Calvin-Benson-Bassham cycle

traitmech:000020 · CLASS · REVIEWED

An autotrophic carbon-fixation pathway (the reductive pentose phosphate cycle) that fixes CO2 using ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). It is the most widespread CO2-fixation pathway, used by plants, algae, cyanobacteria, and many proteobacteria.

Trait evidence (2)

DOI:10.1128/AEM.02473-10
Berg review identifies the Calvin-Benson-Bassham (reductive pentose phosphate) cycle as the reference autotrophic pathway against which the other five are distinguished.
DOI:10.1146/annurev-marine-120709-142712
Hügler & Sievert support the Calvin cycle as the most common/widespread CO2-fixation pathway, including among marine cyanobacteria and proteobacteria.

Calvin-Benson-Bassham cycle fixes CO2 via RuBisCO

Evidence-backed causal sketch linking RuBisCO-catalyzed CO2 carboxylation of RuBP to the reductive pentose phosphate cycle.

MECHANISTIC · This is a composite bacterial CBB graph. The Cupriavidus example grounds the RuBisCO and cbb-regulatory branch, whereas carboxysome and carbonic-anhydrase edges describe source-backed cyanobacterial or other bacterial CO2-concentrating modules that are not asserted to occur in C. necator H16.

Edge evidence

RuBisCO enables Calvin-Benson cycle RO:0002327
RuBisCO catalyzes the CO2-fixing carboxylation step of the cycle.
- DOI:10.1128/AEM.02473-10 Berg identifies the CBB cycle as the reference autotrophic pathway.
Calvin-Benson cycle confers Calvin-Benson-Bassham cycle METPO:2007700
The reductive pentose phosphate cycle realizes the trait.
- DOI:10.1146/annurev-marine-120709-142712 Hügler & Sievert support the Calvin cycle as the most widespread CO2-fixation pathway.
carbon dioxide fixed by Calvin-Benson-Bassham cycle METPO:2007404
CO2 is the substrate fixed by the CBB cycle.
- DOI:10.1128/AEM.02473-10 Berg identifies the CBB cycle as a CO2-fixing pathway.
RuBisCO converts 3-phospho-D-glycerate
RuBisCO catalyzes the first CBB reaction, carboxylating RuBP with CO2 to yield 3-phosphoglycerate.
- DOI:10.1111/ppl.14140 Kurkela 2024: "RubisCo... catalyses the first CBB reaction (RuBP + CO2 -> 3-PGA)."
ribulose-1,5-bisphosphate substrate of RuBisCO
RuBP is the CO2-acceptor substrate carboxylated by RuBisCO.
- DOI:10.1111/ppl.14140 Kurkela 2024: RuBP + CO2 -> 3-PGA is the first CBB reaction catalysed by RubisCo.
carboxysome contains RuBisCO
Carboxysomes encapsulate RuBisCO as part of the CO2-concentrating mechanism.
- DOI:10.1111/ppl.14140 Kurkela 2024: "carboxysomes that encapsulate RuBisCo and carbonic anhydrase."
carboxysome concentrates carbon dioxide
Carboxysomes concentrate CO2 near RuBisCO, bridging environmental DIC supply to enzymatic demand.
- DOI:10.1128/AEM.01557-23 Scott 2024: "carboxysomes... concentrate CO2 near Rubisco to bridge environmental DIC supply to enzymatic demand."
carboxysomal carbonic anhydrase converts carbon dioxide
Carboxysomal carbonic anhydrase converts HCO3- to CO2 to elevate luminal CO2 and promote RuBisCO fixation.
- DOI:10.1126/sciadv.adk7283 Pulsford 2024: within carboxysomes, CA converts HCO3- to CO2 to elevate luminal CO2 and promote Rubisco-catalyzed CO2 fixation.
bicarbonate substrate of carboxysomal carbonic anhydrase
Bicarbonate is dehydrated by carboxysomal carbonic anhydrase to supply CO2 to RuBisCO.
- DOI:10.1126/sciadv.adk7283 Pulsford 2024: carboxysomal CA converts HCO3- to CO2 to promote Rubisco-catalyzed CO2 fixation.
CbbR transcriptional regulator activates transcription of cbb operon transcription
CbbR is the master transcriptional activator that elevates expression of bacterial cbb CO2-fixation operons.
- DOI:10.1128/JB.00442-15 Dangel 2015: "CbbR, the Master Regulator for Microbial Carbon Dioxide Fixation"; elevates expression of the cbb CO2 fixation operons. Broad evidence across bacteria.
cytoplasmic carbonic anhydrase can cause CO2 leakage
Cytosolic (non-vectorial) carbonic anhydrase activity can dissipate the CCM by causing CO2 leakage.
- DOI:10.1128/AEM.01557-23 Scott 2024: "cytosolic (non-vectoral) CA activity can dissipate the CCM by causing CO2 leakage."
cbb operon transcription part of Calvin-Benson cycle biolink:part_of
The cbb operon specifies enzymes of the CBB pathway.
- DOI:10.1128/JB.00442-15 cbb operons that specify enzymes of the Calvin-Bassham-Benson (CBB) pathway Verified against the public PMC full text.
CO2 leakage negatively regulates Calvin-Benson cycle RO:0002212
CO2 leakage opposes the carboxysome concentration mechanism that supplies Rubisco for the CBB pathway.
- DOI:10.1371/journal.pone.0007521 the carboxysome shell constitutes a diffusional barrier for CO2, thereby preventing leakage of this Ci species out of the microcompartment Verified against the public PMC full text; the same introduction identifies Rubisco as catalyzing the first CBB-cycle step.

