title: Unusual marine unicellular symbiosis with the nitrogen-fixing cyanobacterium UCYN-A.
abstract: 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.
authors.affiliation: Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.
authors.forename: Jonathan P
authors.identifier: 0000-0002-5691-5408
authors.initials: JP
authors.lastname: Zehr
authors.affiliation: Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.
authors.forename: Irina N
authors.initials: IN
authors.lastname: Shilova
authors.affiliation: 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.
authors.forename: Hanna M
authors.initials: HM
authors.lastname: Farnelid
authors.affiliation: Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.|Departamento de Bioquímica y Biología Molecular, Edificio Severo Ochoa, Universidad de Córdoba, 14071-Córdoba, Spain.
authors.forename: Maria Del Carmen
authors.initials: MD
authors.lastname: Muñoz-Marín
authors.affiliation: Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.
authors.forename: Kendra A
authors.initials: KA
authors.lastname: Turk-Kubo