Sphaerocavum: a coccoid morphogenus identical to Microcystis in terms of 16S rDNA and ITS sequence phylogenies

Sphaerocavum is a typical planktonic coccoid cyanobacterium that co-occurs with Microcystis species, and it is commonly found in blooms. Sphaerocavum is a morphogenus distinguishable from Microcystis, as it presents hollow colonies and two-plane cell division. However, Sphaerocavum phylogenetic position based on marker gene(s) has not yet been established. Here, we combined morphological and molecular analyses to address the taxonomic and phylogenetic placement of Sphaerocavum isolated from Brazilian eutrophic lakes. Phylogenetic analysis revealed that Sphaerocavum and Microcystis clustered together in a well-established clade, thus not supporting the Sphaerocavum designation as a divergent genus. The type strain and isolated lineages from Brazilian tropical regions were grouped in the ITS phylogenetic analysis apart from other Microcystis strains. As such, although the infrageneric recognition of Microcystis by molecular markers is still unclear, our analyses strongly indicate that Sphaerocavum must be considered as a morphotype of Microcystis. Thus, taking together our morphological and molecular analyses, we propose the inclusion of Sphaerocavum within the genus Microcystis. In addition, we suggest that the strains used in this study should be named as Microcystis brasiliensis (Azevedo & Sant'Anna) Rigonato et al. comb. nov., with CCIBt3094 as the type strain.