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Disproportionation

METPO:1000806 · CLASS · REVIEWED

A metabolism in which a single substrate simultaneously undergoes both oxidation and reduction reactions, with part of the substrate serving as the electron donor and another part serving as the electron acceptor.

Sulfur disproportionation redox split

DOI-backed graph linking a single sulfur intermediate to simultaneous oxidation and reduction products under anoxic conditions.

Sulfur disproportionation redox split Interactive directed graph showing evidence-backed causal relationships for Disproportionation.

Edge evidence

  • Disproportionation occurs in anoxic condition biolink:occurs_in

    Sulfur disproportionation is commonly observed as anaerobic microbial metabolism.

    • DOI:10.1080/17415990802105770 microbiologically catalyzed chemolithotrophic process Supports microbial sulfur disproportionation under non-oxygenated sulfur cycling contexts.
  • inorganic sulfur intermediate serves as electron donor and acceptor Disproportionation METPO:2007603

    A single sulfur intermediate provides both donor and acceptor halves.

    • DOI:10.1080/17415990802105770 serve as both electron donor and acceptor Directly supports the defining redox split.
  • elemental sulfur example of inorganic sulfur intermediate rdfs:subClassOf

    Elemental sulfur is a disproportionation substrate.

    • DOI:10.1016/j.gca.2013.03.013 elemental sulfur disproportionation Supports elemental sulfur as a substrate.
  • thiosulfate example of inorganic sulfur intermediate rdfs:subClassOf

    Thiosulfate is a disproportionation substrate.

    • DOI:10.1080/17415990802105770 elemental sulfur, thiosulfate, and sulfite Supports thiosulfate among disproportionated sulfur intermediates.
  • inorganic sulfur intermediate reduced to sulfide

    One fraction of the substrate is reduced to sulfide.

    • DOI:10.1080/17415990802105770 generate hydrogen sulfide and sulfate Supports sulfide as reduced product.
  • inorganic sulfur intermediate oxidized to sulfate METPO:2007405

    One fraction of the substrate is oxidized to sulfate.

    • DOI:10.1080/17415990802105770 generate hydrogen sulfide and sulfate Supports sulfate as oxidized product.
  • reverse sulfate reduction pathway participates in Disproportionation biolink:participates_in

    Some disproportionating microbes use reversed sulfate-reduction enzymes.

    • DOI:10.1080/17415990802105770 reverse the sulfate reduction pathway Supports the proposed pathway mechanism for some isolates.
  • thiosulfate derives into sulfate RO:0001001

    Thiosulfate disproportionation yields sulfate as the oxidized product.

    • DOI:10.1128/msystems.00954-22 Thiosulfate disproportionation follows the stoichiometry S2O3(2-) + H2O -> SO4(2-) + HS- + H+.
  • thiosulfate derives into sulfide RO:0001001

    Thiosulfate disproportionation yields sulfide as the reduced product.

    • DOI:10.1128/msystems.00954-22 Thiosulfate disproportionation follows the stoichiometry S2O3(2-) + H2O -> SO4(2-) + HS- + H+.
  • TsdA thiosulfate dehydrogenase catalyzes oxidation of thiosulfate

    TsdA oxidizes thiosulfate to tetrathionate, the entry step of the S4 intermediate pathway.

    • DOI:10.3389/fmicb.2024.1426584 tsdA (and doxDA) catalyze conversion of thiosulfate (S2O3 2-) to tetrathionate (S4O6 2-).
  • TsdA thiosulfate dehydrogenase produces tetrathionate METPO:2007800

    Product of TsdA-catalyzed thiosulfate oxidation is tetrathionate.

    • DOI:10.3389/fmicb.2024.1426584 tsdA catalyzes conversion of thiosulfate to tetrathionate (S4O6 2-).
  • TetH tetrathionate hydrolase disproportionates tetrathionate METPO:2007811

    TetH hydrolyzes/disproportionates tetrathionate into elemental sulfur, thiosulfate, and sulfate.

    • DOI:10.3389/fmicb.2024.1426584 Tetrathionate can be subsequently disproportionated via tetH to form S0, S2O3 2-, and SO4 2-.
  • SOR sulfur oxygenase reductase disproportionates elemental sulfur METPO:2007811

    SOR catalyzes disproportionation of elemental sulfur, releasing sulfite and sulfide.

    • DOI:10.1007/978-3-031-54306-7_15 The SOR catalyzes an O2-dependent S0 disproportionation releasing SO3 2- and HS-.
  • SOR sulfur oxygenase reductase produces sulfite METPO:2007800

    Sulfite is the oxidized product of SOR-catalyzed elemental sulfur disproportionation.

