Table of contents Chapter 1 General introduction and thesis outline Chapter 2 A genomic comparison of syntrophic and non-syntrophic butyrate-and propionate-degrading bacteria points to a key role of formate in syntrophy Chapter 3 Metabolic flexibility of Syntrophobacter fumaroxidans: Syntrophic vs sulfate-reducing lifestyle during growth on propionate Chapter 4 Comparative proteome analysis of propionate degradation by Syntrophobacter fumaroxidans in pure culture and in coculture with methanogens Chapter 5 Proteomic analyses of Methanospirillum hungatei and Methanobacterium formicicum grown in a syntrophic partnership with Syntrophobacter fumaroxidans and in pure culture with H2/CO2 or formate. Chapter 6 Microbial community analysis and performance of a mesophilic anaerobic membrane bioreactor treating pot ale Chapter 7 General discussion