title: Comparative genomics reveals surprising divergence of two closely related strains of uncultivated UCYN-A cyanobacteria.
abstract: Marine planktonic cyanobacteria capable of fixing molecular nitrogen (termed 'diazotrophs') are key in biogeochemical cycling, and the nitrogen fixed is one of the major external sources of nitrogen to the open ocean. Candidatus Atelocyanobacterium thalassa (UCYN-A) is a diazotrophic cyanobacterium known for its widespread geographic distribution in tropical and subtropical oligotrophic oceans, unusually reduced genome and symbiosis with a single-celled prymnesiophyte alga. Recently a novel strain of this organism was also detected in coastal waters sampled from the Scripps Institute of Oceanography pier. We analyzed the metagenome of this UCYN-A2 population by concentrating cells by flow cytometry. Phylogenomic analysis provided strong bootstrap support for the monophyly of UCYN-A (here called UCYN-A1) and UCYN-A2 within the marine Crocosphaera sp. and Cyanothece sp. clade. UCYN-A2 shares 1159 of the 1200 UCYN-A1 protein-coding genes (96.6%) with high synteny, yet the average amino-acid sequence identity between these orthologs is only 86%. UCYN-A2 lacks the same major pathways and proteins that are absent in UCYN-A1, suggesting that both strains can be grouped at the same functional and ecological level. Our results suggest that UCYN-A1 and UCYN-A2 had a common ancestor and diverged after genome reduction. These two variants may reflect adaptation of the host to different niches, which could be coastal and open ocean habitats.
authors.affiliation: Ocean Sciences Department, University of California, Santa Cruz, CA, USA.
authors.forename: Deniz
authors.initials: D
authors.lastname: Bombar
authors.affiliation: Biomolecular Engineering Department, University of California, Santa Cruz, CA, USA.
authors.forename: Philip
authors.initials: P
authors.lastname: Heller
authors.affiliation: Schools of Biological and Geographical Sciences, University of Bristol, Bristol, UK.
authors.forename: Patricia
authors.initials: P
authors.lastname: Sanchez-Baracaldo
authors.affiliation: Ocean Sciences Department, University of California, Santa Cruz, CA, USA.
authors.forename: Brandon J
authors.initials: BJ
authors.lastname: Carter
authors.affiliation: Ocean Sciences Department, University of California, Santa Cruz, CA, USA.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Zehr
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics; D000458:Cyanobacteria / Q000145:classification / Q000235:genetics*; D016680:Genome, Bacterial; D023281:Genomics; D009586:Nitrogen Fixation / Q000235:genetics; D009792:Oceans and Seas; D010802:Phylogeny; D012623:Seawater / Q000382:microbiology
chemical_list: D001426:Bacterial Proteins