CO2 reduction to CH4 by Methanosarcina barkeri and a mixed methanogenic culture using humin as sole electron donor

In this study, we show that wetland sediment-sourced humin (HMWS) could act as the sole electron donor for promoting the reduction of CO2 to CH4 by Methanosarcina barkeri and a mixed methanogenic culture (ME consortium) with maximum methanogenic rates of 0.0025 ± 0.001 and 0.026 ± 0.003 mmol/d, respectively. 93.27% ± 2.06% and 96.35% ± 3.18% of the electrons derived from HMWS were used to produce CH4 by M. barkeri and the ME consortium. Spectroscopic analyses revealed that multiple redox groups were involved in the electron-donating process of HMWS. Molecular microbial analyses indicated that the archaeon Methanosarcina and bacteria Desulfovibrio were dominant in the ME consortium and that cytochrome c played an important role in the bio-oxidation of HMWS. Methanogenesis of M. barkeri with HMWS as an electron donor strongly depended on the HMWS dosage and NaCl and NH4Cl concentrations. Apart from these factors, the ME consortium was also sensitive to NaHCO3 concentration. The results showed that HMWS might relieve NaHCO3, NaCl, and NH4Cl stresses on methanogenesis with ME consortium, and HMWS could be regenerated to promote methanogenesis, indicating its potential for practical applications.