title: Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga.
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 N(2) fixation.
authors.affiliation: Ocean Sciences, University of California, Santa Cruz, CA 95064, USA.
authors.forename: Anne W
authors.initials: AW
authors.lastname: Thompson
authors.forename: Rachel A
authors.initials: RA
authors.lastname: Foster
authors.forename: Andreas
authors.initials: A
authors.lastname: Krupke
authors.forename: Brandon J
authors.initials: BJ
authors.lastname: Carter
authors.forename: Niculina
authors.initials: N
authors.lastname: Musat
authors.forename: Daniel
authors.initials: D
authors.lastname: Vaulot
authors.forename: Marcel M M
authors.initials: MM
authors.lastname: Kuypers
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Zehr
mesh_terms: D002113:Calcification, Physiologic; D002244:Carbon / Q000378:metabolism; D000458:Cyanobacteria / Q000166:cytology / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D020459:Genes, rRNA; D016680:Genome, Bacterial; D058087:Haptophyta / Q000166:cytology / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D008969:Molecular Sequence Data; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation; D010140:Pacific Ocean; D010788:Photosynthesis; D010839:Phytoplankton / Q000166:cytology / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D012623:Seawater / Q000382:microbiology*; D013559:Symbiosis
chemical_list: D002244:Carbon; D009584:Nitrogen