nitroplast

cellstructuremech:nitroplast · Membrane organelle · Membrane bounded organelle · Proposed

A membrane-bounded nitrogen-fixing organelle derived from a UCYN-A cyanobacterium within a Braarudosphaera haptophyte; characterized in B. bigelowii by coordinated division and evidence for host-encoded protein import. [DOI:10.1126/science.adk1075]

Image (not hosted — OTHER): 10.1080/00318884.2026.2660260 Figures 28-29

Figures 28-29: FR-21 cells with one or two bodies labeled Nit. Scale bar: 1 micrometer. Adjacent Figures 30-31 show OTR22-B, a different host, and are not the example asserted here. TEM · Braarudosphaera bigelowii (NCBITaxon:373042) · Hagino et al. (2026), Phycologia; copyright International Phycological Society. Linked, not reproduced. · OTHER · source: OTHER 10.1080/00318884.2026.2660260 Figures 28-29 · DOI:10.1080/00318884.2026.2660260

Synonyms: UCYN-A 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	Stoichiometry	Essential	Role
membrane	Other	part of	`GO:0016020`			UNKNOWN	Boundary and internal membrane system. `DOI:10.1080/00318884.2026.2660260` Figures 28-29 show FR-21 nitroplast boundaries and internal lamellae.
nitrogenase complex	Protein complex	part of	`GO:0016610`			UNKNOWN	Nitrogen-fixing enzyme machinery; organelle assembly essentiality untested. `DOI:10.1126/science.adk1075` NifH enrichment and nitrogenase activity support assignment; no subunit stoichiometry asserted.

Taxonomic distribution

NCBITaxon:373042 Braarudosphaera bigelowii — VARIABLE: Sensu stricto, 18S genotype III. Table 2 distinguishes nitroplast-bearing FR-12/FR-21 from culture KC1-P2, which lost it; not a wild-population frequency estimate. [DOI:10.1080/00318884.2026.2660260]

Canonical examples

NCBITaxon:373042 Braarudosphaera bigelowii — FR-21 (NIES-4399), containing UCYN-A2; supplementary Methods specify the experimental strain. [DOI:10.1126/science.adk1075]

Functions

nitrogen fixation GO:0009399 — Fixes dinitrogen within the host cell.
- DOI:10.1126/science.adk1075 Supplementary Methods and Figure S3 report acetylene-reduction activity in FR-21 cultures.

Discussions

CURATION_TODO (RESOLVED): Keep the nitroplast distinct from a whole host, a microbial taxon and the diazoplast. A compartment record must not turn a lineage name into an unrestricted structural synonym.
CURATION_TODO (RESOLVED): Separate organelle-level import evidence from individual candidate validation. Protein enrichment and sequence prediction have different evidential strength.
CURATION_TODO (OPEN): Resolve import machinery, exchanged substrates and vesicle cargo experimentally. Static architecture cannot establish transport direction or regulatory causality.

Evidence

DOI:10.1126/science.adk1075 Primary Results and supplementary Methods read via publicly readable article mirrors; title, authors, journal, DOI and PMID 38603509 cross-checked. Publisher access returned 403.
DOI:10.1080/00318884.2026.2660260 Author-hosted final PDF read; Table 2 and Figures 28-31 visually inspected. FR-21 is B. bigelowii sensu stricto, not B. parkeae.
GO:0043229 Broad intracellular-organelle parent avoids encoding an exact membrane count. GO and host taxon labels verified at QuickGO and ENA/NCBI.

Curation history

2026-10-03T15:58:06Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-03T15:59:20Z codex · EXPANDED_RECORD — Added verified host scope, core constituents, nitrogen-fixation evidence, import and preprint boundaries, and inspected link-only TEM provenance.

Source: data/structures/membrane_organelle/nitroplast.yaml