Methanococcus thermolithotrophicus can use either H2 or formate as the electron donor for methanogenesis from CO2. Resuspended-cell experiments revealed that the ability to use H2 as the source of electrons for methanogenesis was constitutive; cells grown on formate or H2-CO2 were equally capable of H2-CO2 methanogenesis. The ability to metabolize formate at high rates was observed only in cells previously grown on formate. Two such strains were distinguished: strain F and strain HF. Strain F was repeatedly grown exclusively on formate for over 3 years; this strain showed a constitutive capacity to metabolize formate to methane, even after subsequent repeated transfers to medium containing only H2-CO2. Strain HF could only metabolize formate to methane when grown in the presence of formate with no H2 present; this strain was recently derived from another strain (H) that had been exclusively grown on H2-CO2 and which upon initial transfer to formate medium could only metabolize formate to methane at a very slow rate. Initial adaptation of strain H to growth on formate was preceded by a long lag. The specific activities of hydrogenase and formate dehydrogenase in cell extracts derived from these different strains confirmed these findings. Similar levels of hydrogenase were observed in all strains, independent of the presence of H2 in the growth medium medium. High levels of formate dehydrogenase were also constitutive in strain F. Only low formate dehydrogenase activities were observed in strain H. High levels of formate dehydrogenase were observed in strain HF only when these cells were grown with formate in the absence of H2. In all strains the two- to threefold fluctuations of both hydrogenase and formate dehydrogenase cell-free activities were observed during growth, with peak activities reached in the middle of the exponential phase
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