putative future oceanic scenarios was investigated. The results of our study suggest that the combined effect of ocean acidification and increased temperature might lead to a significant reduction in aggregate formation and sinking velocities of marine aggregates. We suggest that a combination of ocean acidification and global warming may severely impact the vertical transport of particulate organic matter in a future ocean.