Diversity and function of microbial communities in sediments from different deep-sea habitats

Abstract

Deep-sea floors are diverse environments that range from permanently cold (desert-like plains) to hot systems (hydrothermal vents). In hot systems, primary productivity is performed by microbial communities which use chemical energy generated by geological processes (lithotrophy). This energy transfer from mantle to the ocean is as yet poorly understood, and the diversity and activity of microbes at these sites is therefore an interesting target for microbial ecologists. However, the vast majority of all globally distributed deep-sea sediments is permanently cold. The distribution of microorganisms in deep-sea floors and the factors controlling it at small and large scales are important for the understanding of the mechanisms that regulate biodiversity. During this thesis, hydrothermally influenced sediments of the peridotite-hosted Logatchev hydrothermal vent field were investigated in an interdisciplinary study to reveal the diversity and activity of the associated microbial communities. In situ microprofiles showed that these sediments were controlled by diffusive transport, instead of previously reported advective processes. White mats on top of these sediments resemble Beggiatoa-mats from the basalt- hosted field in the Guaymas Basin. However, fluorescence in situ hybridization revealed that the overlying sulfur-mats were dominated by filamentous Epsilonproteobacteria or a vibrioid Arcobacter-type. The microbial community of the surface layer was predominantly composed of Epsilonproteobacteria (7-21%), Deltaproteobacteria (20-21%), and Bacteroidetes (19- 20%). Comparative 16S rRNA gene sequence analyses identified various bacteria related to those found in basaltic systems. The presence of an active microbial community in these sediment surface layers was confirmed by high oxygen consumption rates. Geochemical analyses detected metal-sulfides in the sediments, elemental sulfur in the mats and an intensive sulfide flux from below. Ex situ incubations and turnover rate experiments revealed that sulfide is consumed and that sulfate-reduction is performed by the surface sediment microbial community. This was consistent with the detection of aprA-genes and soxB-genes, which are both key genes of the sulfur cycle. Further metabolic capabilities such as denitrification and CO2-fixation were indicated by primary analysis of metagenomic data retrieved by pyrosequencing. So far, our analyses suggest that sulfur cycling is one of the driving forces for primary production and biomass formation in surface sediments of the ultramafic-hosted Logatchev hydrothermal vent fields. Therefore, major differences in microbial composition between basalt- and peridotite-hosted fields were not detected. Hydrothermally influenced sediments from the Mid-Atlantic Ridge and permanently cold sediments from three basins of the eastern South Atlantic Ocean were investigated to examine the ability of microorganisms to disperse in the deep-sea. Besides spatial distance, the structuring effect of the physical barrier Walvis Ridge, which separates the Cape Basin from the other two basins, was determined. The analysis of 16S rRNA gene sequences of the deep- sea sediments revealed phylotypes affiliated with Gammaproteobacteria, Deltaproteobacteria and Acidobacteria, which were present in all three basins. The distribution of these shared phylotypes seemed to be influenced neither by the Walvis Ridge nor by different deep water masses, suggesting a high dispersal capability, as also indicated by low distance decay relationships. In contrast, the comparison of the total bacterial diversity of the cold sediments as well as of the hydrothermally influenced sediments revealed significant differences between the microbial communities. Within the Logatchev field and therefore for small distances (<10 km) microbial biogeography was primarily controlled by environmental heterogeneity. In contrast, the analysis of the permanently cold sediments revealed that at intermediate (10 3000 km) and large scales (>3000 km), both factors influenced bacterial diversity, indicating a complex interplay of local contemporary environmental effects and dispersal limitation.