Acetate and electricity generation from methane in conductive fiber membrane- microbial fuel cells

Current trends in electromicrobiology of methane oxidation

With many methane oxidation processes now recognized as being electrochemically driven, microbial methane oxidation is becoming an emerging focus in electromicrobiology. This review examines the current trends in the electromicrobiology of methane oxidation. We begin by reviewing recent advances in the understanding of the microbial and physiological diversity involved in microbial methane oxidation. We highlight the versatile role of aerobic methane-oxidizing bacteria in electrochemically driven methane oxidation, and the non-syntrophic lifestyle of anaerobic methanotrophic archaea (ANME) enabled by their extracellular electron transfer (EET) pathways. These aspects are followed by a review of recent findings on the potential reversibility of methanogen metabolism, with a focus on the proposed EET pathways that may facilitate their shift to a methane-oxidizing phenotype, a topic that remains under active investigation and debate. Finally, we examine the biogeochemical cycles and the application potential involving electrochemically driven methane oxidation.

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

Simultaneous denitrification and electricity generation in a methane-powered bioelectrochemical system

Bioelectrochemical system is a novel method for controlling down nitrate pollution, yet the feasibility of using methane as the electron donors for denitrification in this system remains unknown. In this study, using the effluent from mother BESs as inocula, a denitrifying anaerobic methane oxidation bioelectrochemical system was successfully started up in 92 days. When operated with 50 mmol/L phosphate buffer solution at pH 7 and 30°C, the maximum methane consumption, nitrate, and total nitrogen removal load reached 0.23 ± 0.01 mmol/d, 551.0 ± 22.1 mg N/m3 /d, and 64.0 ± 18.8 mg N/m3 /d, respectively. Meanwhile, the peak voltage of 93 ± 4 mV, the anodic coulombic efficiency of 6.99 ± 0.20%, and the maximum power density of 219.86 mW/m3 were obtained. The metagenomics profiles revealed that the dominant denitrifying bacteria in the cathodic chamber reduced most nitrate to nitrite through denitrification and assimilatory reduction. In the anodic chamber, various archaea including methanotrophs and methanogens converted methane via reverse methanogenesis to form formate (or H2 ), acetate, and methyl compounds, which were than utilized by electroactive bacteria to generate electricity. PRACTITIONER POINTS: A denitrifying anaerobic methane oxidation BES was successfully started up in 92 d. Simultaneous removal of methane and nitrate was achieved in the DAMO-BES. Functional genes related to AMO and denitrification were detected in the DAMO-BES. Methylocystis can mediate AMO in the anode and denitrification in the cathode.

Combining biological denitrification and electricity generation in methane-powered microbial fuel cells

Methane has been demonstrated to be a feasible substrate for electricity generation in microbial fuel cells (MFCs) and denitrifying anaerobic methane oxidation (DAMO). However, these two processes were evaluated separately in previous studies and it has remained unknown whether methane is able to simultaneously drive these processes. Here we investigated the co-occurrence and performance of these two processes in the anodic chamber of MFCs. The results showed that methane successfully fueled both electrogenesis and denitrification. Importantly, the maximum nitrate removal rate was significantly enhanced from (1.4 ± 0.8) to (18.4 ± 1.2) mg N/(L·day) by an electrogenic process. In the presence of DAMO, the MFCs achieved a maximum voltage of 610 mV and a maximum power density of 143 ± 12 mW/m². Electrochemical analyses demonstrated that some redox substances (e.g. riboflavin) were likely involved in electrogenesis and also in the denitrification process. High-throughput sequencing indicated that the methanogen Methanobacterium, a close relative of Methanobacterium espanolae, catalyzed methane oxidation and cooperated with both exoelectrogens and denitrifiers (e.g., Azoarcus). This work provides an effective strategy for improving DAMO in methane-powered MFCs, and suggests that methanogens and denitrifiers may jointly be able to provide an alternative to archaeal DAMO for methane-dependent denitrification.

Electro-stimulated anaerobic oxidation of methane with synergistic denitrification by adding AQS: Electron transfer mode and mechanism

Denitrifying anaerobic methane-oxidizing (DAMO) processes, which link anaerobic methane oxidation (AMO) and denitrification, have a promising prospect in anaerobic wastewater treatment. In bioelectrochemical systems (BES), DAMO consortium presented potent metabolic activity. However, the extracellular electron transfer (EET) in BES was poorly understood. This study investigated the EET mechanisms and modes of electron transport in BES dominated by anaerobic methanotrophic bacteria. In the bioreactors with the auxiliary voltage of 0.5 and 1.1 V, named EMN-0.5 and EMN-1.1, respectively, biological voltages of 0.198 and 0.329 V were generated with power densities of 0.6 and 1.20 mW/m², after removing the voltage. High throughput and metagenome analyses demonstrated that main methanotrophs were DAMO bacteria and Methylocystis sp. The electroactive bacteria detected were Pseudomonas sp., Hypomicrobium sp., Thiobacillus sp, and Rhodococcus sp. The pil, cytochrome c, hdr, and he/fp genes related to EET were present on the electrode surfaces. Carbon 13 isotope tracing and chemicals analysis by GC-MS exhibited that methanol was an intermediate product released to extracellular environment and acted as the electronic carrier to drive the EET in methane oxidation. Extracellular electron transfer was achieved through the collaboration of DAMO bacteria, Methylocystis sp., and Pseudomonas sp. Anthraquinone 2-sulfonic acid ester (AQS) could improve the rate of electron transfer to the extracellular space, especially in the EMN-0.5 reaction system. This study provides a new understanding of AMO consortium metabolism in BES and may provide a scientific basis for developing methane control technology.

Study on performance and mechanisms of anaerobic oxidation of methane-microbial fuel cells (AOM-MFCs) with acetate-acclimatizing or formate-acclimatizing electroactive culture

Anaerobic oxidation of methane-microbial fuel cells with acetate-acclimatizing or formate-acclimatizing electroactive culture (A-AOM-MFC and F-AOM-MFC) were designed and operated at room temperature in this study to evaluate and explore the electrochemical performance and mechanisms of methane conversion and electricity generation. The results indicated that A-AOM-MFC output a higher voltage (0.526 ± 0.001 V) and F-AOM-MFC started up in a shorter time (51 d), resulting from different mechanisms of methane-electrogen caused by discrepant microbial alliances. Specifically, in A-AOM-MFC, acetoclastic methanogens (e.g., Methanosaeta) converted methane into intermediates (e.g., acetate) through reversing methanogenesis and carried out the direct interspecific electron transfer (DIET) with Geobacter-predominated electricigens which can oxidize the intermediates to carbon dioxide and transfer electrons to the electrodes. Differently, the intermediate-dependent extracellular electron transfer (EET) existed in F-AOM-MFC between hydro-methanogens (e.g., Methanobacterium) and electricigens (e.g., Geothrix), which was more difficult than DIET. Additionally, hydro-methanogens metabolized methane to produce formate-dominant intermediates more quickly.