diazoplast

cellstructuremech:diazoplast · Membrane organelle · Membrane bounded organelle · Proposed

An obligate, membrane-bounded nitrogen-fixing compartment of Epithemia diatoms containing a reduced, nonphotosynthetic cyanobacterial endosymbiont; a metabolically integrated proto-organelle whose genetic integration remains unresolved. [DOI:10.1073/pnas.2507237122]

NanoSIMS isotope imaging of Epithemia clementina after 12 or 24 hours of labeling. Arrowheads identify diazoplasts; lower panels compare nitrogen and carbon enrichment in diazoplast and host regions. Complete multi-panel Figure 2, including quantification. OTHER · *Epithemia clementina (nom. ined.)* (`NCBITaxon:3042617`) · Solène L Y Moulin, Sarah Frail, Thomas Braukmann, Jon Doenier, Melissa Steele-Ogus, Jane C Marks, Matthew M Mills, Ellen Yeh; CC BY, via PubMed Central · CC_BY_4_0 · source: PMC PMC11070190.1 f2 · `DOI:10.1093/ismeco/ycae055`

Synonyms: spheroid body

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
membrane	Other	part of	`GO:0016020`		UNKNOWN	Boundary and retained internal membranes; no photosynthetic role is asserted. `DOI:10.1093/ismeco/ycae055` Figure 4D-E TEM shows diazoplast boundaries and internal thylakoids.
nitrogenase complex	Protein complex	part of	`GO:0016610`	nifH, nifD, nifK	UNKNOWN	Nitrogen-fixing enzyme machinery. `DOI:10.1093/ismeco/ycae055` Intact nif cluster, NifDK immunoblots and acetylene-reduction activity support assignment; no assembly-essentiality test.
Uptake hydrogenase large subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	hupL	UNKNOWN	Hydrogen-recycling enzyme subunit; functional assignment, not a diazoplast-specific perturbation result. `DOI:10.1093/ismeco/ycae055` Figure 3B reports HupL immunoblot detection, not in situ localization or a hydrogen-flux assay.

Taxonomic distribution

NCBITaxon:420972 Epithemia — COMMON: Reported in all described Epithemia species; physiological uniformity is not implied. [DOI:10.1073/pnas.2507237122]

Canonical examples

NCBITaxon:3042617 Epithemia clementina (nom. ined.) — Butano-area isolate studied by isotope imaging, immunoblots and physiological perturbations. Cultures were monoalgal, not axenic. Taxon label verified at ENA/NCBI. [DOI:10.1093/ismeco/ycae055]

Functions

nitrogen fixation GO:0009399 — Fixes dinitrogen within the host cell.
- DOI:10.1093/ismeco/ycae055 Figures 2-3: isotope incorporation and nitrogenase activity.
Fixed-nitrogen provision to the host — Transfers fixed nitrogen to the diatom host.
- DOI:10.1093/ismeco/ycae055 Figure 2: isotope enrichment extends into host compartments; transferred chemical species remain unidentified.

Mechanism graphs

Host carbon supports diazoplast nitrogen fixation (FUNCTION)

Subject	Predicate	Object	Evidence
nitrogenase complex	catalyzes	nitrogen fixation	`GO:0016610` Nitrogenase reaction; diazoplast membership supported in components.
diazoplast	performs	nitrogen fixation	`DOI:10.1093/ismeco/ycae055` Figures 2-3; compartment isotope enrichment and culture activity.
nitrogen fixation	supplies	Fixed nitrogen in host biomass	`DOI:10.1093/ismeco/ycae055` Figure 2B supports nitrogen transfer; no export species assigned.
Host photosynthesis	produces	Host-fixed carbon	`DOI:10.1093/ismeco/ycae055` Bicarbonate labeling and bulk IRMS distinguish daytime carbon fixation.
Host-fixed carbon	is transferred into	diazoplast	`DOI:10.1093/ismeco/ycae055` Figure 2C supports carbon transfer; wash losses limit temporal interpretation.
Host photosynthesis	supports	nitrogen fixation	`DOI:10.1093/ismeco/ycae055` Figure 3D daytime perturbations support physiological coupling.
Daytime darkness or DCMU treatment	reduces	nitrogen fixation	`DOI:10.1093/ismeco/ycae055` Results report about 60% reduction, not complete abolition.

Discussions

CURATION_TODO (RESOLVED): Distinguish the diazoplast compartment from nitroplast and a genetically integrated organelle. Dedicated intracellular compartments qualify structurally without assuming demonstrated genetic integration.
CURATION_TODO (RESOLVED): Do not promote enriched host proteins to confirmed diazoplast constituents. Enrichment can reflect contamination.
CURATION_TODO (RESOLVED): Keep perturbation, diel and metabolic claims within demonstrated scope. Abstract wording and metabolic models exceed some direct measurements.
CURATION_TODO (OPEN): Resolve an exact source-neutral uptake-hydrogenase large-subunit family. Broad nickel-hydrogenase families do not isolate the uptake-specific component.
CURATION_TODO (OPEN): Identify transport substrates, transporters and division machinery directly. Metabolic and cellular integration do not establish molecular mechanisms.

Evidence

DOI:10.1093/ismeco/ycae055 Moulin et al. 2024, PMID 38707843/PMC11070190. Final primary Results and Discussion read; Figures 2-4 inspected with PMC metadata MD5 verification.
DOI:10.1073/pnas.2507237122 Frail et al. 2025, PMID 40794833/PMC12377750. Final primary proteomics and Discussion read, not the differently titled 2024 preprint.
GO:0043229 Broad structural parent; no genetic-integration criterion.

Curation history

2026-10-03T14:32:11Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-03T14:33:31Z codex · EXPANDED_RECORD — Added three constituent classes, verified Epithemia scope/example, nitrogen-fixation exchange graph, explicit proto-organelle/import boundaries, two follow-ups and inspected CC BY 4.0 NanoSIMS figure via the PMC adapter.

Source: data/structures/membrane_organelle/diazoplast.yaml