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Anaerobic oxidation of methane coupled with extracellular electron transfer to electrodes
Yaohuan Gao, Jangho Lee, Josh D. Neufeld, Joonhong Park +2
KI-Zusammenfassung
82 % KonfidenzThis paper demonstrates anaerobic oxidation of methane (AOM) coupled to extracellular electron transfer (EET) to a carbon-fiber anode in a dual-chamber microbial electrochemical cell. A biofilm dominated by Methanobacterium spp. (67% of archaeal reads) and Geobacter spp. (38% of bacterial reads) generated steady-state current of 11.0 ± 1.3 mA/m² at anode potential −0.4 V vs Ag/AgCl (−0.2 V vs SHE) under anaerobic conditions. Isotopic analysis (δ¹³C fractionation 1.0153–1.0174) and alternating gas tests confirmed methane-dependent current. Metagenomics and FISH revealed syntrophic partnership: Methanobacterium catalyzes reverse methanogenesis while Geobacter transfers electrons via formate or H₂ intermediates, not direct interspecies electron transfer.
Zentrale Erkenntnisse
Mechanismus Methanobacterium spp. (67% of archaeal community) and Geobacter spp. (38% of bacterial community) form a syntrophic consortium where methanogens oxidize methane and bacteria transfer electrons to the anode via formate or H₂ intermediates, not direct contact.
Was heißt das. Design biofilm anodes expecting syntrophic partnerships rather than single-species dominance; monitor for formate and H₂ accumulation as indicators of inter-species electron transfer efficiency.
Schwellenwert Anode potential of −0.4 V vs Ag/AgCl (−0.2 V vs SHE) sustained AOM-EET current of 11.0 ± 1.3 mA/m² for >300 days under continuous methane sparging in anaerobic conditions, with no exogenous electron acceptor.
Was heißt das. Operate AOM-EET anodes at potentials ≤−0.4 V vs Ag/AgCl to maintain stable current; avoid more positive potentials that may favor competing metabolisms.
Methode Isotopic fractionation factors (1.0153–1.0174) for CO₂ produced from methane oxidation fall within the known range for AOM (1.012–1.039), confirming anaerobic rather than aerobic methane oxidation despite 0.8% aerobic methanotroph sequences in the metagenome.
Was heißt das. Use carbon isotope fractionation as a diagnostic tool to distinguish AOM-EET from aerobic methanotrophy in mixed biofilms; fractionation >1.012 indicates anaerobic oxidation.
Methode Metagenome detected no pmo genes (particulate methane monooxygenase, essential for aerobic oxidation) despite 0.8% aerobic methanotroph sequences, and 24.1% of methane-metabolism reads were methanogenesis genes (reverse methanogenesis pathway for AOM).
Was heißt das. Confirm AOM pathway by screening for mcrA (methyl-coenzyme M reductase) and methanogenesis genes rather than pmo; absence of pmo in a methane-oxidizing biofilm does not rule out aerobic methanotrophs if they are minor constituents.
Methode Abiotic non-Faradaic current was 1 ± 0.2 mA/m², requiring subtraction from total current to isolate biological AOM-EET contribution; this correction reduced reported current from ~12–14 mA/m² to net 11.0 ± 1.3 mA/m².
Was heißt das. Always run abiotic controls with identical electrode geometry and medium; subtract non-Faradaic current to avoid overestimating biological activity by ~10%.
Widersprüche3
Nur Konflikte, die die Publikation selbst zeigt, jeweils am Text geprüft.
Innerhalb der Publikation Aerobic methanotrophs detected in metagenome but pmo gene (essential for aerobic oxidation) absent; current generation confirmed anaerobic.
