Carbon isotope fractionation by Methanobacterium thermoautotrophicum

Abstract

The fractionation of carbon isotopes by Methanobacterium thermoautotrophicum was studied during growth of the bacterium on H₂ plus CO₂ as sole carbon and energy sources. A 80% H₂/20% CO₂ gas mixture was continuously bubbled through the culture. At high gassing rates, in the absence of a “closed system effect”, cells and methane were found to be depleted in ¹³C relative to CO₂ in the gas mixture by 2.4% and 3.4%, respectively. At low gassing rates, when more than 90% of the CO₂ was converted to methane, the cells were enriched in ¹³C by 1.3% and methane was depleted in ¹³C by 0.5%; residual CO₂ was enriched in ¹³C by 3.4%. The magnitude of isotope fractionation suggests that CO₂ rather than bicarbonate is the active species of CO₂ mainly utilized in both CO₂ assimilation and CO₂ reduction to methane. The apparent positive ¹³C-discrimination in cell carbon synthesis, which was observed at low gassing rates, indicates that most of the CO₂ assimilated into cell material is not incorporated via reactions involved in CO₂ reduction to methane.