Freshwater wetlands are a major source of the greenhouse gas methane but can also act as carbon sink, storing currently more than one third of the terrestrial organic carbon. Understanding their intertwined biogeochemistry and microbiology is therefore indispensable to foresee their influence to positive and negative climate feedback cycles. This proposal aims to elucidate the identity and ecophysiology of sulfate reducing microorganisms (SRM) driving a highly active but hidden sulfur cycle in wetlands, which is not apparent from the low standing pools of sulfate and thus has been severely understudied. Since sulfate reduction effectively competes with methanogenic degradation of organic matter, SRM have an important control function on methane production in wetlands. Little is known about wetland SRM. This stands in contrast to the high diversity of evolutionary deep-branching dsrAB genes, which are functional markers for SRM, indicating a large number of yet undiscovered SRM in wetlands. The expected scientific outcome of this proposal comprise: (i) identification of microorganisms involved in the hidden sulfur cycle with focus on SRM, (ii) elucidating their substrate preferences and preferred environmental conditions, (iii) identifying their interaction partners, and (iv) linking their function to their genome as well as their major transcripts and proteins. This will be achieved by an interdisciplinary approach, where (i) structure-function methods like stable isotope probing are linked to meta-genomics and -proteomics, (ii) high-throughput amplicon sequencing of phylogenetic markers and metatranscriptomics is combined with controlled experimental setups under monitored biogeochemical parameters, and (iii) by targeted cultivation of novel microorganisms. This proposal will be vital for me to build up an own research group, to strengthen my international collaborative network, and to maximize my integration in the European Research Area.