Effects of CO on hydrogenotrophic methanogenesis under thermophilic and extreme-thermophilic conditions: Microbial community and biomethanation pathways

[Display omitted] •5% (v/v) CO supplementation enhanced CH4 production at both 55 °C and 70 °C.•Methanothermobacter thermoautotrophicus was the dominant methanogen.•Lag phase significantly shortened after acclimation with 5% (v/v) CO.•Biomethanation processes followed the order: H2/CO2, H2/CO and CO/H2O. Coke oven gas is considered as a potential hydrogen source for biogas bio-upgrading. In this study, the effects of CO on biomethanation performance and microbial community structure of hydrogenotrophic mixed cultures were investigated under thermophilic (55 °C) and extreme-thermophilic (70 °C) conditions. 5% (v/v) CO did not inhibit hydrogenotrophic methanogenesis during semi-continuous operation, and 83–97% CO conversion to CH4 was achieved. Methanothermobacter thermoautotrophicus was the dominant methanogen at both temperatures and was the main functional archaea associated with CO biomethanation. Specific methanogenic activity test results showed that long-term 5% CO acclimation shortened the lag phase from 5 h to 1 h at 55 °C and 15 h to 3 h at 70 °C. CO2 was the preferred carbon source over CO for hydrogenotrophic methanogens and CO consumption only started when CO2 was completely depleted. M. thermoautotrophicus dominated mixed cultures showed a great potential in simultaneous hydrogenotrophic methanogenesis and CO biomethanation.