Synchronously improving intracellular electron transfer in electron-donating bacteria and electron-accepting methanogens for facilitating direct interspecies electron transfer during anaerobic digestion of kitchen wastes

Two major issues generally limit the effectiveness of direct interspecies electron transfer (DIET) during anaerobic digestion: 1) lack of essential electrical connection component, electrically conductive pili (e-pili) or multi-heme c-type cytochrome (MHC); 2) the thermodynamic limitations of combining electrons with protons for reducing carbon dioxide to methane. To address both issues, a strategy for facilitating DIET via combining ethanol-type fermentation pretreatment (EFP) with NaCl addition during anaerobic digestion of kitchen wastes was proposed. Combining EFP with NaCl addition dramatically increased methane yield rates (531.3 ± 11.4 vs 410.5 ± 8.7 mL/gVS_added/d) and efficiencies of conversion of organic substrates to methane (571.2 ± 13.8 vs 488.7 ± 11.7 mL/gVS_removal). The increased performances by combining EFP with NaCl addition were higher than that by independent EFP or NaCl addition. Paludibacter sp., Petrimonas sp. and Syntrophomonas wolfei were the predominant and metabolically active electron-donating bacteria, and their expression of genes for e-pili/MHCs by combining EFP with NaCl addition was higher than that by independent EFP or NaCl addition. Methanothrix soehngenii and Methanoculleus bourgensis were the predominant and metabolically active electron-accepting methanogens, and their expression of genes for carbon dioxide reduction pathway for methanogenesis by combining EFP with NaCl addition was higher than that by independent EFP or NaCl addition. In addition, EFP specifically increased the transcript abundance of genes for NAD⁺/NADH transformation closely associated with the formation of e-pili/MHCs, while NaCl addition specifically increased the transcript abundance of genes for coenzyme F₄₂₀/F₄₂₀H₂ transformation known to participate in reduction of carbon dioxide to methane.