polyhydroxyalkanoate granule

traitmech:000067 · CLASS · REVIEWED

An intracellular storage inclusion composed of polyhydroxyalkanoate (e.g. polyhydroxybutyrate, PHB), a carbon and energy reserve accumulated as cytoplasmic granules.

Trait evidence (2)

DOI:10.1128/mr.54.4.450-472.1990
Anderson & Dawes describe polyhydroxyalkanoates (chiefly PHB) as carbon/energy reserves stored as cytoplasmic granules.
DOI:10.1038/s41579-020-0413-0
Greening & Lithgow include storage granules such as PHA bodies among bacterial intracellular organelles.

PHA granules store carbon and energy as polyhydroxyalkanoate

Evidence-backed causal sketch linking polyhydroxyalkanoate biosynthesis to intracellular storage granules serving as carbon and energy reserves.

MECHANISTIC · The taxon-matched protein example anchors one experimentally supported causal branch; it is not presented as a universal mechanism for every taxon or every contextual branch in this graph.

Edge evidence

polyhydroxyalkanoate located in polyhydroxyalkanoate granule biolink:located_in
PHA polymer accumulates inside cytoplasmic granules.
- DOI:10.1128/mr.54.4.450-472.1990 Anderson & Dawes describe PHAs (chiefly PHB) as carbon/energy reserves stored as cytoplasmic granules.
polyhydroxyalkanoate granule enables carbon and energy storage RO:0002327
PHA granules realize the carbon/energy storage function.
- DOI:10.1038/s41579-020-0413-0 Greening & Lithgow include PHA bodies among bacterial intracellular organelles.
nutrient limitation / high C:N ratio increases polyhydroxyalkanoate accumulation RO:0002213
High C/N ratio and N/S/P depletion stimulate PHA accumulation.
- DOI:10.3390/molecules29102293 PHA accumulation is stimulated by a high C/N ratio, depletion of N, S, and P, or a low rate of respiration.
nutrient limitation / high C:N ratio induces polyhydroxyalkanoate accumulation
Nutrient limitation with excess carbon induces PHA accumulation.
- DOI:10.1016/j.jbc.2024.107523 Environmental triggers include nutrient limitation and excess carbon that induce PHA accumulation.
PHA synthase (PhaC) catalyzes polyhydroxyalkanoate biolink:catalyzes
PhaC polymerizes hydroxyacyl-CoA into PHA/PHB polymer.
- DOI:10.3390/polym15143027 PhbC polymerizing beta-hydroxybutyryl-CoA into PHB.
phasin (PhaP) stabilizes polyhydroxyalkanoate
Phasins coat and stabilize PHB chains at the granule surface.
- DOI:10.3390/polym15143027 Phasins are GAPs that coat and stabilize PHB chains.
phasin (PhaP) controls PHB granule size and number RO:0002211
Phasins control the size and number of PHB granules.
- DOI:10.3390/polym15143027 Phasins control the size of the PHB granules.
PhaR regulator represses transcription of phaP transcription
PhaR binds phaP promoters and represses transcription.
- DOI:10.1016/j.jbc.2024.107523 PhaR binds phaP promoters and represses their transcription PhaR is a transcriptional repressor that binds phaP promoters and represses their transcription.
phaP transcription produces phasin (PhaP)
Expression of phaP produces the phasin protein coating PHA granules.
- DOI:10.1016/j.jbc.2024.107523 phaP encodes the granule-associated phasin PhaP The study links phaP transcriptional regulation to phasin production.
PHA depolymerase (PhaZ) mediates PHB mobilization
PHB depolymerase cleaves PHB to 3-hydroxybutyrate, mediating mobilization.
- DOI:10.3390/polym15143027 PHB mobilization involves a PHB depolymerase (PhbZ/PhaZ) that cleaves PHB to beta-hydroxybutyrate.
carbon starvation triggers PHB degradation
Starvation or exogenous carbon depletion triggers PHB degradation.
- DOI:10.3390/polym15143027 PHB is degraded during starvation/exogenous carbon depletion or stationary phase.
polyhydroxyalkanoate accumulation contributes to polyhydroxyalkanoate granule RO:0002326
Intracellular accumulation of amorphous PHA produces the polymer body represented by the granule trait.
- DOI:10.1016/j.jbc.2024.107523 When synthetized, PHA aggregates in an amorphous state surrounded by several PHA granule-associated proteins forming the so called carbonosomes Verified against the open Europe PMC full text.
PHB mobilization contributes to carbon and energy storage RO:0002326
Mobilization makes stored PHA available for utilization and is part of the granule's carbon-and-energy reserve function.
- DOI:10.1016/j.jbc.2024.107523 Phasins and PHA degrading enzymes are important for PHA storage and utilization Verified against the open Europe PMC full text; the edge covers reserve utilization rather than claiming degradation creates storage.
PHB degradation contributes to PHB mobilization RO:0002326
Degradation of accumulated PHB contributes to reserve mobilization.
- DOI:10.1016/j.jbc.2024.107523 reduced ppGpp alarmone levels leading to enhanced degradation of PHA suggesting that ppGpp acts as repressor of the PHA mobilization system (PhaZ1) Verified against the open Europe PMC full text; the connector links the existing starvation/degradation and PhaZ/mobilization branches.

