H2-Independent Growth of the Hydrogenotrophic Methanogen Methanococcus maripaludis

Hydrogenotrophic methanogenic Archaea require reduced ferredoxin as an anaplerotic source of electrons for methanogenesis. H-2 oxidation by the hydrogenase Eha provides these electrons, consistent with an H-2 requirement for growth. Here we report the identification of alternative pathways of ferredoxin reduction in Methanococcus maripaludis that operate independently of Eha to stimulate methanogenesis. A suppressor mutation that increased expression of the glycolytic enzyme glyceraldehyde-3-phosphate:ferredoxin oxidoreductase resulted in a strain capable of H-2-independent ferredoxin reduction and growth with formate as the sole electron donor. In this background, it was possible to eliminate all seven hydrogenases of M. maripaludis. Alternatively, carbon monoxide oxidation by carbon monoxide dehydrogenase could also generate reduced ferredoxin that feeds into methanogenesis. In either case, the reduced ferredoxin generated was inefficient at stimulating methanogenesis, resulting in a slow growth phenotype. As methanogenesis is limited by the availability of reduced ferredoxin under these conditions, other electron donors, such as reduced coenzyme F-420, should be abundant. Indeed, when F-420-reducing hydrogenase was reintroduced into the hydrogenase-free mutant, the equilibrium of H-2 production via an F-420-dependent formate:H-2 lyase activity shifted markedly toward H-2 compared to the wild type. IMPORTANCE Hydrogenotrophic methanogens are thought to require H-2 as a substrate for growth and methanogenesis. Here we show alternative pathways in methanogenic metabolism that alleviate this H-2 requirement and demonstrate, for the first time, a hydrogenotrophic methanogen that is capable of growth in the complete absence of H-2. The demonstration of alternative pathways in methanogenic metabolism suggests that this important group of organisms is metabolically more versatile than previously thought.