carbon fixation

traitmech:000019 · CLASS · REVIEWED

A metabolic process in which an organism assimilates inorganic carbon (CO2 or bicarbonate) into organic compounds (autotrophy). Six distinct natural autotrophic carbon-fixation pathways are currently recognized.

Trait evidence (2)

DOI:10.1128/AEM.02473-10
Berg review of the distribution of autotrophic CO2-fixation pathways establishes that, besides the Calvin-Benson-Bassham cycle, five further autotrophic carbon-fixation pathways are known, parent of the six pathway sub-variants proposed here.
DOI:10.1146/annurev-marine-120709-142712
Hügler & Sievert, "Beyond the Calvin cycle", supports multiple autotrophic carbon-fixation pathways operating among ocean microorganisms.

Autotrophic carbon fixation assimilates CO2 into organic carbon

Evidence-backed causal sketch linking inorganic CO2 to organic carbon via six recognized autotrophic carbon-fixation pathways.

MECHANISTIC · This composite graph spans multiple independent carbon-fixation pathways. The C. necator H16 RuBisCO example supports the CBB branch only; the rTCA, 3HP/4HB, and Wood-Ljungdahl branches require different pathway-specific proteins.

Edge evidence

carbon dioxide fixed by carbon fixation METPO:2007404
CO2 is the substrate fixed by autotrophic carbon-fixation pathways.
- DOI:10.1128/AEM.02473-10 Berg reviews the distribution of autotrophic CO2-fixation pathways.
carbon fixation confers carbon fixation METPO:2007700
The autotrophic carbon-fixation process realizes the trait.
- DOI:10.1146/annurev-marine-120709-142712 Hügler & Sievert support multiple autotrophic carbon-fixation pathways operating among ocean microorganisms.
carbonic anhydrase accelerates interconversion of carbon dioxide
Carbonic anhydrase speeds interconversion of CO2 and bicarbonate, bridging DIC supply to demand.
- DOI:10.1128/aem.01557-23 Scott 2024: CA activity speeds the interconversion of CO2 and HCO3-.
ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) uses substrate carbon dioxide
RubisCO fixes CO2 as its carboxylation substrate in CBB-associated fixation.
- DOI:10.1128/aem.01557-23 Scott 2024: RubisCO substrate is CO2; applies broadly to CBB-associated fixation.
Calvin-Benson-Bassham cycle consumes ATP biolink:consumes
CBB carbon fixation consumes ATP (nine ATP per three CO2 fixed to one GAP).
- DOI:10.1186/s40643-023-00705-9 Bahrle 2023: three CO2 to one GAP at the cost of nine ATP and six NADPH.
Calvin-Benson-Bassham cycle consumes NADPH biolink:consumes
CBB carbon fixation consumes NADPH (six NADPH per three CO2 fixed).
- DOI:10.1186/s40643-023-00705-9 Bahrle 2023: three CO2 to one GAP at the cost of nine ATP and six NADPH.
reverse (reductive) tricarboxylic acid cycle requires reduced ferredoxin
The reverse TCA carbon-fixation cycle requires reduced ferredoxin as a cofactor.
- DOI:10.4014/jmb.2306.06005 Kang 2023: rTCA cycle cofactors include NAD(P)H, ATP, Fdred, FADH.
3-hydroxypropionate/4-hydroxybutyrate cycle requires NADPH
The 3HP/4HB carbon-fixation cycle requires NADPH and ATP.
- DOI:10.4014/jmb.2306.06005 Kang 2023: 3-HP/4-HB cycle NADPH and ATP required.
Wood-Ljungdahl pathway fixes CO2 to produce acetyl-CoA
The Wood-Ljungdahl pathway fixes CO2 to produce acetyl-CoA.
- DOI:10.3390/bioengineering10121357 Kurt 2023: WLP is employed for CO2 fixation and acetyl-CoA production.
ATP citrate lyase (aclAB) enables reverse (reductive) tricarboxylic acid cycle RO:0002327
ATP citrate lyase (aclAB) is a key enzyme enabling the reverse TCA carbon-fixation cycle.
- DOI:10.1038/s41564-024-01704-y Mitchell 2024: aclAB ATP citrate lyase is one of four key enzymes to run the TCA cycle in reverse.
carbonic anhydrase produces bicarbonate METPO:2007800
Carbonic anhydrase interconverts CO2 and bicarbonate, supplying inorganic carbon to fixation.
- DOI:10.1128/AEM.02473-10 Connecting edge wiring the enrichment node into the graph.
Calvin-Benson-Bassham cycle contributes to carbon fixation RO:0002326
The Calvin-Benson cycle is an autotrophic carbon-dioxide-fixation mechanism.
- DOI:10.1128/AEM.02473-10 The reductive pentose phosphate cycle is the quantitatively most important mechanism of autotrophic CO2 fixation in nature. Verified against the free public article text.
reverse (reductive) tricarboxylic acid cycle contributes to carbon fixation RO:0002326
The reductive TCA cycle is an autotrophic carbon-dioxide-fixation route.
- DOI:10.1128/AEM.02473-10 the second autotrophic CO2 fixation cycle (the reductive citric acid cycle) had been discovered Verified against the free public article text.
Wood-Ljungdahl pathway contributes to carbon fixation RO:0002326
The Wood-Ljungdahl pathway fixes carbon dioxide into acetyl-CoA.
- DOI:10.1128/AEM.02473-10 The Wood-Ljungdahl pathway is a noncyclic pathway that results in the fixation of two CO2 molecules to form acetyl-CoA Verified against the free public article text.

