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Unicellular Cyanobacterium Symbiotic with a Single-Celled Eukaryotic Alga

Science  21 Sep 2012:
Vol. 337, Issue 6101, pp. 1546-1550
DOI: 10.1126/science.1222700

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Abstract

Symbioses between nitrogen (N)2–fixing prokaryotes and photosynthetic eukaryotes are important for nitrogen acquisition in N-limited environments. Recently, a widely distributed planktonic uncultured nitrogen-fixing cyanobacterium (UCYN-A) was found to have unprecedented genome reduction, including the lack of oxygen-evolving photosystem II and the tricarboxylic acid cycle, which suggested partnership in a symbiosis. We showed that UCYN-A has a symbiotic association with a unicellular prymnesiophyte, closely related to calcifying taxa present in the fossil record. The partnership is mutualistic, because the prymnesiophyte receives fixed N in exchange for transferring fixed carbon to UCYN-A. This unusual partnership between a cyanobacterium and a unicellular alga is a model for symbiosis and is analogous to plastid and organismal evolution, and if calcifying, may have important implications for past and present oceanic N2 fixation.

  • Present address: Department of Isotope Biogeochemistry, UFZ–Helmholtz Centre for Environmental Research, Leipzig 04318, Germany.

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