Protein and taxon examples

Graph node Protein Taxon UniProt status Role and evidence

RuBisCO

Graph node	Protein	Taxon	UniProt status	Role and evidence
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)

carbon fixation traitmech:000019

Synonyms (2)

Calvin cycle EXACT_SYNONYM · DOI:10.1128/AEM.02473-10
reductive pentose phosphate cycle RELATED_SYNONYM · DOI:10.1128/AEM.02473-10

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/calvin_benson_bassham_cycle-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: Calvin–Benson–Bassham cycle

## Trait record and scope

- **Trait:** Calvin–Benson–Bassham (CBB) cycle
- **Identifier:** `traitmech:000020`
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** `traitmech:000019`
- **Synonyms:** Calvin cycle; reductive pentose-phosphate cycle

### Recommended operational definition

This trait represents the genetically encoded and physiologically operable **autocatalytic pathway that incorporates inorganic carbon into biomass through Rubisco-dependent carboxylation of ribulose-1,5-bisphosphate (RuBP), ATP/NADPH-dependent reduction of 3-phosphoglycerate (3-PGA), and phosphoribulokinase (PRK)-dependent regeneration of RuBP**. Fixation of three CO2 to one net glyceraldehyde-3-phosphate requires nine ATP and six NADPH. Rubisco and PRK are the most diagnostic enzymes; much of the remaining chemistry overlaps glycolysis, gluconeogenesis, and the pentose-phosphate pathway. (berg2011ecologicalaspectsof pages 3-4, wang2023microbialconversionand pages 2-3, meloni2023ribulose15bisphosphateregenerationin pages 1-2, prywes2023rubiscofunctionevolution pages 10-13)

The CBB cycle is quantitatively the dominant autotrophic pathway and occurs in cyanobacteria and diverse Proteobacteria, among other bacteria. It can be powered by oxygenic photosynthesis or by chemolithotrophic oxidation of compounds such as H2, reduced sulfur, Fe(II), ammonia, or nitrite. A 2024 environmental synthesis estimated that CBB accounts for **>99% of planetary autotrophy**, although that estimate is dominated by oxygenic phototrophs rather than microbes alone. (berg2011ecologicalaspectsof pages 2-3, harrison2024prevalenceofthe pages 1-5)

### Inclusion and exclusion boundaries

**Include:** complete native or engineered CBB operation; Rubisco carboxylation; ATP/NADPH-dependent 3-PGA reduction; RuBP regeneration through PRK; directly supporting carbon-concentrating mechanisms (CCMs), activases, regulators, and phosphoglycolate salvage when represented as modifiers.

**Do not infer the trait from Rubisco alone.** Forms III and some II/III Rubiscos can participate in nucleoside salvage or other pathways, while Form IV Rubisco-like proteins generally do not carboxylate RuBP. Proteobacteria may encode several Rubisco forms, so gene context and PRK evidence are important. (prywes2023rubiscofunctionevolution pages 10-13, harrison2024prevalenceofthe pages 1-5)

**Nearby but distinct traits/processes:**

1. rTCA, Wood–Ljungdahl, 3-hydroxypropionate, 3HP/4HB, DC/4HB, reductive-glycine, and synthetic carbon-fixation cycles.
2. Isolated anaplerotic CO2 incorporation by PEP or pyruvate carboxylase.
3. C4 and CAM carbon-concentrating adaptations, which deliver CO2 to CBB but are not the CBB cycle itself.
4. Carboxysomes and bicarbonate uptake: supporting CCM modules, not universal defining components.
5. Rubisco oxygenation and phosphoglycolate salvage: competing/repair processes, not positive evidence of productive CBB flux.
6. Photoheterotrophic CBB activity as an electron sink: genuine cycle activity, but not necessarily autotrophic growth. (berg2011ecologicalaspectsof pages 3-4, berg2011ecologicalaspectsof pages 2-3)

