Electron carriers increase electricity production in methane microbial fuel cells that reverse methanogenesis

Capturing methane with recombinant soluble methane monooxygenase and recombinant methyl-coenzyme M reductase

Methane capture via oxidation is considered one of the 'Holy Grails' of catalysis (Tucci and Rosenzweig, 2024). Methane is also a primary greenhouse gas that has to be reduced by 1.2 billion metric tonnes in 10 years to decrease global warming by only 0.23°C (He and Lidstrom, 2024); hence, new technologies are needed to reduce atmospheric methane levels. In Nature, methane is captured aerobically by methanotrophs and anaerobically by anaerobic methanotrophic archaea; however, the anaerobic process dominates. Here, we describe the history and potential of using the two remarkable enzymes that have been cloned with activity for capturing methane: aerobic capture via soluble methane monooxygenase and anaerobic capture via methyl-coenzyme M reductase. We suggest these two enzymes may play a prominent, sustainable role in addressing our current global warming crisis.

Detection and Characterization of Electrogenic Bacteria from Soils

Soil microbial fuel cells (SMFCs) are bioelectrical devices powered by the oxidation of organic and inorganic compounds due to microbial activity. Seven soils were randomly selected from Bergen Community College or areas nearby, located in the state of New Jersey, USA, were used to screen for the presence of electrogenic bacteria. SMFCs were incubated at 35-37 °C. Electricity generation and electrogenic bacteria were determined using an application developed for cellular phones. Of the seven samples, five generated electricity and enriched electrogenic bacteria. Average electrical output for the seven SMFCs was 155 microwatts with the start-up time ranging from 1 to 11 days. The highest output and electrogenic bacterial numbers were found with SMFC-B1 with 143 microwatts and 2.99 × 109 electrogenic bacteria after 15 days. Optimal electrical output and electrogenic bacterial numbers ranged from 1 to 21 days. Microbial DNA was extracted from the top and bottom of the anode of SMFC-B1 using the ZR Soil Microbe DNA MiniPrep Protocol followed by PCR amplification of 16S rRNA V3-V4 region. Next-generation sequencing of 16S rRNA genes generated an average of 58 k sequences. BLAST analysis of the anode bacterial community in SMFC-B1 demonstrated that the predominant bacterial phylum was Bacillota of the class Clostridia (50%). However, bacteria belonging to the phylum Pseudomonadota (15%) such as Magnetospirillum sp. and Methylocaldum gracile were also part of the predominant electrogenic bacterial community in the anode. Unidentified uncultured bacteria accounted for 35% of the predominant bacterial community. Bioelectrical devices such as MFCs provide sustainable and clean alternatives to future applications for electricity generation, waste treatment, and biosensors.

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.

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.

Advancements in Bioelectricity Generation Through Nanomaterial-Modified Anode Electrodes in Microbial Fuel Cells

The use of nanotechnology in bioelectrochemical systems to recover bioelectricity and metals from waste appears to be a potentially appealing alternative to existing established procedures. This trend exactly characterizes the current renewable energy production technology. Hence, this review focuses on the improvement of the anode electrode by using different functional metal oxide-conducting polymer nanocomposites to enhance microbial fuel cell (MFC) performance. Enhancement of interfacial bioelectrocatalysis between electroactive microorganisms and hierarchical porous nanocomposite materials could enhance cost-effective bioanode materials with superior bioelectrocatalytic activity for MFCs. In this review, improvement in efficiency of MFCs by using iron oxide- and manganese oxide-based polypyrrole hybrid composites as model anode modifiers was discussed. The review also extended to discussing and covering the principles, components, power density, current density, and removal efficiencies of biofuel cell systems. In addition, this research review demonstrates the application of MFCs for renewable energy generation, wastewater treatment, and metal recovery. This is due to having their own unique working principle under mild conditions and using renewable biodegradable organic matter as a direct fuel source.

