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tagsalgae, marine-biology, endosymbiosis, organelle, evolution

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Braarudosphaera bigelowii

Braarudosphaera bigelowii is a coastal coccolithophore (haptophyte alga, class Prymnesiophyceae) and the only known host of the nitroplast — the first nitrogen-fixing organelle found in a eukaryote. It is the first eukaryote known to pull nitrogen directly from the air.

Description

  • Diploid phase is covered by a calcareous shell of 12 pentaliths (five-fold-symmetric scales), giving a regular dodecahedral structure ~10 µm across.
  • Fossil record extends back ~100 million years to the Late Cretaceous.

Endosymbiosis

  • Carries two cyanobacterium-derived organelles: a chloroplast from secondary endosymbiosis, and the nitroplast from a second, primary endosymbiosis event ~100 Mya. The nitroplast’s ancestor was a different cyanobacterium from the chloroplast’s ancestor; both were photosynthetic, but the nitroplast lineage lost photosynthesis during organelle development.
  • The nitroplast is the UCYN-A2 lineage of Candidatus Atelocyanobacterium thalassa.

Cell biology

  • Fixed organelle numbers: two chloroplasts and one nitroplast. Replication order is mitochondria → nitroplast → nucleus → chloroplasts, with division synchronized so each daughter cell inherits the correct complement.
  • Cell division occurs at night, when photosynthesis and nitrogen fixation pause; LAC (linear absorption coefficient) measurements show low energy availability in mitochondria and nitroplasts at fission time, so division and N₂ fixation are staggered.
  • Mitochondria cluster around the nitroplast, presumably supplying ATP for nitrogen fixation — an arrangement likened to other energy-demanding cells such as vertebrate neurons.

Life cycle and history

  • Motile flagellated (haploid) and non-motile calcified (diploid) phases; Kyoko Hagino’s decade of culturing the motile phase enabled the 2024 organelle discovery.
  • Genus named in honour of Norwegian marine botanist Trygve Braarud (1903–1985).

References

  • Hagino, K. et al. (2013). PLoS ONE 8(12):e81749. DOI: 10.1371/journal.pone.0081749
  • Coale, T. H. et al. (2024). Science 384(6692):217–222. DOI: 10.1126/science.adk1075
  • Massana, R. (2024). Science 384(6692):160–161. DOI: 10.1126/science.ado8571
  • Wong, C. (2024). Nature 628:702. DOI: 10.1038/d41586-024-01046-z

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