There are several methane-fermenting microorganisms including Methanobacterium thermoautotrophicum, Methanothermobacter thermoautotrophicus, M. barkeri, Methanosarcina acetivorans strain C2A, R. rubrum, and M. formicum (O’Brien et al., 1984) that have been isolated for biomethane production from syngas. In syngas-to-methane fermentation, CO acts as an electron donor and CO2 as an electron acceptor, which gets reduced to methane (CH4). O’Brien et al. (1984) reported hydrogen production during the growth of M. barkeri on CO when the CO partial pressure exceeded 20 kPa. The authors further revealed a net consump­tion of hydrogen below CO partial pressure 20 kPa. Kluyver and Schnellen (1947) reported the production of intermediates such as H2 and CO2 in their suggested CO to methane pathway. Several studies reported the low growth rates of M. barkeri and M. thermoautotrophicus on CO compared to the growth on H2 as the electron donor (O’Brien et al., 1984). The possible chemi­cal reactions and the relevant Gibbs free energy contents of the conversion of CO to methane are given in Equations (8) and (9).

From 100% CO, 4CO + 2H2O! CH4 + 3CO2 AG° = -53.0kJ/mole CO (8)

From H2 and CO, CO + 3H2CH4 + H2O AG° = -151.0kJ/mole CO (9)

Sipma et al. (2003) reported the use of several granular anaerobic sludges to produce meth­ane from CO at 30 and 55 °C. The authors found a significant increase in the CO to methane conversion efficiency (up to 90%). But the authors did not fully characterize the microbial communities in the sludge. According to some studies, methanogenesis is highly sensitive to CO concentration in the liquid phase (Klasson et al., 1990). However, successive transfers could enhance the ability of the microorganisms to grow on 100% CO (O’Brien et al., 1984). CO fermentation to methane opens up new area of syngas bioconversion to methane gas, which may overcome some of the challenges of syngas-to-ethanol fermentation.