Eine SeiteAerobic methanotrophs (Methylococcaceae and Methylocystaceae) comprised 0.8% of classified ribosomal RNA genes
“aerobic methanotrophs (Methylococcaceae and Methylocystaceae) comprised 0.8% of the classified ribosomal RNA genes (data not shown)”
Andere SeiteThe pmo gene, encoding particulate methane monooxygenase, which is mainly essential for aerobic oxidation of methane, was not detected
“The pmo gene, encoding particulate methane monooxygenase, which is mainly essential for aerobic oxidation of methane, was not detected, suggesting that O2-dependent methane oxidation would not occur in the MxC”
Einordnung der Autoren. Authors resolve this by noting that the 0.8% aerobic methanotrophs are minor constituents and lack the functional genes to perform aerobic oxidation; the anaerobic chamber environment (H₂ at 5% partial pressure) suppresses aerobic metabolism. The presence of these sequences likely reflects inoculum carryover or dormant cells.
Innerhalb der Publikation Methanobacterium spp. catalyze AOM but lack multi-heme cytochromes and EET genes; Geobacter carries all EET genes but no AOM genes.
Eine SeiteMost AOM-pathway genes were affiliated with Methanobacterium-related archaea...none of these genes were associated with the Geobacter genus
“According to taxonomic identification of the essential AOM-pathway genes detected by shotgun sequencing, the AOM-pathway genes belong to the family Methanobacteriaceae, such as Methanobacterium and Methanothermobacter, but none of these gen”
Andere SeiteGeobacter-related bacteria accounted for 89% of cytochrome c annotations and 94% of type IV pili annotations...Methanobacterium spp. did not contribute any of these genes
“Geobacter-related bacteria accounted for 89% of cytochrome c annotations and 94% of type IV pili annotations...In contrast, Methanobacterium spp. did not contribute any of these genes to the metagenome”
Einordnung der Autoren. Authors resolve this by proposing syntrophic division of labor: Methanobacterium performs reverse methanogenesis to oxidize methane and generate formate/H₂; Geobacter oxidizes these intermediates and transfers electrons to the anode via cytochromes and pili. This is intermediate-dependent inter-species electron transfer (IDET), not direct interspecies electron transfer (DIET).
Innerhalb der Publikation H₂ generated at cathode could diffuse to anode and be oxidized, confounding AOM-EET attribution.
Eine SeiteBecause H₂ is generated at the cathode, it is possible that a very small amount of H₂ gas diffused to the anode and might have affected current density in the biofilm anode
“Because H₂ is generated at the cathode, it is possible that a very small amount of H₂ gas diffused to the anode and might have affected current density in the biofilm anode”
Andere SeiteThe effect of H₂ diffusion has to have been very small because the H₂ generated at the cathode came from the oxidation of methane at the anode. As H₂ diffusion from the cathode could not have been a significant fraction of the H₂ generated at the cathode, the relative impact of H₂ oxidation at the anode would have been negligible
“The effect of H₂ diffusion has to have been very small because the H₂ generated at the cathode came from the oxidation of methane at the anode”
Einordnung der Autoren. Authors acknowledge the concern but argue it is negligible because H₂ at the cathode is produced from electrons derived from methane oxidation at the anode; any H₂ diffusion back to the anode would represent a small fraction of the total H₂ produced and would not significantly alter the net current attribution to AOM.
Ansatzpunkte6
Konkrete Lücken in dieser Publikation, jeweils mit einem Experiment, das sie schließen würde.
Mittel No measurement of dissolved methane concentration in the anode chamber over time; only headspace saturation (~25 mg CH₄/L) confirmed at start. Methane limitation or accumulation during operation unknown.
Paper states 'We monitored the dissolved methane concentration with a headspace method and confirmed saturation of aqueous methane concentration (~25 mg CH₄/L) in the medium during experiments' but provides no time-series data. Current density stability (Fig. 1b) suggests methane was not limiting, but quantitative profile would strengthen mechanistic understanding.
Versuch. Measure dissolved CH₄ concentration weekly via headspace GC; correlate with current density to identify methane limitation threshold and optimize sparging rate.
Hoch No measurement of formate or H₂ accumulation in the anode chamber; syntrophic intermediate-dependent electron transfer (IDET) mechanism inferred from gene presence but not validated by metabolite quantification.
Metagenome shows formate dehydrogenase and hydrogenase genes >50% Geobacter-affiliated, and authors propose IDET via formate or H₂ as the electron transfer mechanism. However, no liquid chromatography or gas chromatography data for these intermediates provided. Direct evidence would confirm the proposed mechanism.
