A proposal for the unification of five species of the cyanobacterial genus Microcystis Kützing ex Lemmermann 1907 under the rules of the Bacteriological Code.

Genomic DNA homologies were examined from six Microcystis (cyanobacteria) strains, including five different species, Microcystis aeruginosa, Microcystis ichthyoblabe, Microcystis novacekii, Microcystis viridis and Microcystis wesenbergii. All DNA-DNA reassociation values between two strains of M. aeruginosa and the other four species exceeded 70%, which is considered high enough for them to be classified within the same bacterial species. It is proposed to unify these five species into M. aeruginosa under the Rules of the Bacteriological Code and NIES843T (= IAM M-247T) is proposed as the type strain. Two other species, Microcystis flos-aquae and Microcystis pseudofilamentosa, should be regarded as morphological variations of this unified M. aeruginosa. The current taxonomy of cyanobacteria depends too much upon morphological characteristics and must be reviewed by means of bacteriological methods as well as traditional botanical methods.