The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Reliable operational procedures for the control of Microbial Electrochemical Technologies (METs) to produce suitable substrates for economically viable downstream applications were investigated. We aimed at levelling key operational conditions in a model electrosynthetic reactor in order to obtain biofuels from carbon dioxide (CO₂). The cathodes of two tubular bioelectrochemical systems (BESs) were inoculated with an enriched culture of a carboxydotrophic strain and were operated until stable conversion of CO₂ into acetate and ethanol. Both reactors averaged a production of 76 ± 9 mg_ethanol m⁻² d⁻¹ and 120 ± 29 mg_acetate m⁻² d⁻¹, which were considered crucial for triggering the production of longer carbon chain carboxylates and alcohols in a chain elongation bioreactor. Changes in the current signal and pH were correlated with CO₂ depletion and the transition from acetogenesis to solventogenesis in the enriched culture. We show that in-line monitoring of pH and electron consumption are meaningful operational variables to differentiate between the two biological processes, and open the door to develop new approaches to control the bioelectro-recycling of CO₂ into biofuels by METs.

Electromethanogenesis is the bioreduction of carbon dioxide (CO₂) to methane (CH₄) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F₄₂₀-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp.

Carbon oxidation reaction enables a membrane-less bioelectrochemical system for microbial electrosynthesis.