Identity and function of key bacterial groups in Arctic deep-sea surface sediments

The deep-sea floor covers about 65% of the Earth s surface and benthic biomass is dominated by highly diverse bacterial communities. Bacterial carbon cycling in deep-sea sediments plays a crucial role in global biogeochemical cycles, and remineralization efficiency of organic carbon can be more than 97%. However, key bacteria relevant for carbon turnover and ecosystem functioning remain unknown. Benthic bacteria mainly depend on organic carbon supply from the surface ocean, and will therefore likely be affected by changing surface ocean conditions. The Arctic Ocean is already impacted by environmental changes more rapidly here than in any other ocean region and will be impacted even more in the future. This turns the Arctic Ocean into an important study site to understand the effects of environmental changes on bacterial communities and ecosystem functioning, such as carbon cycling. At the same time, the Arctic Ocean remains to a large extent understudied, and little is known about the identity of key bacterial groups, which could be useful as indicators to describe the state of the ecosystem and to monitor community response to changing environmental conditions. Consequently, the goals of this thesis include the identification of indigenous key bacteria in deep-sea sediments and their metabolic potential, as well as the development of a better understanding of the specific response of Arctic deep-sea bacterial communities to changes in the supply of organic matter. The Long-Term Ecological Research site HAUSGARTEN (HG) is one out of two open ocean, long-term observatories in a polar region, and therefore provided a unique opportunity to study key bacterial groups from Arctic deep-sea sediments. Chapters I and II present one of the first characterizations of a globally sequence-abundant sediment bacterial group, the JTB255 marine benthic group (JTB255). Cell counts with newly designed probes evidenced high cell abundances in coastal (Chapter I) and deep-sea sediments (Chapter II). Labeling experiments together with metatranscriptomic data suggested a chemolithoautotrophic lifestyle, with a potential high importance for sulfur-based carbon fixation in coastal sediments (Chapter II). Furthermore, genomic analyses of single cells emerged as a powerful means to provide first insights into the metabolic potential of JTB255 representatives in deep-sea sediments, suggesting a heterotrophic lifestyle with oxygen as terminal electron acceptor (Chapter II). Genomic analysis showed that JTB255 encode enzymes for the oxidative degradation of polymeric cell material such as membranes and cell walls, suggesting recalcitrant organic carbon sources in marine sediments. Therefore, it is hypothesized for the first time that some representatives of JTB255 might be involved in the cycling of a major class of refractory sediment organic matter, potentially explaining their global ecological success. In an ex situ experimental approach, the response of Arctic benthic bacterial deep-sea communities at HG to different types of detritus was explored (Chapter III). This is the first experimental study investigating the response of bacterial deep-sea communities to the addition of natural food sources by combining measurements of community function with the analysis of high resolution taxonomic community structure. Our results provide evidence that differences in organic matter composition lead to significant changes in bacterial community structure and function at the seafloor, which can affect carbon turnover and retention in the deep sea. In addition, opportunistic groups of bacteria were identified that may serve as indicator taxa for different organic matter sources at this site. In Chapter IV, a pilot study is presented which addresses an issue often discussed in deep-sea research, i.e. the unknown effects of sample retrieval from high-pressure environments on bacterial communities. Therefore, the influence of de- and recompression on deep-sea sediment bacteria, as inherently imposed during sediment retrieval and subsequent laboratory experiments, was studied in a small-scale experiment. Results indicated few effects of de- and recompression on bacterial community structure within the experimental time frame, but contained evidence for changes in the metabolic activity of specific taxa, after the retrieval of decompressed samples from the seafloor. These observations remain to be verified with further sample replication. In summary, this thesis contributes to the identification of candidate key bacterial groups. It further provides valuable insights into bacterial diversity and function in Arc-tic deep-sea sediments and will help to assess impacts of future climate scenarios on pelago-benthic coupling in the Arctic.