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Nitroplast

The nitroplast is a nitrogen-fixing organelle in the marine alga braarudosphaera-bigelowii — the first nitrogen-fixing organelle found in a eukaryotic cell. It descends from a cyanobacterial endosymbiont, Candidatus Atelocyanobacterium thalassa (UCYN-A), whose integration into its host has progressed so far that it is treated as an organelle rather than a symbiont (Coale et al., 2024, Science). Despite the “-plast” suffix, it is not a derived plastid.

Discovery

  • 1998 — Jonathan Zehr (UC Santa Cruz) and colleagues amplify unknown nifH (nitrogenase) sequences from Pacific/Atlantic water: “UCYN-A” (unicellular cyanobacteria group A).
  • Genome analyses revealed a massively reduced ~1.44 Mb genome lacking photosystem II, RuBisCO, and the TCA cycle (~80% gene loss) — yet the organism fixes nitrogen.
  • Kyoko Hagino (Kochi University) spent a decade culturing the host B. bigelowii, providing the biological material for the key experiments.
  • April 2024 — Coale et al. (Science 384:217–222) show that many proteins required by UCYN-A are now encoded in the host nucleus and imported into the cell; the team named the structure the nitroplast.

Organelle hallmarks

  • Protein import from the host nucleus: roughly half of the nitroplast’s proteins are host-encoded and imported, guided by uTP (UCYN transition peptide) sequences.
  • Synchronized division: nitroplast replication is coordinated with host cell division, exactly like mitochondria and chloroplasts.
  • Host control of physiology: host cryptochrome proteins keep the organelle’s light–dark cycle in sync; several metabolic pathways are only complete with host proteins.
  • These are the same hallmarks that define mitochondria and chloroplasts, placing the nitroplast on the endosymbiosis-to-organelle trajectory (see endosymbiosis-organelle-evolution).

Metabolism and exchange

  • The nitroplast reduces N₂ to ammonium for the host; the host supplies fixed carbon. Isotope experiments show it receives ~16% of the host’s total carbon and returns ~85–95% of its fixed nitrogen.
  • It fixes nitrogen during the day — unusual among diazotrophs, but possible because it lacks photosystem II, so photosynthesis cannot generate the O₂ that would poison nitrogenase.
  • Mitochondria of the host wrap around the nitroplast, presumably delivering ATP for the energy-intensive fixation (soft X-ray tomography; Larabell lab, LBNL).

Lineages and age

  • Four main sublineages (UCYN-A1–A4) occupy different niches. UCYN-A2 is the nitroplast of B. bigelowii; UCYN-A4 is “nitroplast-like” in a B. bigelowii genotype; UCYN-A1 associates with an unnamed Chrysochromulina species in the open ocean.
  • Molecular clocks date the A1/A2 divergence to the late Cretaceous (~90 Mya), consistent with B. bigelowii fossils back to ~100 Mya — a young organelle compared with mitochondria and chloroplasts.

Significance

  • First demonstration that nitrogen fixation is not an exclusively prokaryotic function.
  • Raises the prospect of engineering nitrogen-fixing organelles into crop plants, potentially reducing reliance on synthetic nitrogen fertilizer (Liu et al., 2024).
  • Recognized among the top science discoveries of 2024 (AAAS).

References

  • Coale, T. H. et al. (2024). Nitrogen-fixing organelle in a marine alga. Science 384(6692):217–222. DOI: 10.1126/science.adk1075
  • Massana, R. (2024). The nitroplast: A nitrogen-fixing organelle. Science 384(6692):160–161. DOI: 10.1126/science.ado8571
  • Hagino, K. et al. (2013). Discovery of an endosymbiotic nitrogen-fixing cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae). PLoS ONE 8(12):e81749. DOI: 10.1371/journal.pone.0081749
  • Wong, C. (2024). Scientists discover first algae that can fix nitrogen — thanks to a tiny cell structure. Nature 628:702. DOI: 10.1038/d41586-024-01046-z

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