Plasmid content and distribution in the toxic cyanobacterial genus Microcystis Kutzing ex Lemmermann (Cyanobacteria : Chroococcales)

Fourteen axenic and six non-axenic strains of Microcystis Kutzing ex Lemmermann, isolated from 19 Australian blooms, and 11 strains from European, Japanese and North American blooms were screened for microcystin content by HPLC and plasmid content by agarose gel electrophoresis. As no plasmids were detected in any of the Australian strains with high microcystin content, it is unlikely that a plasmid is involved in microcystin production. Two different plasmids were found in two different Australian strains with low microcystin content: a 2.20 kb plasmid from a strain isolated from Griffith, NSW, and a 4.24 kb plasmid from a strain isolated from Mt Bold Reservoir, SA. There were no differences in the plasmid contents of axenic subcultures isolated from a parent, uni-algal culture. The low prevalence of plasmids in Australian strains (two of 20 strains) is in contrast with the prevalence in Microcystis strains from other parts of the world (11 of 19 strains). The data from this study, combined with previous reports, indicate that plasmid content may be a relatively stable character that could be useful for resolving taxa within the genus Microcystis.