Quantification of total microbial biomass and metabolic activity in subsurface sediments Metabolically active microbial communities are present in a wide range of subsurface environments. Techniques like enumeration of microbial cells activity measurements with radiotracer assays and the analysis of porewater constituents are currently being used to explore the subsurface biosphere alongside with molecular biological analyses. However many of these techniques reach their detection limits due to low microbial activity and abundance. Direct measurements of microbial turnover not just face issues of insufficient sensitivity they only provide information about a single specific process but in sediments many different process can occur simultaneously. Therefore the development of a new technique to measure total microbial activity would be a major improvement. A new tritium based hydrogenase enzyme assay appeared to be a promising tool to quantify total living biomass even in low activity subsurface environments. In this PhD project total microbial biomass and microbial activity was quantified in different subsurface sediments using established techniques (cell enumeration and pore water geochemistry) as well as a new tritium based hydrogenase enzyme assay. By using a large database of our own cell enumeration data from equatorial Pacific and north Pacific sediments and published data it was shown that the global geographic distribution of subseafloor sedimentary microbes varies between sites by 5 to 6 orders of magnitude and correlates with the sedimentation rate and distance from land. Based on these correlations global subseafloor biomass was estimated to be 4.1 petagram C and ~0.6 % of Earths total living biomass which is significantly lower than previous estimates. Despite the massive reduction in biomass the subseafloor biosphere is still an important player in global biogeochemical cycles. To understand the relationship between microbial activity abundance and organic matter flux into the sediment an expedition to the equatorial Pacific upwelling area and the north Pacific Gyre was carried out. Oxygen respiration rates in subseafloor sediments from the north Pacific Gyre which are deposited at sedimentation rates of 1 mm per 1000 years showed that microbial communities could survive for millions of years without fresh supply of organic carbon. Contrary to the north Pacific Gyre oxygen was completely depleted within the upper few millimeters to centimeters in sediments of the equatorial upwelling region due to a higher supply of organic matter and higher metabolic activity. So occurrence and variability of electron acceptors over depth and sites make the subsurface a complex environment for the quantification of total microbial activity. Recent studies showed that electron acceptor processes which were previously thought to thermodynamically exclude each other can occur simultaneously. So in many cases a simple measure of the total microbial activity would be a better and more robust solution than assays for several specific processes for example sulfate reduction rates or methanogenesis. Enzyme or molecular assays provide a more general approach as they target key metabolic compounds. Since hydrogenase enzymes are ubiquitous in microbes the recently developed tritium based hydrogenase radiotracer assay is applied to quantify hydrogenase enzyme activity as a parameter of total living cell activity. Hydrogenase enzyme activity was measured in sediments from different locations (Lake Van Barents Sea Equatorial Pacific and Gulf of Mexico). In sediment samples that contained nitrate we found the lowest cell specific enzyme activity around 10^( 5) nmol H_(2) cell^( 1) d^( 1). With decreasing energy yield of the electron acceptor used cell specific hydrogenase activity increased and maximum values of up to 1 nmol H_(2) cell^( 1) d^( 1) were found in samples with methane concentrations of >10 ppm. Although hydrogenase activity cannot be converted directly into a turnover rate of a specific process cell specific activity factors can be used to identify specific metabolism and to quantify the metabolically active microbial population. In another study on sediments from the Nankai Trough microbial abundance and hydrogenase activity data show that both the habitat and the activity of subseafloor sedimentary microbial communities have been impacted by seismic activities. An increase in hydrogenase activity near the fault zone revealed that the microbial community was supplied with hydrogen as an energy source and that the microbes were specialized to hydrogen metabolism