Symbiogenesis
Nitroplasts

A unicellular marine alga, Braarudosphaera bigelowii (a coccolithophore, which is a eukaryote), has been found with a cyanobacterium as an endosymbiont. The cyanobacterium forms a nitrogen-fixing structure, dubbed the nitroplast. It divides evenly when the host cell undergoes mitosis, and many of its proteins derive from the host alga, implying that the endosymbiont has proceeded far along the path towards becoming an organelle. The cyanobacterium is named Candidatus Atelocyanobacterium thalassa, and is abbreviated UCYN-A. The alga is the first eukaryote known to have the ability to fix nitrogen.
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