Protein and taxon examples

Graph node Protein Taxon UniProt status Role and evidence

ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO)

Graph node	Protein	Taxon	UniProt status	Role and evidence
ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO)	`UniProtKB:P42721` Ribulose bisphosphate carboxylase large chain, plasmid (cbbL2)	Cupriavidus necator H16 `NCBITaxon:381666`	`REVIEWED` retrieved 2026-08-24 · entry v153 · sequence v3	Large-chain component of the plasmid-encoded RuBisCO in one of the two active CBB operons of C. necator H16. `DOI:10.1186/s12934-020-01494-y` both CBB operons were active and contributed almost equally to the carbon fixation process The strain-level perturbation study supports activity of both H16 CBB operons; UniProtKB P42721 identifies the reviewed plasmid cbbL2 large chain in that strain.

UniProtKB:P42721
Ribulose bisphosphate carboxylase large chain, plasmid (cbbL2)

Cupriavidus necator H16
NCBITaxon:381666

REVIEWED
retrieved 2026-08-24 · entry v153 · sequence v3

Large-chain component of the plasmid-encoded RuBisCO in one of the two active CBB operons of C. necator H16.

DOI:10.1186/s12934-020-01494-y both CBB operons were active and contributed almost equally to the carbon fixation process The strain-level perturbation study supports activity of both H16 CBB operons; UniProtKB P42721 identifies the reviewed plasmid cbbL2 large chain in that strain.

Provenance

Identifier source: TraitMech local identifier
Definition source: DOI:10.1128/AEM.02473-10

Parent traits (1)

metabolism METPO:1000060

Children (6)

Synonyms (5)

CO2 fixation RELATED_SYNONYM · DOI:10.1128/AEM.02473-10
autotrophic carbon assimilation RELATED_SYNONYM · DOI:10.1146/annurev-marine-120709-142712
autotrophic CO2 fixation EXACT_SYNONYM · http://purl.obolibrary.org/obo/go/releases/2026-07-26/go-basic.obo
autotrophic CO2 fixation pathway EXACT_SYNONYM · http://purl.obolibrary.org/obo/go/releases/2026-07-26/go-basic.obo
autotrophy EXACT_SYNONYM · http://purl.obolibrary.org/obo/go/releases/2026-07-26/go-basic.obo

Cross-references

GO:0015977

kg-microbe context

Matched 1 kg-microbe node via parent_proxy.

METPO:1000060 [-1.052, -1.766, -1.194, +0.291, …]

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).

metabolism manganese oxidation 1.000
metabolism sulfur oxidation 1.000
metabolism starch degradation 1.000
metabolism reductive tricarboxylic acid cycle 1.000
metabolism proteorhodopsin phototrophy 1.000
metabolism proteolysis 1.000
metabolism phototrophy 1.000
metabolism photosynthesis 1.000

Deep research

Generated by just research-trait; source: research/traits/metabolism/carbon_fixation-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 report: microbial carbon fixation

## 1. Trait record and scope

- **Trait:** carbon fixation
- **Identifier:** **traitmech:000019**
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** METPO:1000060
- **Synonyms:** CO2 fixation; autotrophic carbon assimilation

### Recommended scope

The trait should represent the **physiological capacity for net assimilation of inorganic carbon—CO2 and/or bicarbonate—into central organic metabolites and biomass through an autotrophic pathway**. Reduction of CO2 to average cellular carbon requires reducing equivalents and energy, usually ATP; a carboxylase links CO2 or HCO3− to an organic acceptor that the pathway regenerates. Carbon-species availability is pH dependent, with bicarbonate favored under mildly alkaline conditions such as seawater. (berg2011ecologicalaspectsof pages 1-2)

