Morphological,biochemical and phylogenetic assessments of water‐bloom‐forming tropical morphospecies of Microcystis (Chroococcales,Cyanobacteria)

Previous polyphasic analyses of five morphospecies of the water‐bloom‐forming cyanobacterial genus Microcystis, Microcystis aeruginosa (Kützing) Lemmermann (=Microcystis aeruginosa (Kützing) Kützing), Microcystis ichthyoblabe Kützing, Microcystis novacekii (Komárek) Compère, Microcystis viridis (A. Braun) Lemmermann, and Microcystis wesenbergii (Komárek) Komárek in Kondratieva, have shown them to be conspecific and they have been proposed to be included under the binomial Microcystis aeruginosa (Kützing) Lemmermann. However, several morphospecies from tropical regions, such as Microcystis bengalensis Banerji, Microcystis panniformis Komárek, Komárková‐Legnerová, Sant'anna, Azevedo & Senna, Microcystis protocystis Crow, Microcystis pseudofilamentosa Crow, Microcystis ramosa Bharadwaya, and Microcystis robusta (Clark) Nygaard, have never been analyzed biochemically or phylogenetically; consequently, their taxonomic status is uncertain. To resolve this issue, we collected 57 strains of Microcystis from Vietnam for taxonomic analysis using a polyphasic approach. Strains were assigned to the six tropical morphospecies listed above or to four morphospecies with cosmopolitan distributions (M. aeruginosa, M. ichthyoblabe, M. novacekii, and M. wesenbergii). Several strains produced colony variants in different culture media; some of these variants had forms that overlapped with those of other morphospecies. Cell diameters varied widely between strains (2.6–9.3 µm) and were unrelated to morphospecies discrimination criteria. Strains of the 10 morphospecies examined had similar fatty acid compositions and closely similar 16S rRNA gene sequences (>99.2% similar). Phylogenetic analyses using 16S rRNA gene and 16S–23S internal transcribed spacer sequences did not identify any clear separations corresponding to morphospecies concepts or microcystin‐producing abilities. Thus, the six tropical morphospecies (M. bengalensis, M. panniformis, M. protocystis, M. pseudofilamentosa, M. ramosa, and M. robusta) are not natural taxonomic units within the genus Microcystis and should be included under M. aeruginosa.