How Low Can You Go: Methane Production of Methanobacterium congolense at Low CO(2) Concentrations

Autotrophic hydrogenotrophic methanogens use H(2)/CO(2) as sole carbon and energy source. In contrast to H(2), CO(2) is present in high concentrations in environments dominated by methanogens e.g., anaerobic digesters (AD), and is therefore rarely considered to be a limiting factor. Nonetheless, potential CO(2) limitation can be relevant in the process of biomethanation, a power-to-gas technology, where biogas is upgraded by the addition of H(2) and ideally reduce the CO(2) concentration in the produced biogas to 0–6%. H(2) is effectively utilized by methanogens even at very low concentrations, but little is known about the impact of low CO(2) concentrations on methanogenic activity. In this study, CO(2) consumption and CH(4) production kinetics under low CO(2) concentrations were studied, using a hydrogenotrophic methanogen, Methanobacterium congolense, as model organism. We found that both cellular growth and methane production were limited at low CO(2) concentrations (here expressed as Dissolved Inorganic Carbon, DIC). Maximum rates (V(max)) were reached at [DIC] of 100 mM (extrapolated), with a CO(2) consumption rate of 69.2 fmol cell(−1) d(−1) and a CH(4) production rate of 48.8 fmol cell(−1) d(−1). In our experimental setup, 80% of V(max) was achieved at [DIC] >9 mM. DIC half-saturation concentrations (K(m)) was about 2.5 mM for CO(2) consumption and 2.2 mM for CH(4) production. No CH(4) production could be detected below 44.4 μM [DIC]. These data revealed that the limiting concentration of DIC may be much higher than that of H(2) for a hydrogenotrophic methanogen. However, DIC is not a limiting factor in ADs running under standard operating conditions. For biomethanation, the results are applicable for both in situ and ex situ biomethanation reactors and show that biogas can be upgraded to concentrations of 2% CO(2) (98% CH(4)) while still retaining 80% V(max) at pH 7.5 evaluated from M. congolense. Since DIC concentration can vary significantly with pH and pCO(2) during biomethanation, monitoring DIC concentration through pH and pCO(2) is therefore important for keeping optimal operational conditions for the biomethanation process.