Protein and taxon examples

Graph node Protein Taxon UniProt status Role and evidence

PHA synthase (PhaC)

Graph node	Protein	Taxon	UniProt status	Role and evidence
PHA synthase (PhaC)	`UniProtKB:P23608` Polyhydroxyalkanoate synthase PhaC (phaC)	Cupriavidus necator H16 `NCBITaxon:381666`	`REVIEWED` retrieved 2026-08-25 · entry v137 · sequence v1	H16 PhaC polymerizes hydroxyacyl-CoA into PHA and directly supplies polymer to storage granules. `PMID:16964242` PhaC catalyzes polyhydroxyalkanoate polymerization The cited source supports the represented protein-to-trait branch; UniProt verifies this current strain-matched protein entry.

UniProtKB:P23608
Polyhydroxyalkanoate synthase PhaC (phaC)

Cupriavidus necator H16
NCBITaxon:381666

REVIEWED
retrieved 2026-08-25 · entry v137 · sequence v1

H16 PhaC polymerizes hydroxyacyl-CoA into PHA and directly supplies polymer to storage granules.

PMID:16964242 PhaC catalyzes polyhydroxyalkanoate polymerization The cited source supports the represented protein-to-trait branch; UniProt verifies this current strain-matched protein entry.

Provenance

Identifier source: TraitMech local identifier
Definition source: DOI:10.1128/mr.54.4.450-472.1990

Parent traits (1)

intracellular inclusion traitmech:000066

Synonyms (3)

PHB granule RELATED_SYNONYM · DOI:10.1128/mr.54.4.450-472.1990
polyhydroxybutyrate inclusion RELATED_SYNONYM · DOI:10.1128/mr.54.4.450-472.1990
PHA granule EXACT_SYNONYM · http://purl.obolibrary.org/obo/go/releases/2026-07-26/go-basic.obo

Cross-references

GO:0070088

kg-microbe context

Matched 1 kg-microbe node via parent_proxy.

