Zehr Jonathan P, Shilova Irina N, Farnelid Hanna M, Muñoz-Marín Maria Del Carmen, Turk-Kubo Kendra A
Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.
Centre for Ecology and Evolution in Microbial Model Systems, Linnaeus University, 392 34 Kalmar, Sweden.
Nat Microbiol. 2016 Dec 20;2:16214. doi: 10.1038/nmicrobiol.2016.214.
Nitrogen fixation - the reduction of dinitrogen (N) gas to biologically available nitrogen (N) - is an important source of N for terrestrial and aquatic ecosystems. In terrestrial environments, N-fixing symbioses involve multicellular plants, but in the marine environment these symbioses occur with unicellular planktonic algae. An unusual symbiosis between an uncultivated unicellular cyanobacterium (UCYN-A) and a haptophyte picoplankton alga was recently discovered in oligotrophic oceans. UCYN-A has a highly reduced genome, and exchanges fixed N for fixed carbon with its host. This symbiosis bears some resemblance to symbioses found in freshwater ecosystems. UCYN-A shares many core genes with the 'spheroid bodies' of Epithemia turgida and the endosymbionts of the amoeba Paulinella chromatophora. UCYN-A is widely distributed, and has diversified into a number of sublineages that could be ecotypes. Many questions remain regarding the physical and genetic mechanisms of the association, but UCYN-A is an intriguing model for contemplating the evolution of N-fixing organelles.
固氮作用——将氮气（N₂）还原为生物可利用的氮（N）——是陆地和水生生态系统中氮的重要来源。在陆地环境中，固氮共生关系涉及多细胞植物，但在海洋环境中，这些共生关系发生在单细胞浮游藻类中。最近在贫营养海洋中发现了一种未培养的单细胞蓝细菌（UCYN - A）与一种定鞭藻浮游微藻之间的异常共生关系。UCYN - A具有高度简化的基因组，并与其宿主交换固定氮以获取固定碳。这种共生关系与淡水生态系统中的共生关系有一些相似之处。UCYN - A与膨胀艾氏藻的“球状体”以及变形虫嗜色保利娜的内共生体共享许多核心基因。UCYN - A分布广泛，并已分化为多个可能是生态型的亚谱系。关于这种关联的物理和遗传机制仍有许多问题，但UCYN - A是思考固氮细胞器进化的一个有趣模型。
