Nitrogen-fixing organelle in a marine alga
Tyler H. Coale, Valentina Loconte, Kendra A. Turk‐Kubo, Bieke Vanslembrouck, Wingkwan Mak, Shunyan Cheung, Axel Ekman, Jian-Hua Chen, Kyoko Hagino, Yoshihito Takano, Tomohiro Nishimura, Masao Adachi, Mark Le Gros, Carolyn A. Larabell, Jonathan P. Zehr
Abstract
Symbiotic interactions were key to the evolution of chloroplast and mitochondria organelles, which mediate carbon and energy metabolism in eukaryotes. Biological nitrogen fixation, the reduction of abundant atmospheric nitrogen gas (N 2 ) to biologically available ammonia, is a key metabolic process performed exclusively by prokaryotes. Candidatus Atelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N 2 -fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga. Here we show that UCYN-A has been tightly integrated into algal cell architecture and organellar division and that it imports proteins encoded by the algal genome. These are characteristics of organelles and show that UCYN-A has evolved beyond endosymbiosis and functions as an early evolutionary stage N 2 -fixing organelle, or “nitroplast.”