Review Article

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

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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.

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Author information

Affiliations

  1. Department of Ocean Sciences, University of California, Santa Cruz, California 95064, USA.

    • Jonathan P. Zehr
    • , Irina N. Shilova
    • , Hanna M. Farnelid
    • , Maria del Carmen Muñoz-Marín
    •  & Kendra A. Turk-Kubo
  2. Centre for Ecology and Evolution in Microbial Model Systems, Linnaeus University, 392 34 Kalmar, Sweden.

    • Hanna M. Farnelid
  3. Departamento de Bioquímica y Biología Molecular, Edificio Severo Ochoa, Universidad de Córdoba, 14071-Córdoba, Spain.

    • Maria del Carmen Muñoz-Marín

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Contributions

J.P.Z. conceptualized and drafted the manuscript. I.N.S. performed genome comparisons and prepared accompanying figure. M.M.M. compiled photomicrographs, and prepared conceptual figures. K.T.K. performed phylogenetic analysis and prepared accompanying figure. J.P.Z., I.N.S., H.M.F., M.M.M. and K.T.K. drafted and edited the manuscript and figures. All authors read and approved the final manuscript.

Competing interests

The authors declare no competing financial interest.

Corresponding author

Correspondence to Jonathan P. Zehr.