Physiological and molecular characterizations of the interactions in two cellulose-to-methane cocultures

BACKGROUND The interspecies interactions in a biomethanation community play a vital role in substrate degradation and methane (CH4) formation. However, the physiological and molecular mechanisms of interaction among the microbial members of this community remain poorly understood due to the lack of an experimentally tractable model system. In this study, we successfully established two coculture models combining the cellulose-degrading bacterium Clostridium cellulovorans 743B with Methanosarcina barkeri Fusaro or Methanosarcina mazei Gö1 for the direct conversion of cellulose to CH4. RESULTS Physiological characterizations of these models revealed that the methanogens in both cocultures were able to efficiently utilize the products produced by C. cellulovorans during cellulose degradation. In particular, the simultaneous utilization of hydrogen, formate, and acetate for methanogenesis was observed in the C. cellulovorans-M. barkeri cocultures, whereas monocultures of M. barkeri were unable to grow with formate alone. Enhanced cellulose degradation was observed in both cocultures, and the CH4 yield of the C. cellulovorans-M. barkeri cocultures (0.87 ± 0.02 mol CH4/mol glucose equivalent) was among the highest compared to other coculture studies. A metabolic shift in the fermentation pattern of C. cellulovorans was observed in both cocultures. The expression levels of genes in key pathways that are important to the regulation and metabolism of the interactions in cocultures were examined by reverse transcription-quantitative PCR, and the expression profiles largely matched the physiological observations. CONCLUSIONS The physiological and molecular characteristics of the interactions of two CH4-producing cocultures are reported. Coculturing C. cellulovorans with M. barkeri or M. mazei not only enabled direct conversion of cellulose to CH4, but also stabilized pH for C. cellulovorans, resulting in a metabolic shift and enhanced cellulose degradation. This study deepens our understanding of interspecies interactions for CH4 production from cellulose, providing useful insights for assembling consortia as inocula for industrial biomethanation processes.