For compatibility with the reviewed definition and existing graph, retain the classical six natural autotrophic pathways as child mechanisms:

1. Calvin–Benson–Bassham cycle (CBB)
2. reductive tricarboxylic-acid cycle (rTCA)
3. Wood–Ljungdahl/reductive acetyl-CoA pathway (WL)
4. 3-hydroxypropionate bicycle (3HP bicycle)
5. 3-hydroxypropionate/4-hydroxybutyrate cycle (3HP/4HB)
6. dicarboxylate/4-hydroxybutyrate cycle (DC/4HB)

Berg’s authoritative review states: “Besides the well-known Calvin-Benson cycle, five other totally different autotrophic mechanisms are known today.” A 2024 Great Salt Lake study used the same six-pathway classification when screening metagenome-assembled genomes. (berg2011ecologicalaspectsof pages 1-2, shoemaker2024wood–ljungdahlpathwayencoding pages 2-3)

### Boundary cases

- **Exclude anaplerotic fixation alone.** Carboxylation by phosphoenolpyruvate carboxylase, pyruvate carboxylase, or related enzymes can replenish central-metabolic intermediates in heterotrophs without supporting autotrophic growth.
- **Do not infer the trait from `rbcL` or another marker alone.** A recent genome survey explicitly warns that CBB genes do not necessarily establish autotrophic growth; in some aerobic anoxygenic phototrophs the cycle supplements heterotrophic metabolism. (nishihara2025exploringthediversity pages 5-8)
- **Distinguish fixation from an electron-balancing sink.** In some purple photoheterotrophs the CBB cycle consumes excess reducing power rather than establishing autotrophic carbon assimilation. (berg2011ecologicalaspectsof pages 2-3)
- **Exclude dissimilatory CO2 reduction by itself.** Methanogenesis or other CO2-reducing energy metabolism does not necessarily assimilate carbon into biomass through one of the trait’s pathways.
- **Exclude carbon capture, storage, and sequestration as environmental outcomes** unless organism-level inorganic-carbon assimilation is demonstrated.
- **Partial or engineered pathways** may be represented as experimental mechanisms, but they should not automatically imply the organism-level autotrophic trait.
- **Pathway-count warning:** newer literature sometimes describes seven or eight mechanisms by adding the reductive glycine pathway and/or reverse oxidative TCA variant. These should be modeled as proposed extensions rather than silently changing the reviewed six-pathway definition. (nishihara2025exploringthediversity pages 1-5, li2024productionofsuccinate pages 1-2)

## 2. Current mechanistic understanding

The CBB cycle is quantitatively dominant and unusually tolerant of oxygen, but Rubisco is slow, has limited CO2 affinity, and also catalyzes an oxygenase reaction that generates 2-phosphoglycolate. Berg reported a Rubisco turnover range of approximately 1–12 s−1 and explained why carbon-concentrating mechanisms are advantageous. (berg2011ecologicalaspectsof pages 2-3)

Anaerobic pathways commonly exploit low-potential reduced ferredoxin, whereas aerobic pathways more often use NAD(P)H. Reviews consequently classify CBB, 3HP, and 3HP/4HB as broadly oxygen-compatible and rTCA, WL, and DC/4HB as anaerobic or microaerobic, although this is a pathway-level generalization rather than an absolute taxonomic rule. The WL and DC/4HB mechanisms contain especially oxygen-sensitive chemistry. (liang2020recentadvancesin pages 3-5, liang2020recentadvancesin pages 1-2)

A 2025 computational—not experimental—comparison concluded that anaerobic pathways generally incur lower ATP costs and that rTCA and WL are efficient across broad simulated CO2 and H2 conditions. This is useful expert guidance for graph interpretation and engineering prioritization, but not evidence that a particular organism expresses those pathways in vivo. (taha2025bioenergetictradeoffscan pages 1-2)

## 3. Candidate nodes

### Trait and biological-process nodes

| Candidate node | Suggested grounding | Curation note |
|---|---|---|
| carbon fixation | **traitmech:000019**; GO:0015977 | Root trait/process; preserve supplied CURIE verbatim. |
| autotrophic growth | GO label candidate | Phenotypic outcome; require growth or biomass-assimilation evidence. |
| carbon-concentrating mechanism | Label-only candidate | Mechanistic module supporting CBB fixation, especially in cyanobacteria. |
| photorespiration | GO:0009853 | Competing/consequent process caused by Rubisco oxygenation. |
| inorganic-carbon sensing | Label-only candidate | Regulatory module, not fixation itself. |

### Pathway/module nodes

Use label-only nodes unless the project’s selected pathway ontology has been validated:

- Calvin–Benson–Bassham cycle

Showing the first 60 of 254 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.

