Carbon fixation is colour-coded in green, fermentation in blue and complete oxidation of fermentation products in red. (a) Metabolic interactions between seven microorganisms. Carbon fixation is carried out by a sulfate-reducing bacterium (Desulfobulbaceae c16a) and by a non-sulfate reducing bacterium (Rhodospirillaceae c57). Both use H2 as an electron source. All other organisms are heterotrophic. A fermenting bacterium (Hyphomonas c22) can oxidize organic macromolecules to acetate, while sulfate-reducing bacteria (Peptococcaceae c4a, c8a, c23 and Desulfatitalea c12) oxidizes acetate to CO2. All protein data used to build this metabolic interaction are listed in Supplementary Data 6. (b) A simplified model of this carbon loop based on the presence of CO2, H2 and sulfate is illustrated.