Cable bacteria metabolism

Deep research

Generated by just research-trait; source: research/traits/metabolism/cable_bacteria_metabolism-deep-research-falcon.md

# Curation-focused research report: Cable bacteria metabolism

## Trait record and scope

- **Trait label:** Cable bacteria metabolism
- **Trait identifier:** **METPO:1002003**
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** METPO:1000060
- **Operational definition:** a filament-level metabolism in which oxidation and reduction half-reactions occur in spatially separated cells and are coupled by electronic transport over millimetre-to-centimetre distances.

The canonical phenotype is **electrogenic sulfur oxidation**: cells in deeper anoxic sediment oxidize sulfide, electrons travel through a multicellular filament, and cells near an oxic boundary reduce oxygen. Nitrate can substitute as an acceptor in some taxa or conditions. This division of labor distinguishes the trait from ordinary intracellular respiration and short-range extracellular electron transfer. Direct Raman experiments showed a cytochrome-redox gradient along individual living filaments; removing oxygen or laser-cutting the filament immediately collapsed the gradient, establishing that electron flow depends on an intact connection between donor and acceptor zones (bjerg2018longdistanceelectrontransport pages 1-2).

The trait should be represented at the **whole-filament level**, because no single cell necessarily contains both terminal half-reactions. Genomic and physiological models distinguish anoxic “anodic” cells from oxic “cathodic” cells and place energy conservation principally in sulfide-oxidizing cells; cathodic oxygen reduction without energy conservation remains a well-supported model rather than a completely resolved molecular mechanism (kjeldsen2019ontheevolution pages 1-1, wang2024electrogenicsulfuroxidation pages 2-3).

### Boundary cases

1. **Sulfur disproportionation** by groundwater cable bacteria is metabolic capacity adjacent to, but not identical with, the defining long-distance metabolism. Curate it as a taxon/condition-specific branch rather than as necessary for `METPO:1002003`.
2. **Nitrate reduction/DNRA** is an alternative cathodic module, not required for every cable-bacterium filament. The 2024 synthesis describes nitrate reduction toward ammonium, but taxon and assay context should be retained (wang2024electrogenicsulfuroxidation pages 2-3).
3. **Electrode respiration/EET** is an experimentally demonstrated extension of cable-bacterium metabolism, but electrode use is not part of the trait definition. In 2024, living filaments moved toward +200 mV carbon electrodes and withdrew when the potential was removed (bonne2024interactionofliving pages 1-2, bonne2024interactionofliving pages 2-5).
4. Generic nanowire production, conductive biofilms, sulfate reduction, and sulfur oxidation by non-cable filamentous bacteria are insufficient. The diagnostic feature is centimeter-scale conduction through a multicellular cable-bacterium filament.

## Current mechanistic model

Anoxic cells oxidize reduced sulfur and pass electrons into a periplasmic transport system. A continuous network of parallel conductive fibers traverses cells and cell–cell junctions. Direct electrode measurements detected nanoampere currents across intact filaments as long as 10.1 mm and more than 2,000 cells; isolated fiber networks reached conductivities up to 79 S cm⁻¹. Conductance persisted under vacuum but fell to approximately 2% after 30 minutes in air, demonstrating electronic rather than ionic charge transport (meysman2019ahighlyconductive pages 1-2).

At the acceptor end, electrons are discharged to oxygen, or in some organisms to nitrate. Periplasmic c-type cytochromes show redox gradients and are plausible interfaces between metabolism and the conductive fibers, but their status as the principal long-range carrier is not established. Likewise, the precise terminal oxygen reductase is unresolved and may vary among cable-bacterium lineages (wang2024electrogenicsulfuroxidation pages 2-3, wang2024electrogenicsulfuroxidation pages 3-3).

The leading 2019 metabolic reconstruction proposed sulfide oxidation by reversal of the canonical dissimilatory sulfate-reduction pathway and autotrophic carbon fixation through the Wood–Ljungdahl pathway. Because more than half of the genes in the analyzed genomes were then unknown and pure cultures were unavailable, these pathway directions should be encoded as proposed rather than definitive catalytic steps (kjeldsen2019ontheevolution pages 1-1). Recent genomic work continues to reveal lineage-specific alternatives; for example, a 2025 strain encodes a complete reductive Dsr repertoire, Psr/PhsABC, a Nap system, and a truncated hemoglobin proposed for oxygen reduction, underscoring that genome presence alone does not establish reaction direction (hiralal2025anovelcable pages 10-13).

A major development is the identification of a **sulfur-ligated nickel cofactor** in the conductive protein core. Oxidation or removal of nickel reduces conductivity, and 2024 comparative genomics found marked adaptations in nickel import, export, binding, and chaperoning. These results support a causal nickel-dependent conduction module, although the exact conductive protein sequence and electron-transfer chemistry remain unresolved (zhuang2024electrontransferin pages 6-8, wang2024electrogenicsulfuroxidation pages 3-3).

## Candidate nodes grouped by type

### Trait, organism, and habitat nodes

| Candidate node | Suggested grounding | Curation note |
|---|---|---|
| Cable bacteria metabolism | **METPO:1002003** | Root trait node; quote identifier verbatim in YAML. |
| Cable bacteria | NCBITaxon label search required | Include *Candidatus Electrothrix* and *Candidatus Electronema* as organism contexts; do not invent taxon CURIEs. |
| Multicellular cable-bacterium filament | Label-only candidate | Functional unit spanning donor and acceptor zones. |
| Anodic cell / cathodic cell | Label-only candidates | Physiological states/locations, not established stable cell types. |
| Aquatic sediment | ENVO grounding recommended after registry lookup | Marine, estuarine, freshwater, and groundwater contexts should remain distinguishable. |
| Anoxic sulfidic sediment zone | ENVO label candidate | Donor-side microenvironment. |
| Oxic sediment surface zone | ENVO label candidate | Oxygen-acceptor microenvironment. |
| Suboxic zone | ENVO label candidate | Geochemical phenotype between O₂ and H₂S fronts. |

Cable bacteria occur in marine and freshwater sediments and can generate 1–4 cm zones lacking both detectable oxygen and sulfide, with characteristic pH separation between cathodic and anodic regions (kjeldsen2019ontheevolution pages 1-1).

### Chemicals and electron-transfer roles

| Node | Suggested CURIE | Role |
|---|---|---|
| Hydrogen sulfide | CHEBI:16136 | Canonical electron donor/substrate. |
| Sulfide | CHEBI registry verification required for protonation-specific form | Reduced-sulfur donor pool. |
| Oxygen | CHEBI:15379 | Canonical terminal electron acceptor. |
| Nitrate | CHEBI:17632 | Alternative acceptor in supported taxa/conditions. |
| Nitrite | CHEBI:16301 | Candidate DNRA intermediate. |
| Ammonium | CHEBI:28938 | Candidate nitrate-reduction product. |
| Sulfate | CHEBI:16189 | Net oxidized sulfur product. |
| Elemental sulfur | CHEBI:33403 | Taxon/assay-specific donor or disproportionation substrate. |

Showing the first 60 of 214 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.