primary endosymbiosis 3 (a new one?)
"primary endosymbiosis" -- simply put, it's the merging of two organisms into one (basically if you could eat a whole, living animal, and then that animal would function as a bonus internal organ that let you do new things).
It's happened twice (that we know of): an ancient unicellular organism absorbed another ancient unicellular organism, and the one that got absorbed performed functions for the one that absorbed it ; eventually leading to the mitochondria, the powerhouse of the cell! The other time, the same thing happened with a different set of organisms, and the absorbed organism became the chloroplast, leading to plants. Both of these events are proposed origin-points for all multicellular life on Earth so primary endosymbiosis is a big deal.
So given that context, we think we may have spotted a third case of this happening! A paper (linked below) states: A nitrogen-fixing bacteria , UCYN-A, appears to be functioning as a nitrogen-fixing organelle for the algae, Braarudosphaera bigelowii. The UCYN-A "nitroplast" lives interior to the algae, providing nitrogen for the algae's growth, while the photosynthesis by the algae nourishes the UCYN-A. Given how useful nitrogen-fixation is, and how we haven't seen nitroplasts all over the place, the paper posits that it's a recent (...late-Cretaceous) symbiosis.
The paper is here for the curious: Cornejo-Castillo, F. M., Inomura, K., Zehr, J. P., & Follows, M. J. Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis. Cell 2024, 187:1762–1768.
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