Versuch. Measure formate and H₂ concentrations in anode chamber via HPLC and GC respectively during steady-state operation; correlate with current density and gene expression (qPCR of formate dehydrogenase and hydrogenase).
Mittel No cathode characterization or cathode potential control; only anode potential fixed at −0.4 V vs Ag/AgCl. Cathode potential, H₂ production rate, and pH changes in cathode chamber not reported.
Dual-chamber MEC with anion exchange membrane separating anode and cathode. Cathode potential and H₂ production rate determine the thermodynamic driving force for the overall cell and the H₂ diffusion risk mentioned by authors. Catholyte pH and composition affect ion transport across membrane.
Versuch. Install reference electrode in cathode chamber; measure cathode potential, H₂ production rate (gas chromatography), and pH over time. Vary cathode potential to optimize overall cell voltage and current.
Hoch No variation of anode potential; only −0.4 V vs Ag/AgCl tested. Operating window for AOM-EET (e.g., −0.6 to −0.2 V) and trade-off between current density and methane oxidation rate unknown.
Single anode potential used throughout 1,052-day experiment. For comparison, acetate-fed biofilm achieved 7.3 A/m² at same potential. No data on whether more negative potentials increase current or more positive potentials favor competing metabolisms (e.g., methanogenesis).
Versuch. Chronoamperometry at anode potentials −0.6, −0.5, −0.4, −0.3, −0.2 V vs Ag/AgCl; measure steady-state current density and biofilm composition (FISH, qPCR) at each potential to identify optimal window.
Niedrig No measurement of N₂O, NO₂⁻, or NO₃⁻ in anode or cathode chambers; nitrogen cycling not characterized despite focus on bioelectrochemical systems.
Paper is about AOM-EET, not nitrogen cycling. However, inoculum from wastewater treatment plant likely contains nitrifiers and denitrifiers. Ammonium oxidation explicitly ruled out (Figure S5), but no data on nitrite/nitrate reduction or N₂O production, which are common in MES.
Versuch. Measure NO₃⁻, NO₂⁻, NH₄⁺, and N₂O in anode and cathode chambers via ion chromatography and GC; assess whether biofilm performs simultaneous AOM and denitrification.
Mittel No long-term stability test after 1,052 days; final current density and biofilm viability at end of experiment not reported. Decline in current density visible in Fig. 1b after day ~800 not explained.
Figure 1b shows current density fluctuations and apparent decline after ~800 days. Paper states operation continued for >300 days with methane alone, but final steady-state current and reason for fluctuations (methane limitation, biofilm detachment, electrode fouling) not discussed.
Versuch. Continue operation for additional 6–12 months; measure current density, biofilm thickness (confocal microscopy), electrode impedance (electrochemical impedance spectroscopy), and community composition (FISH, qPCR) monthly to assess long-term stability and degradation mechanisms.
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Was gemacht wurde
- Systemtyp
- FUNDAMENTAL
Was funktioniert hat
Zusammenfassung
Anaerobic oxidation of methane (AOM) is an important process for understanding the global flux of methane and its relation to the global carbon cycle. Although AOM is known to be coupled to reductions of sulfate, nitrite, and nitrate, evidence that AOM is coupled with extracellular electron transfer (EET) to conductive solids is relatively insufficient. Here, we demonstrate EET-dependent AOM in a biofilm anode dominated by Geobacter spp. and Methanobacterium spp. using carbon-fiber electrodes as the terminal electron sink. The steady-state current density was kept at 11.0 ± 1.3 mA/m 2 in a microbial electrochemical cell, and isotopic experiments supported AOM-EET to the anode. Fluorescence in situ hybridization images and metagenome results suggest that Methanobacterium spp. may work synergistically with Geobacter spp. to allow AOM, likely by employing intermediate (formate or H 2 )-dependent inter-species electron transport. Since metal oxides are widely present in sedimentary and terrestrial environments, an AOM-EET niche would have implications for minimizing the net global emissions of methane.
Schlagwörter
Kennungen
- PubMed
- 28698657
- Zeitschrift
- Scientific Reports
- Jahr
- 2017
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