A polyphasic approach leading to the revision of the genus Planktothrix (Cyanobacteria) and its type species, P. agardhii, and proposal for integrating the emended valid botanical taxa, as well as three new species, Planktothrix paucivesiculata sp. nov.ICNP, Planktothrix tepida sp. nov.ICNP, and Planktothrix serta sp. nov.ICNP, as genus and species names with nomenclatural standing under the ICNP.

Twenty strains of Planktothrix and five of 'Oscillatoria' were characterized by a polyphasic approach, for clarification of their taxonomic relationships. Emphasis was given to the strains (17) of the Pasteur Culture Collection of Cyanobacteria (PCC). Phenotypic characters analyzed comprised morphology, phycobiliprotein composition, temperature and salinity tolerance. The gvpA gas vesicle gene was detected by PCR in all strains, and transmission electron microscopy confirmed gas vesicle formation in the strains of 'Oscillatoria'. MALDI-TOF mass spectrometry revealed 13 chemotypes, nine of which produce microcystins. A multi-locus sequence typing (MLST) analysis was conducted using individual and concatenated nucleotide sequences of the 16S rDNA, internal transcribed spacer (ITS), gyrB, rpoC1 and rpoB. The results highlighted an unexpected diversity within the genus Planktothrix, showing that the five strains of 'Oscillatoria' need to be included in this taxon. Consequently, the genus consists of seven phylogenetic clusters, three of which represent new species, named Planktothrix paucivesiculata sp. nov.ICNP (type strain: PCC 8926T), Planktothrix tepida sp. nov.ICNP (type strain: PCC 9214T) and Planktothrix serta sp. nov.ICNP (type strain: PCC 8927T). These, together with the emended genus Planktothrix and its type species P. agardhii, valid taxa under the ICN, are described/re-described for gaining nomenclatural standing under the ICNP.