Microbes are key drivers of global biogeochemical cycles and do so by interacting in complex communities. These interactions are based upon different needs, for the incorporation (1) of energy sources and (2) of essential building block metabolites. I am particularly interested in the investigation of metabolic interdependencies that originate from the inability to synthesize essential anabolic metabolites the so-called auxotrophy. Auxotroph microbes are streamlined and thus save energetic costs by taking advantage of the metabolism of other community members. At the same time auxotrophs obligately rely on the metabolic production of another fraction in the community. In this sense, auxotrophs cheat on microbes that are syntrophic for their own lost function. My research focusses on the question if true `cheaters` exist or if `cheaters` are actually ‘traders’ and functionally complement each other by reciprocally exchange metabolites with other microbes in the community. I want to find out how auxotrophies and functional complementarity impact taxonomic and metabolic complexity in interacting cohorts and community stability.