Canonical examples (2)

Organisms cited as exemplars of this trait. Taxon ids are NCBITaxon and link out to the NCBI record.

Cupriavidus necator NCBITaxon:106590 DOI:10.1038/nbt1244 Model chemolithoautotroph fixing CO2 via the Calvin-Benson-Bassham cycle on H2+CO2 (strain H16).
Cupriavidus necator H16 NCBITaxon:381666 DOI:10.1186/s12934-020-01494-y Strain-level CBB model paired to the reviewed plasmid cbbL2 RuBisCO large-chain example.

Curation history

2026-05-30T00:00:00Z · PROPOSED_FROM_RESEARCH · claude
Proposed candidate METABOLISM trait (autotrophic carbon fixation) from literature research to fill the carbon-fixation-pathway coverage gap. Parent of the six pathway-specific sub-variants.
2026-06-10T14:00:00Z · CURATED_CAUSAL_GRAPH · claude
Added evidence-backed causal graph (CO2 assimilation axis) with CHEBI/GO node groundings and RO/METPO predicate groundings; promoted PROPOSED to REVIEWED.
2026-06-23T00:00:00Z · ENRICH_CAUSAL_GRAPH · claude
Added 8 evidence-backed generic edges (12 new nodes) from the deep-research report.
2026-06-23T00:00:00Z · FIX_ORPHAN_NODE · claude
Connected orphaned node 'bicarbonate' via carbonic_anhydrase -[produces]-> bicarbonate.
2026-06-24T17:21:12Z · GROUND_CAUSAL_PREDICATES · claude
Grounded 4 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:consumes×2, RO:0002327×1, METPO:2000202×1).
2026-06-24T17:21:18Z · GROUND_CAUSAL_NODES · claude
Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (GO:0030634×1, CHEBI:16474×1).
2026-06-24T17:23:32Z · GROUND_CAUSAL_NODES · claude
Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:17544×1, CHEBI:17513×1).
2026-06-26T07:15:18Z · GROUND_CAUSAL_NODES · claude
Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (UniProtKB:A0A009PMS8×1).
2026-07-20T04:21:11Z · RETRACT_DEAD_UNIPROT_GROUNDINGS · claude
Retracted 1 UniProtKB grounding(s) whose accessions are deleted from UniProt; nodes demoted to label-only pending re-grounding (docs/GROUNDING_POLICY.md)
2026-07-20T04:23:53Z · GROUND_CAUSAL_NODES · claude
Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (InterPro:IPR001765×1).
2026-08-06T00:00:00Z · MIGRATE_ENABLES_TRAIT_EDGES · claude
Migrated 1 causal edge(s) off enables/RO:0002327 with a TRAIT object (1 to confers), issue 302. RO:0002327 has range 'biological process or activity', which a trait (a disposition) cannot satisfy, so the previous form entailed trait is-a BiologicalProcessOrActivity. The replacements are proposed in proposals/metpo_traitmech_v8 and are placeholder ids until METPO mints them.
2026-08-06T02:00:00Z · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude
Re-grounded 1 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (1 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.
2026-08-24T08:25:00Z · CURATE_PROTEIN_TAXON_EXAMPLE · codex
Added a DOI-backed C. necator H16 cbbL2/RuBisCO example, grounded the generic RuBisCO node to InterPro, recorded the AclAB complex as reviewed label-only, and scoped the example to the CBB branch of this multi-pathway graph.
2026-08-26T06:15:39Z · ADD_EXACT_ONTOLOGY_MATCH · codex
Ontology exact-match review (2026-08-25): approved exact xref(s): GO:0015977; declared exact synonym(s): 'autotrophic CO2 fixation', 'autotrophic CO2 fixation pathway', 'autotrophy'. Evidence is predicate-scoped in the versioned ontology snapshots; OAK cross-checked direct data, and OLS4 spot-checked release deltas and disputed hits.
2026-08-31T12:15:00Z · CONNECT_CAUSAL_GRAPH_COMPONENTS · codex
Resolved issue #183 graph fragmentation (4 components to 1) with 3 public-source connector(s). No paid research service was called.