Bioelectrochemical systems offer a method to microbially harness the energy from unwanted compounds, such as wastewater, and use towards biosynthesis of beneficial compounds. Understanding the effects of the cation exchange membrane and other potential environmental stimuli is critical for developing bioelectrochemical systems that are robust and reliable. In this study, an electron-consuming community enriched from rice paddy soil was fed with a stable electron source via potentiostatic control of the working electrode to -700 mV vs. Ag/AgCl, and carbon dioxide as the sole inorganic carbon source. A subpassage technique was used as a selective pressure to enhance electron consumption and methanogenesis within the system. After relatively stable operation over a 250-day period along with sampling of the community to determine taxonomic composition, the cation exchange membrane was changed from Nafion®-117 to Ultrex®. While methane production remained consistent for an additional 450-day operation, there was a striking decrease in measured current consumption of the system (Fig. 1), which may also correlate to a change in the cathode microbial community and mechanism of energy recovery from the electrode (Fig. 2).

Results suggest that the properties of the cation exchange membrane are correlated to electrical system performance. Nafion is a physically thinner cation exchange membrane than the Ultrex material, and the physical differences in the membrane properties may impact the diffusion of protons and other molecules between the counter and working compartments. For example, it is possible that free radicals are formed at the counter electrode surface during poised-potential operation and may diffuse into the working chamber via the membrane. However, the presence of a thicker member (e.g. the Ultrex) in the system may minimize this diffusion effect and also limit the proton transfer, thereby leading to reduced electrochemical activity and alternative mechanisms for methane formation at the working electrode.

The taxonomic composition of the electromethanogic community was characterized using 16S rRNA gene sequences. The communities associated with the working electrode changed significantly when the membrane was switched from Nafion to Ultrex (Fig. 2).  The relative abundance of methanogens (Euryarchaeota) increased from below detection to occupying nearly 60% of the community during the first subpassage after the membrane change. The relative abundance of the methanogenic population varied throughout operation with Ultrex membranes, but remained detectable and in high relative abundance for several subpassages. Interestingly, the relative abundance of Clostridia, α-,  δ-, and ε-Proteobacteria also significantly changed throughout the duration of operation with the Ultrex membrane.