State-of-the-art management technologies of dissolved methane in anaerobically-treated low-strength wastewaters: A review

The recent advancement in low temperature anaerobic processes shows a great promise for realizing low-energy-cost, sustainable mainstream wastewater treatment. However, the considerable loss of the dissolved methane from anaerobically-treated low-strength wastewater significantly compromises the energy potential of the anaerobic processes and poses an environmental risk. In this review, the promises and challenges of existing and emerging technologies for dissolved methane management are examined: its removal, recovery, and on-site reuse. It begins by describing the working principles of gas-stripping and biological oxidation for methane removal, membrane contactors and vacuum degassers for methane recovery, and on-site biological conversion of dissolved methane into electricity or value-added biochemicals as direct energy sources or energy-compensating substances. A comparative assessment of these technologies in the three categories is presented based on methane treating efficiency, energy-production potential, applicability, and scalability. Finally, current research needs and future perspectives are highlighted to advance the future development of an economically and technically sustainable methane-management technology.

Methane-fueled microbial fuel cells with the formate-acclimating electroactive culture

Methane gas is widespread in natural environments and anaerobic wastewater treatment sites, bringing the risk of the greenhouse effect and energy loss if left unmanaged. A methane-fueled microbial fuel cell (MFC) can convert methane to electricity under mild condition, but faced difficulties in startup. In this study, the new startup strategy and operation performance for methane-fueled MFCs were investigated. After the pre-cultivation of formate-acclimating electroactive culture, the methane-fueled MFC was successfully started up in a short time of 53 d. Increasing concentrations of molybdenum and tungsten in medium facilitated both methane consumption and electricity generation. Under the optimal condition (batch duration of 11 h, 30 °C, pH 7 buffered by phosphate buffer solution), the methane-fueled MFC achieved the maximum power density of 166 mW/m³, a coulomb production of 6.58 ± 0.07C/batch, a CE of 27.4 ± 0.4% and a methane consumption of 31.2 ± 0.3 μmol/batch. This work explored a suitable inoculum (formate-acclimating electroactive culture) for methane-fueled MFCs.

Valorization of methane from environmental engineering applications: A critical review

Wastewater and waste management sectors alone account for 18% of the anthropogenic methane (CH₄) emissions. This study presents a critical overview of methanotrophs ("methane oxidizing microorganisms") for valorizing typically discarded CH₄ from environmental engineering applications, focusing on wastewater treatment plants. Methanotrophs can convert CH₄ into valuable bioproducts including chemicals, biodiesel, DC electricity, polymers, and S-layers, all under ambient conditions. As discarded CH₄ and its oxidation products can also be used as a carbon source in nitrification and annamox processes. Here we discuss modes of CH₄ assimilation by methanotrophs in both natural and engineered systems. We also highlight the technical challenges and technological breakthroughs needed to enable targeted CH₄ oxidation in wastewater treatment plants.

Rhamnolipids and surfactin inhibit the growth or formation of oral bacterial biofilm

Background

Bacteria survive in various environments by forming biofilms. Bacterial biofilms often cause significant problems to medical instruments and industrial processes. Techniques to inhibit biofilm formation are essential and have wide applications. In this study, we evaluated the ability of two types of biosurfactants (rhamnolipids and surfactin) to inhibit growth and biofilm formation ability of oral pathogenic bacteria such as Aggregatibacter actinomycetemcomitans, Streptococcus mutans, and Streptococcus sanguinis.

Results

Rhamnolipids inhibited the growth and biofilm formation ability of all examined oral bacteria. Surfactin showed effective inhibition against S. sanguinis ATCC10556, but lower effects toward A. actinomycetemcomitans Y4 and S. mutans UA159. To corroborate these results, biofilms were observed by scanning electron microscopy (SEM) and confocal microscopy. The observations were largely in concordance with the biofilm assay results. We also attempted to determine the step in the biofilm formation process that was inhibited by biosurfactants. The results clearly demonstrated that rhamnolipids inhibit biofilm formation after the initiation process, however, they do not affect attachment or maturation.

Conclusions

Rhamnolipids inhibit oral bacterial growth and biofilm formation by A. actinomycetemcomitans Y4, and may serve as novel oral drug against localized invasive periodontitis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02034-9.