METPO:1000059 [-2.682, -2.070, -3.656, -0.652, …]

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).

environment cadmium tolerant 1.000
physiology bioluminescence 1.000
environment cobalt tolerant 1.000
environment copper tolerant 1.000
environment desiccation tolerant 1.000
environment piezotolerant 1.000
environment obligately piezophilic 1.000
morphology gas vesicle 1.000

Deep research

Generated by just research-trait; source: research/traits/morphology/polyhydroxyalkanoate_granule-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: polyhydroxyalkanoate granule

## 1. Trait record and scope

- **Trait label:** polyhydroxyalkanoate granule
- **Trait identifier:** `traitmech:000067`
- **Category / kind / status:** MORPHOLOGY / CLASS / REVIEWED
- **Parent:** `traitmech:000066`
- **Synonyms:** PHB granule; polyhydroxybutyrate inclusion

### Recommended operational definition

An intracellular, usually approximately spherical inclusion containing an amorphous polyhydroxyalkanoate (PHA) polymer core and a surface enriched in granule-associated proteins. It functions principally as a dynamic carbon, energy, and reducing-equivalent reservoir. “Carbonosome” is used for the organelle-like granule-plus-protein complex. In *Ralstonia eutropha*/*Cupriavidus necator*, typical granules are reported as approximately **0.2–0.5 µm in diameter**. PHA accumulation is commonly induced when carbon remains available while another nutrient—especially nitrogen or phosphorus—limits growth. (gonzalezrojo2024advancesinmicrobial pages 2-4, bresan2016polyhydroxyalkanoate(pha)granules pages 1-2, santolin2024elucidatingregulationof pages 1-2)

The morphological trait should mean **presence or formation of the intracellular granule**, not merely possession of `pha` genes, detectable PHA monomers, or production of purified polymer. PHA synthesis is the causal process; the granule is its cellular morphological outcome.

### Boundaries

**Include:**

1. Intracellular PHB, PHBV, short-chain-length PHA, or medium-chain-length PHA inclusions.
2. Native and engineered granules when an intracellular inclusion is demonstrated.
3. Granule number, size, localization, protein coating, nucleoid association, and partitioning mechanisms.
4. Carbonosomes containing synthases, depolymerases, phasins, and regulatory/localization proteins.

**Exclude or model separately:**

1. Soluble 3-hydroxyacyl-CoA precursors without polymer or granules.
2. Bulk PHA concentration or extracted bioplastic without evidence of intracellular inclusions.
3. Extracellular polymer, generic protein inclusion bodies, glycogen granules, sulfur globules, lipid droplets, and magnetosomes.
4. Polymer composition—PHB versus PHBV or medium-chain-length PHA—as a material/compositional attribute rather than a separate granule-presence phenotype.
5. “Membrane-bounded organelle” as a defining property. In vivo analysis in three proteobacterial representatives found no phospholipid layer around PHA granules; the observed surface was consistent with proteins rather than a canonical membrane. (bresan2016polyhydroxyalkanoate(pha)granules pages 1-2)

## 2. Current mechanistic model

Under excess carbon and growth limitation, metabolism channels carbon into hydroxyacyl-CoA precursors. In the canonical PHB route, PhaA condenses two acetyl-CoA molecules, PhaB reduces acetoacetyl-CoA, and PhaC polymerizes (R)-3-hydroxybutyryl-CoA. Hydrophobic polymer accumulates as a cytoplasmic core whose interface is populated by phasins and metabolic/regulatory proteins. Phasins constrain coalescence and surface-to-volume ratio, while taxon-specific systems such as PhaM–nucleoid coupling in *C. necator* and PhaF–nucleoid coupling in *P. putida* position and partition granules. PhaZ depolymerases mobilize stored polymer when carbon or energy is required. (gonzalezrojo2024advancesinmicrobial pages 2-4, galan2011nucleoid‐associatedphafphasin pages 1-2, santolin2024elucidatingregulationof pages 1-2, kelly2024comprehensiveproteomicsanalysis pages 1-3)

This model is better supported than older “membrane budding” cartoons. Micelle, budding, and scaffold/mediation-element models have all been proposed, but granule initiation is not sufficiently universal to curate one of them as the general bacterial mechanism. The phospholipid-free in vivo result directly contradicts treating a lipid monolayer as universal. (bresan2016polyhydroxyalkanoate(pha)granules pages 1-2, galan2011nucleoid‐associatedphafphasin pages 9-10)