    • DOI:10.1007/978-3-031-54306-7_15 SOR catalyzes an O2-dependent S0 disproportionation releasing SO3 2- and HS-.
  • iron-sulfide mineral formation scavenges sulfide

    Formation of Fe-sulfide minerals scavenges produced sulfide, relieving product inhibition of sulfur disproportionation.

    • DOI:10.1128/msystems.00954-22 Ferrihydrite scavenges produced sulfide as Fe-sulfide.

Provenance

Source
METPO (2025-11-25)
Author
Jed Dongjin Kim-Ozaeta
Definition source
DOI:10.1080/17415990802105770

Parent traits (1)

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1000806 [-0.155, -1.068, -1.362, +0.081, …]

512-dim DeepWalkSkipGramEnsmallen embedding from kg-microbe (2026-04-25).

Nearest neighbors in embedding space

Top-8 cosine-similar METPO traits from the 2026-04-25 deepwalk (512-D).

Deep research

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

Unreviewed literature output — not curated TraitMech content Ontology identifiers suggested below have not been resolved against their ontologies, and some are known to be wrong. Check any CURIE against the source before using it. 
# Curation-focused research report: microbial disproportionation

## Executive summary

**Target trait:** **Disproportionation**  
**Trait identifier:** **METPO:1000806**  
**Category / kind / status:** METABOLISM / CLASS / REVIEWED  
**Parent:** **METPO:1000060**

The trait should represent a microbial metabolic capacity in which **one intermediate-oxidation-state substrate supplies both the oxidized and reduced product branches**. For the evidence currently available, the best-resolved microbial instance is inorganic sulfur disproportionation: elemental sulfur, thiosulfate, sulfite, or tetrathionate is converted into more oxidized sulfur—usually sulfate—and more reduced sulfur—usually sulfide. It is often termed “inorganic fermentation,” although that phrase is an analogy rather than a mechanistic ontology definition. In canonical anaerobic growth, no external electron donor or terminal acceptor is required for the redox split itself. Sulfide removal can nevertheless be essential to the thermodynamics of elemental-sulfur disproportionation. (thamdrup1993bacterialdisproportionationof pages 1-2, finster2013completegenomesequence pages 1-2)

The strongest graph should contain **substrate-specific reaction modules**, not one universal enzyme pathway. Desulfobulbaceae-like organisms support a model involving Sat–AprAB in an oxidative branch and DsrAB/DsrC in a reductive branch. Conversely, 2023 experiments showed that *Sulfurimonas* and *Sulfurovum* disproportionate sulfur despite lacking `aprAB`, `dsrAB`, `dsrC`, `dsrMKJOP`, and `qmoABC`, demonstrating at least one unresolved alternative mechanism. (wang2023disproportionationofinorganic pages 1-2, hashimoto2022physiologicalandcomparative pages 12-13, hashimoto2022physiologicalandcomparative pages 7-9)

## 1. Trait scope and diagnostic phenotype

### 1.1 Recommended operational definition

Curate **METPO:1000806** when all of the following are demonstrated or explicitly asserted by a reliable source:

1. The same named substrate pool is partitioned into products at both higher and lower oxidation states.
2. Both product branches are measured or otherwise directly supported.
3. The reaction is biologically catalyzed.
4. For a physiological trait assertion, growth, energy conservation, repeated transfer, or substrate-dependent activity is shown.
5. Alternative explanations—ordinary oxidation using oxygen/nitrate, ordinary sulfur reduction using an added donor, and abiotic product formation—are excluded or separately modeled.

A high-confidence sulfur assay therefore measures substrate depletion, sulfate and sulfide formation, and cell growth under anoxic conditions. The 2023 Campylobacterota study used direct cell counts, methylene-blue sulfide assays, ion chromatography for thiosulfate/sulfate, and ferrozine measurements of iron reduction. (wang2023disproportionationofinorganic pages 15-17)

### 1.2 Core reactions

Canonical net reactions supported by culture experiments include:

- **Elemental sulfur:** `4 S0 + 4 H2O → SO4^2− + 3 H2S + 2 H+`.
- **Thiosulfate:** `S2O3^2− + H2O → SO4^2− + HS− + H+`.
- With ferrihydrite as sulfide sink, the observed elemental-sulfur net reaction can be written `3 S0 + 2 Fe(OH)3 → SO4^2− + 2 FeS + 2 H+ + 2 H2O`. (wang2023disproportionationofinorganic pages 9-12, thamdrup1993bacterialdisproportionationof pages 1-2, canfield1998isotopefractionationand pages 7-8)

Thiosulfate and sulfite disproportionation were reported as exergonic under standard conditions, approximately −21.9/−22.3 and −58.9 kJ mol−1 substrate, respectively. Elemental-sulfur disproportionation is unfavorable or nearly neutral under standard conditions but becomes favorable when free sulfide is maintained at low concentration; one analysis estimated approximately −30 kJ mol−1 S0 at `H2S = 10−7 M` and sulfate `2.8 × 10−2 M`. (wang2023disproportionationofinorganic pages 1-2, finster2013completegenomesequence pages 1-2)

