Formate-Dependent H2 Production by the Mesophilic Methanogen Methanococcus maripaludis

Methanococcus maripaludis, an H-2-and formate-utilizing methanogen, produced H-2 at high rates from formate. The rates and kinetics of H-2 production depended upon the growth conditions, and H-2 availability during growth was a major factor. Specific activities of resting cells grown with formate or H-2 were 0.4 to 1.4 U.mg(-1) (dry weight). H-2 production in formate-grown cells followed Michaelis-Menten kinetics, and the concentration of formate required for half-maximal activity (K-f) was 3.6 mM. In contrast, in H-2-grown cells this process followed sigmoidal kinetics, and the Kf was 9 mM. A key enzyme for formate-dependent H-2 production was formate dehydrogenase, Fdh. H-2 production and growth were severely reduced in a mutant containing a deletion of the gene encoding the Fdh1 isozyme, indicating that it was the primary Fdh. In contrast, a mutant containing a deletion of the gene encoding the Fdh2 isozyme possessed near-wild-type activities, indicating that this isozyme did not play a major role. H-2 production by a mutant containing a deletion of the coenzyme F-420-reducing hydrogenase Fru was also severely reduced, suggesting that the major pathway of H-2 production comprised Fdh1 and Fru. Because a Delta fru-Delta frc mutant retained 10% of the wild-type activity, an additional pathway is present. Mutants possessing deletions of the gene encoding the F-420-dependent methylene-H4MTP dehydrogenase (Mtd) or the H-2-forming methylene-H4MTP dehydrogenase (Hmd) also possessed reduced activity, which suggested that this second pathway was comprised of Fdh1-Mtd-Hmd. In contrast to H-2 production, the cellular rates of methanogenesis were unaffected in these mutants, which suggested that the observed H-2 production was not a direct intermediate of methanogenesis. In conclusion, high rates of for matedependent H-2 production demonstrated the potential of M. maripaludis for the microbial production of H-2 from formate.