Methanogenic syntrophies and their potential distribution. (A) The digestive system of ruminants and other animals, fresh water sediments, and anaerobic digesters for the treatment of agricultural waste are representative environments where syntrophy-based methanogenesis is vigorously active, likely as a result of syntrophic associations between methanogenic archaea and hydrogen-producing microorganisms such as D. vulgaris. (B) Schematic representation of an anaerobic trophic cascade. Hydrogen produced in the fermentation of biopolymers and fatty acids serve as the electron donor for the secondary fermenters and subsequently for methanogenic reduction of carbon dioxide. (C) FISH microphotograph of a syntrophic association between M. maripaludis (red) and D. vulgaris (green), the model syntrophy for this study. (D) Diagram representing the main metabolic interactions between D. vulgaris and M. maripaludis in methanogenic co-cultures. Hydrogen, carbon dioxide, and acetate are known substrates for M. maripaludis; the transfer of formate from D. vulgaris to M. maripaludis is proposed based on our data.