## 3. Candidate nodes

### Environmental and experimental factors

- Excess or surplus carbon source
- Nitrogen limitation
- Phosphorus limitation
- Sulfur limitation — candidate, less universal
- Oxygen limitation — context-dependent; PHA can act as an electron sink
- Nutrient imbalance / growth limitation
- Carbon starvation or renewed growth, promoting reserve mobilization
- Fed-batch fermentation
- Engineered high PhaC dosage
- Renewable or waste-derived carbon feedstock

Candidate grounding includes **ENVO** terms for the specific culture environment when available; otherwise retain label-only experimental-condition nodes. Do not collapse nitrogen, phosphorus, sulfur, and oxygen limitation into one causal edge because their effects and taxonomic scope differ. Nitrogen limitation is the most commonly applied experimental induction condition. (santolin2024elucidatingregulationof pages 1-2, manoli2023heterologousconstitutiveproduction pages 1-3, kelly2024comprehensiveproteomicsanalysis pages 1-3)

### Chemicals and metabolites

- Acetyl-CoA — `CHEBI:15351`
- Acetoacetyl-CoA — `CHEBI:15345`

Showing the first 60 of 237 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 PMID:16964242 Model PHA/PHB accumulator (strain H16; formerly Ralstonia eutropha / Alcaligenes eutrophus).
Cupriavidus necator H16 NCBITaxon:381666 PMID:16964242 Strain-level PhaC granule-biosynthesis exemplar.

Curation history

2026-06-03T00:00:00Z · PROPOSED_FROM_RESEARCH · claude
Proposed candidate MORPHOLOGY trait (polyhydroxyalkanoate/PHB granule); storage sub-variant of intracellular inclusion.
2026-06-09T15:00:00Z · CURATED_CAUSAL_GRAPH · claude
Added evidence-backed causal graph (PHA granule carbon/energy storage) with RO/biolink predicate groundings; promoted PROPOSED to REVIEWED.
2026-06-23T00:00:00Z · ENRICH_CAUSAL_GRAPH · claude
Added 8 evidence-backed generic edges (11 new nodes) from the deep-research report.
2026-06-24T17:21:13Z · GROUND_CAUSAL_PREDICATES · claude
Grounded 3 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002213×1, biolink:catalyzes×1, RO:0002211×1).
2026-08-26T03:23:00Z · CURATE_PROTEIN_TAXON_EXAMPLE · claude
Backfilled provenance (review issue 517) for the codex protein-taxon review tranche of 2026-08-24/25, which shipped without a per-record event. In this record the tranche: set graph scope pha_granule_carbon_energy_storage=MECHANISTIC with scope_notes; marked 4 GENE_OR_PROTEIN node(s) REVIEWED_LABEL_ONLY with grounding_notes (pha_synthase_phac, phasin_phap, phar_regulator, pha_depolymerase_phaz); added taxon-paired protein example(s) UniProtKB:P23608 on pha_synthase_phac (NCBITaxon:381666); added 1 node(s) and 2 edge(s) (phap_transcription); removed 1 node(s) and 1 edge(s) together with their evidence (phap_gene); added canonical example(s) NCBITaxon:381666.
2026-08-26T06:15:40Z · ADD_EXACT_ONTOLOGY_MATCH · codex
Ontology exact-match review (2026-08-25): approved exact xref(s): GO:0070088; declared exact synonym(s): 'PHA granule'. Evidence is predicate-scoped in the versioned ontology snapshots; OAK cross-checked direct data, and OLS4 spot-checked release deltas and disputed hits.
2026-08-31T06:48:00Z · CONNECT_CAUSAL_GRAPH_COMPONENTS · codex
Resolved issue #183 graph fragmentation (4 components to 1) using 3 source- and verbatim-snippet-backed connector(s). No paid research service was called.