Direct interspecies electron transfer-based simplified microbial consortia for high-efficiency conversion of...

Establishing direct interspecies electron transfer (DIET)-based methanogenic pathway is likely to address the technical bottlenecks involved in long periods and low rates of methanogenesis during anaerobic digestion of lignocellulose. However, the efficiency of DIET is limited by low abundance of electroactive bacteria and electron competition with conventional methanogenic pathway. Here, we combined cow manures with paddy soils/marine sediments as initial inocula, and constructed two simplified microbial consortia (DIETsimp) for conversion of lignocellulose to methane via a 'top-down' selection. Both DIETsimp dramatically shortened periods of methanogenesis (ca. 15-16 vs 25-40 d, this study vs present level) and increased methane production rates (ca. 32 vs 10-25 mL/gVS·d). Lowering pH dramatically increased conductivity of both DIETsimp, similar to that was found in electrically conductive pili of Geobacter sulfurreducens. Meanwhile, the intensities of characteristic peaks in electrochemical Fourier transform infrared spectra associated with c-type cytochrome in both DIETsimp dramatically increased. Metagenomic analysis showed that, Methanosarcina mazei, capable of accepting electrons via DIET, and electroactive species, Sphaerochaeta globosa and Clostridium aceticum, were the dominant archaea and bacteria in both DIETsimp, respectively. The potential DIET-based methanogenic pathway during anaerobic digestion of lignocellulose that S. globosa and C. aceticum metabolized intermediates (e.g. xylose, glucose, pyruvate and acetate) and transferred electrons to M. mazei for the reduction of CO₂ to methane was proposed. At last, we optimized culture conditions (including inoculum ratio, C/N and period) to maximize the performances of both DIETsimp via combining the single-factor experiments with response surface methodology.