### 1.3 Boundaries and nearby traits

- **Not ordinary sulfur oxidation:** oxidation of sulfide, sulfur, or thiosulfate with O2 or nitrate as the external electron acceptor does not satisfy the same-substrate redox split.
- **Not sulfur reduction:** reduction of S0, sulfite, or thiosulfate using H2 or organic carbon as the electron donor is a separate respiratory process.
- **Not comproportionation:** combining sulfur species at different oxidation states to produce an intermediate is the reverse conceptual pattern.
- **SOR is a boundary case:** sulfur oxygenase reductase catalyzes coupled oxidation and reduction of S0, but its reported reaction consumes molecular oxygen. It should be represented as **oxygen-dependent SOR-catalyzed sulfur oxygenation/reduction**, not automatically equated with canonical anaerobic, energy-conserving disproportionation. (guo2016sulfurmetabolismpathways pages 7-8)
- **TetH/TTH is a boundary case:** acidophilic tetrathionate hydrolase converts tetrathionate into thiosulfate, elemental sulfur, sulfate, and related polythionates in the S4-intermediate sulfur-oxidation pathway. This is enzyme-catalyzed bond hydrolysis within sulfur oxidation metabolism; evidence of TetH activity alone is insufficient to assign the physiological trait METPO:1000806. (kanao2024tetrathionatehydrolasefrom pages 1-2, kanao2024tetrathionatehydrolasefrom pages 3-4)
- **Metagenomic prediction alone is insufficient:** `dsr`, `apr`, `sor`, `phs`, `psr`, `ttr`, or `tetH` genes are neither individually necessary nor sufficient for the trait.

## 2. Candidate nodes grouped by type

### Trait and process nodes

- Disproportionation — **METPO:1000806**
- Parent trait — **METPO:1000060**
- Inorganic sulfur disproportionation
- Elemental-sulfur disproportionation
- Thiosulfate disproportionation
- Sulfite disproportionation
- Tetrathionate disproportionation
- Oxidative branch of sulfur disproportionation
- Reductive branch of sulfur disproportionation
- Chemolithoautotrophic growth

Showing the first 60 of 246 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.

Curation history

  1. · SEEDED_FROM_METPO · seed_from_metpo

    imported from data/raw/metpo.owl (CLASS)

  2. · ADDED_CAUSAL_GRAPH · codex

    Added DOI-backed causal graph for sulfur disproportionation substrates, donor/acceptor redox split, sulfide and sulfate products, and reverse sulfate-reduction pathway involvement.

  3. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:participates_in×1).

  4. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (rdfs:subClassOf×2).

  5. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007405×1).

  6. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (ENVO:01001057×1, CHEBI:15138×1).

  7. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:33542×1).

  8. · RENAME_PREDICATE_LABELS · claude

    Renamed 1 causal-edge predicate label(s) to align with existing groundings: occurs under → occurs in ×1.

  9. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:occurs_in×1).

  10. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:26833×1).

  11. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007603×1).

  12. · FIX_NODE_GROUNDING_CURIE · claude

    Overwrote 2 causal-node grounding(s) to corrected CURIEs (phase-2 id-label fix; verified vs OAK).

  13. · ENRICH_CAUSAL_GRAPH · claude

    Added 8 evidence-backed generic edges (6 new nodes) from the deep-research report.

  14. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 4 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2000202×4).

  15. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2000200×2).

  16. · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude

    Re-grounded 6 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (2 to disproportionates, 4 to produces), issue 301 part 2. The previous predicates are transitively rdfs:subPropertyOf METPO:2000001, whose rdfs:domain is METPO:1000525 (microbe), so a causal-graph subject entailed that the subject IS a microbe; CausalNodeTypeEnum has no organism member, so no such edge could ever satisfy the domain. Each replacement is a 1:1 mirror of its source predicate that changes only the domain, so the claim each edge makes is unchanged and directions are unchanged. The replacements are proposed in proposals/metpo_traitmech_v9 and are placeholder ids until METPO mints them.

  17. · REGROUND_CAUSAL_EDGE · claude

    Relabelled the two thiosulfate edges from produces to derives into and re-grounded them from METPO:2007800 to RO:0001001, issue 327. Both have a CHEMICAL subject, and a chemical does not bring another chemical into existence the way an agent does; RO:0001001 is the substrate-to-product relation and fits disproportionation directly. METPO:2007405 (oxidized to, proposals v2) was considered for the sulfate half and rejected because it fits only the oxidised half, which would split a matched pair from one reaction, and because it is an unminted placeholder where RO:0001001 is real.