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 ^sup-_inf3 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 ^sup-_inf3 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 ^sup-_inf3 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.