The nitroplast: A nitrogen-fixing organelle

Eukaryotic cells are notably complex-for example, they have various organelles, which are membrane-bound structures with specific functions. Two of these organelles, mitochondria and chloroplasts, which function in respiration and photosynthesis, evolved from the integration of endosymbiotic bacteria to the eukaryotic cell (1). In marine systems, some nitrogen-fixing bacteria are endosymbionts of microalgae, such as Candidatus Atelocyanobacterium thalassa (UCYN-A), a cyanobacterial symbiont of the unicellular algae Braarudosphaera bigelowii (2). On page 217 of this issue, Coale et al. (3)report a close integration of the endosymbiont into the architecture and function of the host cell, which is a characteristic of organelles. These findings show that UCYN-A has evolved from a symbiont to a eukaryotic organelle for nitrogen fixation-the nitroplast-thereby expanding a function that was thought to be exclusively carried out by prokaryotic cells to eukaryotes.