## Candidate nodes

### Core pathway and processes

- Calvin–Benson–Bassham cycle — `traitmech:000020`; candidate cross-reference **KEGG:M00165**.
- Carbon fixation / carbon assimilation — candidate **GO:0015977**.
- Carboxylation, reduction, and RuBP-regeneration phases.
- Photorespiration/phosphoglycolate salvage — modifier or competing pathway, not part of the positive trait core.
- Carbon-concentrating mechanism — label-only unless a validated ontology term is selected.

### Genes, proteins, enzymes, and complexes

- **Rubisco**; bacterial genes `rbcL/rbcS` or `cbbL/cbbS`, Form-II `cbbM`; **EC:4.1.1.39**, **KEGG:K01601**.
- **Phosphoribulokinase**; `prk`, `prkA`, or `cbbP`; **EC:2.7.1.19**, **KEGG:K00855**.
- Phosphoglycerate kinase, **EC:2.7.2.3**.
- NAD(P)-dependent glyceraldehyde-3-phosphate dehydrogenase; curate the taxon-appropriate isoenzyme only after sequence/context validation.
- Fructose-bisphosphate aldolase, **EC:4.1.2.13**.
- Fructose-1,6-bisphosphatase, **EC:3.1.3.11**.
- Transketolase, **EC:2.2.1.1**.
- Ribose-5-phosphate isomerase; ribulose-phosphate 3-epimerase; sedoheptulose-bisphosphatase or bifunctional FBPase/SBPase where taxonomically appropriate.
- Rubisco activases/chaperones CbbQ, CbbX, and RbcX — accessory and non-universal. Comparative genomics associates CbbQ/CbbX and several regeneration enzymes with CBB-positive genomes. (asplundsamuelsson2021widerangeof pages 12-13, asplundsamuelsson2021widerangeof pages 8-11, asplundsamuelsson2021widerangeof pages 7-8)
- CbbR — bacterial cbb-regulon transcriptional regulator; taxon-specific.
- Cyanobacterial CcmR/NdhR, CmpR, CyAbrB2, and RbcR — CCM/carboxysome regulators, not universal CBB regulators.
- Cyanobacterial phosphoketolase SeXPK — negative flux branch under low ATP in *Synechococcus elongatus* PCC 7942. (lu2023anatpsensitivephosphoketolase pages 1-2)

### Chemicals and metabolites

- Carbon dioxide, bicarbonate, oxygen.
- RuBP, ribulose-5-phosphate, 3-PGA, 1,3-bisphosphoglycerate, glyceraldehyde-3-phosphate.

Showing the first 60 of 234 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 model whose two CBB operons are active during autotrophic carbon fixation; paired here to the reviewed plasmid cbbL2 protein example.

Curation history

2026-05-30T00:00:00Z · PROPOSED_FROM_RESEARCH · claude
Proposed candidate METABOLISM trait (Calvin-Benson-Bassham carbon-fixation cycle); sub-variant of carbon fixation.
2026-06-10T14:00:00Z · CURATED_CAUSAL_GRAPH · claude
Added evidence-backed causal graph (CBB / RuBisCO CO2 fixation) with CHEBI/GO node groundings and RO/METPO predicate groundings; promoted PROPOSED to REVIEWED.
2026-06-14T05:51:45Z · GROUND_CAUSAL_NODES · claude
Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (UniProtKB:A0A075WF79×1).
2026-06-23T00:00:00Z · ENRICH_CAUSAL_GRAPH · claude
Added 8 evidence-backed generic edges (9 new nodes) from the deep-research report.
2026-06-24T17:23:32Z · GROUND_CAUSAL_NODES · claude
Grounded 3 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:17794×1, GO:0031470×1, CHEBI:17544×1).
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:IPR033966×1).
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.
2026-08-24T00:00:00Z · REVIEW_GRAPH_PROTEIN_TAXON · codex
Added a strain-matched C. necator H16 RuBisCO example, grounded both carbonic-anhydrase nodes, retyped the cbb operon carrier as transcription, retained CbbR as reviewed label-only, and documented the composite graph scope.
2026-08-31T09:00:00Z · CONNECT_CAUSAL_GRAPH_COMPONENTS · codex
Resolved issue #183 graph fragmentation (3 components to 1) using 2 public-source, verbatim-snippet-backed connector(s). No paid research service was called.