Impact of low pH shock on anodic community structure and functional gene change in microbial electrolysis cells (MECs)

Microbial electrolysis cells (MECs) has been recently developed as a new technology for hydrogen production. The organics are degraded by exoelectrogens in anode biofilm and transport electrons directly to anode, while hydrogen is produced by combining electrons and protons on the surface of Pt-catalyzing cathode under a small external voltage between anode and cathode. Municipal waste water was used as the same inoculum to enrich functional communities in 15 single chamber MEC reactors. Various gas production (hydrogen and methane) was maintained over 1 month using glucose as the sole carbon source in phosphate buffer solution (50 mmol·L^-1, pH=7.0). The highest hydrogen production rate was up to (3.9±0.6) mol H₂/mol glucose with conversion rate of 32.2% for high-H₂ generation MECs. The highest methane conversion rate was 48.4% in low-H₂ generation MECs. A 48 h low pH shock was put into anode biofilm and MEC performances were recovered to their functions in 10~15 d. The microbial diversities increased and gas production rates were changed after low pH shock. Hydrogen yield was reduced by 1.8 mol H₂/mol glucose in high-H₂ generation MECs, while the methane yield increased by 0.4 mol CH₄/mol glucose. Based on Geochip analysis, cytochrome C genes were mostly enriched in high-H₂ yield MECs. Thus functional genes were still recovered dominantly after low pH shock and it supported the recovery of electron transport. The carbon degradation genes were substantially changed among anodic communities in most MECs. The functional genes of labile carbon degradation and methane production were significantly changed after microbial community recovery. The carbon degradation gene structure was more significantly changed in high-H₂ yield MECs than high-methane yield MECs. The methane increase and hydrogen decrease matched well with their stoichiometric relationship.