Microbial processes and element cycling from micro- to meso-scale : from single cells and aggregates to the whole water column perspective.

Marine microorganisms are important drivers of the C-, N- and S- cycles on earth. They can adapt to various substrates and, thus, inhabit extreme environments, such as sub- and anoxic or even sulfidic waters or micro-zones within sinking particles. The latter still represent a major black box in microbial ecology due to their fragile nature, which makes them inaccessible for detailed micro-scale observations. Moreover, sinking particles and their complex communities represent a biogeochemical link between small- and large-scale processes because they are shaped by a wide variety of physical and biological processes ranging from the micro- to meso-scale. In this thesis, cutting-edge methodologies were applied to investigate microbial processes of free-living and particle-associated organisms at the micro-scale, and to build strategies to integrate these processes into a broader understanding of microbial dynamics at oceanographic scales of relevance to the global ocean.