Enhancing methane oxidation in a bioelectrochemical membrane reactor using a soluble electron mediator

BACKGROUND

Bioelectrochemical methane oxidation catalysed by anaerobic methanotrophic archaea (ANME) is constrained by limited methane bioavailability as well as by slow kinetics of extracellular electron transfer (EET) of ANME. In this study, we tested a combination of two strategies to improve the performance of methane-driven bioelectrochemical systems that includes (1) the use of hollow fibre membranes (HFMs) for efficient methane delivery to the ANME organisms and (2) the amendment of ferricyanide, an effective soluble redox mediator, to the liquid medium to enable electrochemical bridging between the ANME organisms and the anode, as well as to promote EET kinetics of ANME.

RESULTS

The combined use of HFMs and the soluble mediator increased the performance of ANME-based bioelectrochemical methane oxidation, enabling the delivery of up to 196 mA m^-2, thereby outperforming the control system by 244 times when HFMs were pressurized at 1.6 bar.

CONCLUSIONS

Improving methane delivery and EET are critical to enhance the performance of bioelectrochemical methane oxidation. This work demonstrates that by process engineering optimization, energy recovery from methane through its direct oxidation at relevant rates is feasible.

Performance and microbial ecology of methane-driven microbial fuel cells at temperatures ranging from 25 to 5 °C

The effluent of mainstream anaerobic processes is saturated with dissolved methane, representing a lost energy source and potent greenhouse gas emission if left unmanaged. This study investigated the impact of operational temperature on methane-driven microbial fuel cells (MFCs) designed for continuous operation to mitigate dissolved methane emissions in anaerobic effluents. Two bench-scale, single-chamber MFCs were operated sequentially at 25, 20, 15, 10 and 5 °C. Voltage production from both MFCs ranged from approximately 0.463 to 0.512 V over 1 kΩ resistance at temperatures ≥15 °C, but abruptly dropped as temperature decreased to 10 and 5 °C, averaging just 0.156 and 0.190 V for the replicate systems. Dissolved methane removal efficiency remained relatively stable across all operational temperatures, ranging from 53.0% to 63.6%. High-throughput sequencing of 16S rRNA genes and reverse transcription quantitative polymerase chain reaction indicated distinct distribution of methanotrophs (e.g., Methylomonas) and exoelectrogens (e.g., Geobacter) on the cathode and anode, respectively. Spearman's rank correlation suggested that an indirect interaction between methanotrophs and exoelectrogens via fermentative bacteria (e.g., Acetobacterium) may play a role in system function. Notably, diversity of the anode microbial community was positively correlated with both voltage production and Coulombic efficiency, suggesting overall diversity, as opposed to abundance or activity of exoelectrogens, was the primary factor governing performance at varying temperatures.

Humic substances as electron acceptors for anaerobic oxidation of methane driven by ANME-2d

Humic substances (humics) are ubiquitous in terrestrial and aquatic environments where they can serve as electron acceptors for anaerobic oxidation of organic compounds. Methane is a powerful greenhouse gas, as well as the least reactive organic molecule. Anaerobic oxidation of methane (AOM) coupled to microbial reduction of various electron acceptors plays a crucial role in mitigating methane emissions. Here, we reported that humics could serve as terminal electron acceptors for AOM using enriched nitrate-reducing AOM microorganisms. AOM coupled to the reduction of humics was demonstrated based on the production of ¹³C-labelled carbon dioxide, and AOM activity was evaluated with different methane partial pressures and electron acceptor concentrations. After three-cycle reduction, both AOM activity and copy numbers of the archaea 16S rRNA and mcrA genes were the highest when anthraquinone-2,6-disulfonic acid and anthraquinone-2-sulfonic acid were electron acceptors. The high-throughput sequencing results suggested that ANME-2d were the dominant methane oxidation archaea after humics reduction, although the partner bacteria NC10 trended downward, other reported humics reduction bacteria (Geobactor and Anammox) appeared. The potential electron transfer models from ANME-2d to humics were proposed. These results enable a better understanding of available electron acceptors for AOM in natural environments and broaden our insight into the significant role of ANME-2d.

