Vast communities of microorganisms in the sub-seafloor biosphere are responsible for the degradation of deeply buried organic matter (OM) and drive complex metabolic processes of OM mineralization. It remains unknown how the microorganisms subsist at the available energetic limits for life with extremely slow turnover. This project aims to determine the energetic and kinetic controls on the major metabolic processes, in particular the role of small organic acids. These occur in a broad range of concentrations in the pore fluid of recent and old sediments and are key intermediates in the microbial food web. The research will develop and apply new and highly sensitive analytical techniques, including 2-dimensional ion chromatography combined with mass spectrometric detection (2D IC-MS). With new analytical capabilities, combined with measurements of microbial substrate turnover rates, we can analyze both the thermodynamic and the kinetic regulation of predominant microbial processes. In international collaboration, including the molecular biological expertise at the Center for Geomicrobiology, it is the ambitious goal to determine, for the first time, the mean cellular energy flux throughout all biogeochemical zones of a sediment column, from the highly active oxic surface to the deep subsurface and from substrate rich to substrate poor sediments. We will focus on three systems of study: A) Coastal marine sediment of Aarhus Bay, an easily accessible and intensively studied test site. B) The North Pacific where IODP (Integrated Ocean Drilling Program) Expedition 337 will drill two km deep sediment harboring Eocene lignite layers. The low-mature OM encountered here is expected to support diverse microbial communities in spite of the 50 million year age. C) The Baltic Sea where IODP Expedition 347 will drill through several glacial-interglacial sequences with extreme variations in past environmental conditions including OM depleted sedimentary layers.