Endosymbiosis and Organelle Evolution

Endosymbiosis — one cell living inside another — is the engine behind the major organelles of eukaryotic life. Over evolutionary time, endosymbionts can become organelles: structures whose genomes shrink as their functions are taken over by host-encoded proteins imported from the nucleus.

Organelle hallmarks

• Highly reduced genome (genes lost or transferred to the host nucleus)
• Import of nuclear-encoded, organelle-targeted proteins (signal/transit peptides)
• Division synchronized with host cell division
• Host-controlled physiology (e.g., coordination of the light–dark cycle)

The canonical cases

• Mitochondria — α-proteobacterium engulfed ~1.5–2.2 billion years ago; the defining event for eukaryotic life.
• Chloroplasts — cyanobacterium engulfed ~1+ billion years ago (primary endosymbiosis), giving rise to Archaeplastida; later secondary endosymbiosis events (a eukaryote with a chloroplast engulfed by another eukaryote) produced the plastids of algae such as haptophytes — including the chloroplast of B. bigelowii.

Recent and intermediate cases

• Nitroplast (2024) — the UCYN-A2 cyanobacterium in B. bigelowii; the first nitrogen-fixing organelle; young (~100 Mya) but already importing ~half its proteins from the host and dividing in synchrony (Coale et al., 2024). See nitroplast.
• Chromatophore of Paulinella — an independent, ~100–120 Myr-old primary plastid from a different cyanobacterial lineage. A useful comparator: the nitroplast lost genes faster than the chromatophore, possibly because metabolic redundancies (e.g., host- and organelle-encoded PyrC in pyrimidine biosynthesis) relax selection on keeping organelle copies.
• Diazoplasts in rhopalodiacean diatoms — nitrogen-fixing cyanobacterial endosymbionts in Epithemia and Rhopalodia; specialized for nitrogen fixation within a photosynthetic eukaryote but not (yet) counted as full organelles (Moulin et al., 2024; Nakayama & Inagaki, 2017).

The nitroplast as a “textbook case”

Nitrogen fixation was long thought to be an exclusively prokaryotic function. The nitroplast shows a eukaryote fixing N₂ through an organelle, and demonstrates that organelle formation can occur on timescales of ~100 million years — recent enough that the process is still observable (Massana, 2024).

Related

• nitroplast — the nitrogen-fixing organelle
• braarudosphaera-bigelowii — the host alga
• aristotle — Founded systematic biology and comparative anatomy; the scala naturae as precursor to evolutionary thinking

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
• Moulin, S. L. Y. et al. (2024). The endosymbiont of Epithemia clementina is specialized for nitrogen fixation within a photosynthetic eukaryote. ISME Communications 4(1):ycae055. DOI: 10.1093/ismeco/ycae055
• Nakayama, T. & Inagaki, Y. (2017). Genomic divergence within non-photosynthetic cyanobacterial endosymbionts in rhopalodiacean diatoms. Scientific Reports 7:13075. DOI: 10.1038/s41598-017-13578-8