Up-regulated proton pump expression after power off promoted proton transfer to enhance anaerobic digestion

Abstract

Microbial electrolysis cell coupled of anaerobic digestion (MEC-AD) can enhance organics removal and methane production of anaerobic digestion even under the intermittent voltage supply, but the reason remains unclear. In this study, compared to an anaerobic digester free of voltage, the methane production of MEC increased by 26.0 % and the organic decomposition increased by 12.9 ± 1.9 %. Notably, the anaerobic digestion performance of MEC was maintained well after the power applied to the electrode was turned off, and the methane production was similar to the power-on stage. However, the electrochemical properties of the cathode biofilm decreased after power off, which generally restrained microbial interspecies electron transfer to reduce methane production of anaerobic digestion. Accordingly, electrophoresis and ThT tests indicated that the proton transfer of biofilm was enhanced after the power was turned off, which might help the performance of the MEC to remain good. Macrotranscriptomics analysis suggested that the expression of the proton pump genes was up-regulated to promote proton and electron circulation and enhance proton transfer. The EPR analysis indicated that the improved redox signal of carbon-centered radical (i.e., ·C-O-H) after power off promoted the expression of proton pumps thereby intensifying proton transfer and methane production. This study suggested that the up-regulation of proton pump expression promoted proton transfer to compensate for the decrease in electron transfer after power off, which could well maintain anaerobic digestion performance.