Nitrogen fixers have a crucial role in sustaining ecosystems through their unique ability to convert atmospheric nitrogen (N2) into bioavailable ammonia (NH3). N2 fixation in eukaryotes occurs exclusively through symbiotic associations with bacteria or archaea. Symbiotic interactions have been central in the evolution of the chloroplast and mitochondrion organelles, which perform carbon and energy metabolism in eukaryotes; however, the existence of a eukaryote with an N2-fixing organelle has been elusive. In a recent study, Coale et al. report the existence of an N2-fixing organelle in a marine alga called the ‘nitroplast’, which is derived from endosymbiosis.

Previously, the N2-fixing cyanobacterium ‘Candidatus Atelocyanobacterium thalassa’ (UCYN-A) was reported to be an endosymbiont of Braarudosphaera bigelowii algae. Using soft X-ray tomography, the authors show that UCYN-A is tightly incorporated into the structure of algal cells and that it divides in sequence with organelles and cell division, ensuring that it is passed to daughter cells. Proteomics analyses uncovered that it imports proteins that are encoded by the algal genome — a trait typical of organelles. Overall, this study provides remarkable insights into organellogenesis by an endosymbiont.