Unusual marine unicellular symbiosis with the nitrogen-fixing cyanobacterium UCYN-A.
- Author(s): Zehr, Jonathan P;
- Shilova, Irina N;
- Farnelid, Hanna M;
- Muñoz-Marín, Maria Del Carmen;
- Turk-Kubo, Kendra A
- et al.
Published Web Locationhttps://doi.org/10.1038/nmicrobiol.2016.214
Nitrogen fixation - the reduction of dinitrogen (N2) gas to biologically available nitrogen (N) - is an important source of N for terrestrial and aquatic ecosystems. In terrestrial environments, N2-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 N2-fixing organelles.