A syntrophic two-member consortium consisting of Geobacter metallireducens and Methanosarcina barkeri that performs direct interspecies electron transfer (DIET) during ethanol oxidation. G. metallireducens oxidizes ethanol and transfers electrons directly to M. barkeri via electrically conductive pili (e-pili), bypassing the need for diffusible electron carriers like H2 or formate. M. barkeri uses these electrons to reduce CO2 to methane. This coculture forms aggregates and achieves stoichiometric conversion of ethanol to methane via DIET. Unlike hydrogen-mediated syntrophy, DIET relies on biological electrical connections between cells, with pili serving as conduits for long-range electron transfer. M. barkeri is unique as a methanogen capable of using either H2 or electrons from DIET for CO2 reduction, making it a model organism for studying electrical connections in microbial communities.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Geobacter metallireducens | NCBITaxon:28232 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
SYNTROPHIC_PARTNER
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N/A |
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| Methanosarcina barkeri | NCBITaxon:2208 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
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N/A |
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Ecological Interactions
The graph shows only explicitly linked interaction participants. Community members without an asserted participant link are omitted; membership alone does not imply an interaction.
Ethanol Oxidation and Direct Electron Transfer
SYNTROPHYG. metallireducens oxidizes ethanol to acetate and supplies electrons to M. barkeri through DIET. Pilin-deficient cocultures did not establish ethanol-to-methane conversion without amendment, whereas granular activated carbon enabled the tested mutant cocultures. Thus the pili-associated requirement is condition-dependent, not universal in conductive-material-amended cultures. The methanogen entry proteins remain unresolved; the donor reaction should not be described as direct ethanol-carbon oxidation to CO2.
Source Taxon: Geobacter metallireducens
Target Taxon: Methanosarcina barkeri
Metabolites: ethanol (CHEBI:16236 · MediaIngredientMech · CMMMech), acetate (CHEBI:30089 · MediaIngredientMech · CMMMech)
Biological Processes:
- ethanol catabolic process (GO:0006068 · PathwayMech)
DIET delivers ethanol-derived electrons to M. barkeri for CO2 reduction. This electron-supply arrow supports the CO2 route, not an attribution of all bulk methane to that route; acetate conversion is a separate missing branch.
Evidence
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PMID:24837373 - SUPPORT (IN_VITRO)"Cocultures formed aggregates that shared electrons via DIET during the stoichiometric conversion of ethanol to methane"
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PMID:24837373 - SUPPORT (IN_VITRO)"Cocultures could not be initiated with a pilin-deficient G. metallireducens strain, suggesting that long-range electron transfer along pili was important for DIET"
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PMID:24837373 - SUPPORT (IN_VITRO)"Amendments of granular activated carbon permitted the pilin-deficient G. metallireducens isolates to share electrons with M. barkeri"
DIET-Linked Carbon Dioxide Reduction to Methane
M. barkeri accepts DIET electrons for CO2 reduction to methane. Bulk ethanol-to-methane conversion also includes acetate metabolism; its stoichiometry is not an isotope measurement assigning every methane molecule to a route. The 2014 first-report wording is historical, not timeless uniqueness. Whether DIET-driven CO2 reduction alone supports methanogen growth, without acetate metabolism, was proposed for future testing rather than established here.
Participating Taxa: Geobacter metallireducens, Methanosarcina barkeri
Metabolites: methane (CHEBI:16183 · MediaIngredientMech · CMMMech), carbon dioxide (CHEBI:16526 · MediaIngredientMech · CMMMech)
Biological Processes:
- methanogenesis (GO:0015948 · PathwayMech)
Evidence
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PMID:24837373 - SUPPORT (IN_VITRO)"M. barkeri is the second methanogen found to accept electrons via DIET and the first methanogen known to be capable of using either H2 or electrons derived from DIET for CO2 reduction"
Hcp-Associated Delay of DIET Establishment
In the G. metallireducens-M. barkeri comparison, wild-type Geobacter cocultures had a methane-production lag exceeding 30 days, while hcp-deficient cocultures had little lag. This is a measured genotype-associated establishment phenotype, not niche partitioning or demonstrated toxin delivery. Discovery RNA-seq and GAC/AQDS comparisons used the G. sulfurreducens partner, not M. barkeri. Altered extracellular-electron-transfer transcripts and the mutant Fe(III)-oxide phenotype expose pleiotropy. T6SS-specific inhibition of the archaeon remains HYPOTHESIZED; protein secretion or killing was not measured by the lag comparison.
Participating Taxa: Geobacter metallireducens, Methanosarcina barkeri
HYPOTHESIZED NEGATIVE: The wild-type Hcp/T6SS-associated state may delay DIET establishment relative to the hcp deletion strain. The exact coculture demonstrates a lag difference, but altered electron-transfer physiology prevents assigning the delay selectively to T6SS antagonism. The sign refers to establishment timing, not proven archaeal killing.
Evidence
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PMID:37650614 - SUPPORT (IN_VITRO)"there was a lag period of more than 30 d in co-cultures initiated with wild-type G. metallireducens"
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PMID:37650614 - SUPPORT (IN_VITRO)"In contrast, there was very little lag in co-cultures initiated with the Hcp-deficient G. metallireducens strain"
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PMID:37650614 - SUPPORT (IN_VITRO)"The hcp-deficient mutant also established DIET quicker with Methanosarcina barkeri"
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PMID:37650614 - PARTIAL (IN_VITRO)"the mutant also reduced Fe(III) oxide faster than the wild-type strain, a phenotype not expected from the loss of the T6SS"
Knowledge gaps & discussions (2)
KNOWLEDGE_GAP OPEN Which electron-entry components and hcp-dependent changes govern this DIET partnership?
Preserve the positive DIET and mutant-lag observations while separating genotype effects from secretion-specific antagonism. Partner-specific perturbations and complementation in the methanogenic coculture would distinguish proposed mechanisms. The Pelobacter/H2 comparator used a different medium and is not a selective same-medium H2 switch. Methanosaeta radiotracer results must not be imported into this system. Non-graph uniqueness/aggregation claims remain preserved, not certified.
Attaches to: ecological_interactions#Ethanol Oxidation and Direct Electron Transfer, ecological_interactions#DIET-Linked Carbon Dioxide Reduction to Methane, ecological_interactions#Hcp-Associated Delay of DIET Establishment
CURATION_TODO OPEN Complete the separately supported acetate branch after Edison review.
The primary paper supports an acetate contribution, but the graph still lacks that branch. A separate one-community Edison dry run completed without API calls or credits; specific authorization and report inspection remain pending under #1483. Add source-backed structure with its own evidence, without promoting ideal stoichiometry to an isotope-measured fraction or claiming DIET-only growth. This repair remains needs_research.
Attaches to: ecological_interactions#Ethanol Oxidation and Direct Electron Transfer, ecological_interactions#DIET-Linked Carbon Dioxide Reduction to Methane, ecological_interactions#Hcp-Associated Delay of DIET Establishment
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Anaerobic Conditions | Strict anaerobic | N/A |
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| Cell Aggregation | Required for DIET | N/A |
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| Pili Expression | Essential for DIET initiation | N/A |
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| Conductive Material Substitution | Granular activated carbon can replace pili | N/A |
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Growth Media
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