Molecular phylogeny of nitrogen-fixing unicellular cyanobacteria

Molecular phylogenetic study was conducted using maximum likelihood tree in ference methods with small subunit ribosomal RNA sequence data to ascertain the evolutionary relationships among sheathless, single-cell cyanobacteria capable of nitrogen fixation. Cyanobacterial strains of the genus Cyanothe ce (circumscribed by Waterbury and Rippka, 1989) fall into at least three i ndependent lines of descent within a larger assemblage previously designate d the SPM sequence group. No strong correlation between aerobic versus anae robic nitrogen-fixing activity and phylogenetic relationships was observed. The results support a hypothesis of multiple gains and/or losses of nitrog en-fixation abilities among the sheathless, unicellular cyanobacteria.