Anaerobic digestion (AD) of organic matter is a common example of a process that converts waste into a renewable resource, for instance biogas containing methane. Although anaerobic digestion is widely applied on large scale, little quantitative understanding is available about
the exchange of metabolites between the different microbial groups involved in the conversion of complex substrates to biogas. Therefore, the AD process is typically treated as a black box process, and consequently only pragmatic tools are available to analyze it. This thesis contributed to open the black box of AD by getting a detailed understanding of the
electron fluxes occurring in the ultimate steps of the process: the syntrophic interactions between acetogenic bacteria and methanogenic archaea. The research focused on unraveling the dominant mechanisms involved in interspecies electron transfer in syntrophic methanogenic communities. We obtained fundamental knowledge of the key features of
methane formation by studying these communities at molecular, cell and community level.