Energetics and regulations of formate and hydrogen metabolism by Methanobacterium formicicum

Abstract

Accumulation of formate to millimolar levels was observed during the growth of Methanobacterium formicicum species on H2−CO2. Hydrogen was also produced during formate metabolism by M. formicicum. The amount of formate accumulated in the medium or the amount H2 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 H2 plus HCO inf3 sup- or produced H2 from formate to a steady-state level at which point the Gibbs free energy (ΔG′) available for formate synthesis or H2 production was approximately -2 to -3 kJ/reaction. Formate conversion to methane was inhibited in the presence of high H2 pressure. The relative rates of conversion of formate and H2 were apparently controlled by the ΔG′ available for formate synthesis, hydrogen production, methane production from formate and methane production from H2. Results from 14C-tracer tests indicated that a rapid isotopic exchange between HCOO- and HCO inf3 sup- occurred during the growth of M. formicicum on H2−CO2. Data from metabolism of 14C-labelled formate to methane suggested that formate was initially split to H2 and HCO inf3 sup- and then subsequently converted to methane. When molybdate was replaced with tungstate in the growth media, the growth of M. formicicum strain MF on H2−CO2 was inhibited although production of methane was not Formate synthesis from H2 was also inhibited.