Archaea are increasingly recognized as globally abundant organisms that mediate important processes controlling greenhouse gases and nutrients. Our latest work, published in PNAS and Nature, suggests that Archaea dominate the biomass in the subseafloor. Their unique ability to cope with extreme energy starvation appears to be a selecting factor. Marine sediments are of crucial importance to the redox balance and climate of our planet but the regulating role of the deep biosphere remains one of the great puzzles in biogeochemistry. The unique and diverse sedimentary Archaea with no cultured representatives, so-called benthic archaea, are key to understanding this system. Their presumed ability to degrade complex recalcitrant organic residues highlights their relevance for the carbon cycle and as potential targets for biotechnology. I propose to study the role of benthic archaea in the carbon cycle and in the deep biosphere and to explore their life strategies. This task requires an interdisciplinary frontier research approach at the scale of an ERC grant, involving biogeochemistry, earth sciences, and microbiology. Central to my research strategy is the information contained in structural and isotopic properties of membrane lipids from benthic archaea, an area of research spearheaded by my lab. In-depth geochemical examination of their habitat will elucidate processes they mediate. Metagenomic analysis will provide a phylogenetic framework and further insights on metabolism. At the Archaeenzentrum in Regensburg, we will grow model Archaea under a set of environmental conditions and examine the impact on cellular lipid distributions in order to develop the full potential of lipids as proxies for studying nearly inaccessible microbial life. Attempts to enrich benthic archaea from sediments will complement this approach. This frontier research will constrain the role of benthic archaea in the Earth system and examine the fundamental properties of life at minimum energy.