Polyphasic characterization of Planktothrix spiroides sp nov (Oscillatoriales, Cyanobacteria), a freshwater bloom-forming alga superficially resembling Arthrospira

In the present study, 24 cyanobacterial strains with regular helical trichomes were isolated from four different freshwater bodies in China. These strains were initially identified as a freshwater Arthrospira-like cyanobacterium. Analysis of their 16S ribosomal RNA gene sequences revealed that these strains formed a monophyletic group with Planktothrix. Further molecular phylogenetic analyses based on the cpcBA-IGS and rpoC1 genes supported the finding that these strains belonged to Planktothrix, not Arthrospira. In addition, the fatty acid composition showed that the strains contained 18:3 alpha but lacked 18:3 gamma, which was similar to Planktothrix. These results showed that the alga formed a new group within Planktothrix and that this group, which resembled Arthrospira in morphology, was unrelated to Arthrospira. The strains were distinct from previously described Planktothrix species, and therefore a new species, Planktothrix spiroides sp. nov., was established.