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

Abstract

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 K _f 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 Δ fru -Δ 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-H ₄ MTP dehydrogenase (Mtd) or the H ₂ -forming methylene-H ₄ MTP 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 formate-dependent H ₂ production demonstrated the potential of M. maripaludis for the microbial production of H ₂ from formate.