Formate-Dependent H 2 Production by the Mesophilic Methanogen Methanococcus maripaludis

Methanococcus maripaludis , an H2 - and formate-utilizing methanogen, produced H2 at high rates from formate. The rates and kinetics of H2 production depended upon the growth conditions, and H2 availability during growth was a major factor. Specific activities of resting cells grown with formate or H2 were 0.4 to 1.4 U·mg−1 (dry weight). H2 production in formate-grown cells followed Michaelis-Menten kinetics, and the concentration of formate required for half-maximal activity (Kf ) was 3.6 mM. In contrast, in H2 -grown cells this process followed sigmoidal kinetics, and theKf was 9 mM. A key enzyme for formate-dependent H2 production was formate dehydrogenase, Fdh. H2 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. H2 production by a mutant containing a deletion of the coenzyme F420 -reducing hydrogenase Fru was also severely reduced, suggesting that the major pathway of H2 production comprised Fdh1 and Fru. Because a Δfru -Δfrc mutant retained 10% of the wild-type activity, an additional pathway is present. Mutants possessing deletions of the gene encoding the F420 -dependent methylene-H4 MTP dehydrogenase (Mtd) or the H2 -forming methylene-H4 MTP dehydrogenase (Hmd) also possessed reduced activity, which suggested that this second pathway was comprised of Fdh1-Mtd-Hmd. In contrast to H2 production, the cellular rates of methanogenesis were unaffected in these mutants, which suggested that the observed H2 production was not a direct intermediate of methanogenesis. In conclusion, high rates of formate-dependent H2 production demonstrated the potential ofM. maripaludis for the microbial production of H2 from formate.