"Our aim is to understand basic functions of the predominant microbial life on our planet: anaerobic communities buried in the seabed and subsisting at the energetic limit for cellular processes. An estimated 90% of all prokaryotic microorganisms on Earth, comprising 1/10 of all living biomass, exist in the deep subsurface biosphere with a cellular energy flux that is orders of magnitude below anything studied in laboratory cultures so far. The cells are essentially non-growing with mean generation times of hundreds to thousands of years. Yet, these microorganisms drive major processes in the geosphere and control element cycles that affect hydrocarbon reservoirs, ocean chemistry, and global climate on geological time scales.We will use and develop new approaches to study microbial life under extreme energy limitation with the aim to understand the microbial and environmental interactions in the deep biosphere. We will particularly target methanogenesis as a key process in the marine carbon cycle and the great diversity of unknown archaea. We will explore mean cellular energy fluxes of subsurface microbial communities and estimate the fraction of dormant versus active cells. We will determine the turnover rate of living and dead microbial biomass in the deep biosphere and analyze the energetic or kinetic controls on key metabolic processes. We will perform high-capacity genomic sequence analyses and use sensitive chemical and isotopic techniques to search for the coupling between phylogenetic identity and metabolic potential of dominant microorganisms. We will also analyze the genetic potential and physiological activity at the single-cell level to identify this coupling. Finally, we will apply the new microbiological and biogeochemical understanding of subsurface carbon mineralization in a global model of methane cycling in the sea bed."