Facilitating direct interspecies electron transfer in anaerobic digestion via speeding up transmembrane transport of electrons and CO reduction in methanogens by Na adjustment.

The possibility of facilitating direct interspecies electron transfer (DIET) in anaerobic digestion with different concentrations of NaCl was explored. Additional NaCl at 2 or 4 g/L strengthened anaerobic digestion to resist the high-organic loading rate impacts, whereas the higher concentrations of NaCl (6 or 8 g/L) suppressed methanogenesis. Additional MgCl with the same ion strength as NaCl at 2 g/L had no effect on performances. Additional NaCl at 2 or 4 g/L dramatically increased the abundance of Methanosarcina species (20.7%/23.4% vs 8.6%) and stimulated the growth of Sphaerochaeta and Petrimonas species that could transfer electrons to the soluble Fe(III) or elemental sulfur. Electrochemical evidences showed that, additional NaCl at 2 or 4 g/L increased capacitances and decreased charge transfer resistances of Methanosarcina-dominant communities. Metagenomic evidences showed that, additional NaCl at 2 or 4 g/L increased the abundance of genes that encoded the type IV pilus assembly proteins (1.98E-04/1.87E-04 vs 1.85E-04) and cytochrome c-like proteins (5.51E-04/5.60E-04 vs 5.31E-04). In addition, additional NaCl at 2 or 4 g/L increased the abundance of genes for methanophenazine (MP)/MPH transformation (1.04E-05/1.24E-05 vs 8.06E-06) and CO reduction (1.64E-03/1.86E-03 vs 1.06E-03), suggesting a rapid transmembrane transport of electrons and CO reduction in methanogens. Both processes were closely associated with F/FH transformation that required ATP. Additional NaCl at 2 or 4 g/L increased the yield of ATP (256.0/249.3 vs 231.8 nmol/L) that might promote F/FH transformation in methanogens, which overcame the thermodynamic limitations of combining electrons with protons for the reduction of CO to methane and facilitated DIET.