Alternating Polarity as a Novel Strategy for Cultivating Electro-Methanogenic Microbial Communities Capable of Robust Biogas Production

Electro-methanogenic microbial communities can produce biogas with high efficiency. Extensive efforts have been made to cultivate these communities in engineered systems. Conventional cultivation strategies can select electrotrophic methanogens but not their electron-donating partners, resulting in communities that are sensitive to perturbations. Herein, we developed an alternating polarity strategy to simultaneously select both microbial populations. In two-chamber bioelectrochemical systems amended with activated carbon, the electrode potential was alternated between +0.8 V and -0.4 V vs. standard hydrogen electrode every three days. After eight alternating cycles, cultivated activated carbon was transferred into new bioreactors, and the enrichment procedure was repeated four times. Cumulative biogas production under alternating polarity increased from 45 L/L/kg-activated carbon after start-up to 125 L/L/kg after the 4th enrichment, significantly higher than that under intermittent cathode (-0.4 V/open circuit), continuous cathode (-0.4 V), and open circuit. The communities cultivated under alternating polarity were electroactive and structurally different from those cultivated under other conditions. One Methanobacterium population and two Geobacter populations were consistently abundant and active in the communities. Their 16S rRNA was upregulated by electrode potentials. Bayesian networks inferred close associations between these populations. The cultivation strategy can enhance biogas production, and the cultivated communities may serve as a model system for elucidating the mechanisms of extracellular electron uptake. SynopsisAn alternating polarity strategy was developed in this study to cultivate electro-methanogenic microbial communities. The cultivated communities can produce biogas more efficiently and help us understand the ecophysiology of the key microbial populations.