Nitroplast

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A nitroplast is an organelle responsible for nitrogen-fixing. It arose as a cyanobacteria species living within the eukaryotic alga Braarudosphaera bigelowii. It is the first recorded example of such an organelle, designated in 2024. The cyanobacterium is Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, which exists exclusively as an obligate symbiont. Despite being found in measurable quantities throughout the world's oceans, A. thalassa is not known to be free-living in any environment. Unlike typical cyanobacteria, its genome has undergone massive reduction, losing the genes for RuBisCO, photosystem II, and the TCA cycle. Consequently, it possesses no independent means of fixing carbon or generating energy through photosynthesis, rendering it entirely dependent on its host (so far only known to be Braarudosphaera bigelowii and a closely-related unnamed species). This partnership is characterized by a strict metabolic exchange: A. thalassa fixes atmospheric nitrogen...

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