Der bakterielle Schwefelkreislaufim Benguela Auftriebssystem

Coastal upwelling regions are economically important as they contribute to ~50% of the world's fisheries landings. High primary production and extensive mineralization of organic matter often lead to oxygen minimum zones (OMZ) and the transient formation of hydrogen sulfide with severe consequences for coastal ecosystems such as the Benguela Upwelling System (BAG). However, the involved microorganisms catalyzing the cycling of sulfur cycle in the water column are still unknown. In this thesis, the microbial community structure in the BAG was studied in a high spatiotemporal resolution by the 16S rRNA approach to identify potential key players of the sulfur cycle.In a sulfidic lens covering 7,000 km2 on the Namibian Shelf both 16S rRNA gene libraries and FISH identified two discrete populations of Gamma- and Epsilonproteobacteria that accounted for up to 20% of the bacterioplankton. Their cell numbers increased from the nitrate-sulfide transition zone to the sediment that indicated a chemolithotrophic oxidation of sulphide with nitrate. Indeed, one population was closely related to gammaproteobacterial sulfur oxidizing endosymbionts (GSO), whereas the second population affiliated with the sulfur oxidizing Arcobacter sulfidicus suggesting their sulfur oxidizing potential. In addition, the congruent phylogenies of 16S rRNA and of the diagnostic marker genes aprA and dsrA strongly confirmed the sulfur oxidizing potential of the GSO cluster. Apparently both populations can efficiently oxidize sulfide, which is a so far unique example for a physiologically-constrained bacterial group catalyzing a large-scale detoxification process in the open ocean.Besides these organisms, in particular the high dsrA diversity indicated other, still unknown sulfur oxidizers in the water column. Sulfate reducing bacteria were too low to account for the observed sulfide formation. Additional abundant, usually aerobic or phototrophic organisms like the SAR11 cluster and the Roseobacterio lineage also occurred in the OMZ, but their role remains unclear. The occurrence of the GSO cluster in other OMZ such as the Peruvian upwelling system (PUS) may reflect yet unrecognized sulfur cycling in various coastal habitats. Interestingly, the comparative analysis of microbial diversity patterns of the BAG and PUS consistently displayed the highest diversity in the OMZ that suggested similar factors streamlining the community structure.