Facilitating direct interspecies electron transfer in anaerobic digestion via speeding up transmembrane transport of electrons and CO2 reduction in methanogens by Na+ adjustment

Abstract

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 MgCl₂ 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)/MPH₂ transformation (1.04E-05/1.24E-05 vs 8.06E-06) and CO₂ reduction (1.64E-03/1.86E-03 vs 1.06E-03), suggesting a rapid transmembrane transport of electrons and CO₂ reduction in methanogens. Both processes were closely associated with F₄₂₀/F₄₂₀H₂ 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 F₄₂₀/F₄₂₀H₂ transformation in methanogens, which overcame the thermodynamic limitations of combining electrons with protons for the reduction of CO₂ to methane and facilitated DIET.