Much of the organic matter produced on land and in aquatic systems is eventually deposited at the seafloor, making marine sediments the largest organic carbon sink on Earth. Predominantly, this organic matter is either remineralized to CO2 via biological activity or preserved over geologic time scales. A substantial fraction of remineralization is carried out by microbes that live below the sediment surface in the absence of O2. To date, these anaerobic microbes remain largely uncharacterized. A detailed knowledge of these microbes and their activities is, however, essential to understanding the role of marine sediments in the global carbon cycle and in regulating ocean chemistry, atmospheric gas composition, and ultimately Earth’s climate. In the following project, I propose to investigate the fate of several common low-molecular weight carbon compounds (formate, acetate, lactate, methanol, methyl chloride) in anaerobic marine sediments from three different locations, Aarhus Bay off of Jutland (Denmark), the Guaymas Basin off of Baja California (Mexico), and the Bering Sea off of Alaska (USA). By inoculating sediments with media containing single substrates or mixtures of substrates, I will select for growth of microbes specialized to use single substrates and ones adapted to use several substrates simultaneously. The three metabolic groups I will select for are sulfate reducers, methanogens, and acetogens, all three of which are key players in organic matter remineralization in anoxic marine sediments. Substrate consumption will be linked to consumer identity by supplying substrates in 13C-labeled form and by monitoring substrate uptake and incorporation into biomass via stable-isotope probing (SIP). This project would allow me to after 12 years abroad transfer state-of-the-art knowledge from leading laboratories in the USA and Japan to the EU, and carry out an important and unpreceded research project on the global carbon cycle.