Engineering Microbial Consortia for High-Performance Cellulosic Hydrolyzates-Fed Microbial Fuel Cells

Microbial fuel cells (MFCs) are eco-friendly bio-electrochemical reactors that use exoelectrogens as biocatalyst for electricity harvest from organic biomass, which could also be used as biosensors for long-term environmental monitoring. Glucose and xylose, as the primary ingredients from cellulose hydrolyzates, is an appealing substrate for MFC. Nevertheless, neither xylose nor glucose can be utilized as carbon source by well-studied exoelectrogens such as Shewanella oneidensis. In this study, to harvest the electricity by rapidly harnessing xylose and glucose from corn stalk hydrolysate, we herein firstly designed glucose and xylose co-fed engineered Klebsiella pneumoniae-S. oneidensis microbial consortium, in which K. pneumoniae as the fermenter converted glucose and xylose into lactate to feed the exoelectrogens (S. oneidensis). To produce more lactate in K. pneumoniae, we eliminated the ethanol and acetate pathway via deleting pta (phosphotransacetylase gene) and adhE (alcohol dehydrogenase gene) and further constructed a synthesis and delivery system through expressing ldhD (lactate dehydrogenase gene) and lldP (lactate transporter gene). To facilitate extracellular electron transfer (EET) of S. oneidensis, a biosynthetic flavins pathway from Bacillus subtilis was expressed in a highly hydrophobic S. oneidensis CP-S1, which not only improved direct-contacted EET via enhancing S. oneidensis adhesion to the carbon electrode but also accelerated the flavins-mediated EET via increasing flavins synthesis. Furthermore, we optimized the ratio of glucose and xylose concentration to provide a stable carbon source supply in MFCs for higher power density. The glucose and xylose co-fed MFC inoculated with the recombinant consortium generated a maximum power density of 104.7 ± 10.0 mW/m², which was 7.2-folds higher than that of the wild-type consortium (12.7 ± 8.0 mW/m²). Lastly, we used this synthetic microbial consortium in the corn straw hydrolyzates-fed MFC, obtaining a power density 23.5 ± 6.0 mW/m².

Novel Gas Diffusion Cloth Bioanodes for High-Performance Methane-Powered Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising technology that converts chemical energy into electricity. However, up to now only few MFCs have been powered by gas fuels, such as methane, and their limited performance is still challenged by the low solubility and bioavailability of gases. Here, we developed a gas diffusion cloth (GDC) anode to significantly enhance the performance of methane-powered MFCs. The GDC anode was constructed by simply coating waterproof GORE-TEX cloth with conductive carbon cloth in one step. After biofilm enrichment, the GDC anodes obtained a methane-dependent current up to 1130.2 mA m^-2, which was 165.2 times higher than conventional carbon cloth (CC) anodes. Moreover, MFCs equipped with GDC anodes generated a maximum power density of 419.5 mW m^-2. Illumina high-throughput sequencing revealed that the GDC anode biofilm was dominated mainly by Geobacter, in contrast with the most abundant Methanobacterium in planktonic cells. It is hypothesized that Methanobacterium reversed the methanogenesis process by transferring electrons to the anodes, and Geobacter generated electricity via the intermediates (e.g., acetate) of anaerobic methane oxidation. Overall, this work provides an effective route in preparing facile and cost-effective anodes for high-performance methane MFCs.

Methane as a Substrate for Energy Generation Using Microbial Fuel Cells

Methane (CH₄) is a well-known and abundant feedstock for natural gas, and is readily available from various sources. In thermal plants, the CH₄ generated from anthropogenic sources is converted into electrical energy via combustion. Microbial fuel cell (MFC) technology has proven to be an efficient strategy for the biological conversion of a many substrates, including biogas (CH₄), to electricity. MFC technology uses gaseous substrate along with an enriched and selective microbial consortium. Predominantly, methanotrophs and electrochemically active Geobacter were utilized in a syntrophic association on the anode of an MFC. This review focuses on the exploitation of CH₄ as a substrate for bioelectrogenesis via MFCs.