ENERGETICS AND REGULATIONS OF FORMATE AND HYDROGEN METABOLISM BY METHANOBACTERIUM-FORMICICUM

Accumulation of formate to millimolar levels was observed during the growth of Methanobacterium formicicum species on H-2-CO2. Hydrogen was also produced during formate metabolism by M. formicicum. The amount of formate accumulated in the medium or the amount H-2 released in gas phase was influenced by the bicarbonate concentration. The formate hydrogenlyase system was constitutive but regulated by formate. When methanogenesis was inhibited by addition of 2-bromoethane sulfonate, M. formicicum synthesized formate from H-2 plus HCO3- or produced H-2 from formate to a steady-state level at which point the Gibbs free energy (DELTAG') available for formate synthesis or H-2 production was approximately -2 to -3 kJ/reaction. Formate conversion to methane was inhibited in the presence of high H-2 pressure. The relative rates of conversion of formate and H-2 were apparently controlled by the DELTAG' available for formate synthesis, hydrogen production, methane production from formate and methane production from H-2. Results from C-14-tracer tests indicated that a rapid isotopic exchange between HCOO- and HCO3- occurred during the growth of M. formicicum on H-2-CO2. Data from metabolism of C-14-labelled formate to methane suggested that formate was initially split to H-2 and HCO3- and then subsequently converted to methane. When molybdate was replaced with tungstate in the growth media, the growth of M. formicicum strain MF on H-2-CO2 was inhibited although production of methane was not. Formate synthesis from H-2 was also inhibited.