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Unusual marine unicellular symbiosis with the nitrogen-fixing cyanobacterium UCYN-A

2016; Nature Portfolio; Volume: 2; Issue: 1 Linguagem: Inglês

10.1038/nmicrobiol.2016.214

2058-5276

Jonathan P. Zehr, Irina N. Shilova, Hanna Farnelid, Maria del Carmen Muñoz‐Marín, Kendra A. Turk‐Kubo,

Marine and coastal ecosystems

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. The symbiosis between UCYN-A and haptophyte picoplankton plays a major role in oceanic nitrogen cycling. Though it bears some resemblance to freshwater examples, making it an interesting marine model, UCYN-A diversity means that many questions remain.