Epitypification of Merismopedia tranquilla (Cyanobacteria) prompts phylogenetic reassessment of the genus and the description of two morphologically similar genera.

The genus Merismopedia is morphologically characterized by its distinctive colony arrangement yet exhibits considerable variability among species. Recent studies suggested that Merismopedia is polyphyletic, and its most likely true lineage was provisionally placed within the Microcystaceae. However, the available molecular data for most of the extant species were scarce, and notably, its type species, Me. tranquilla, lacked contemporary distributional and molecular data. In the present study, we sampled multiple localities to capture the potential morphological and genetic variability within Merismopedia. Using culture-dependent and -independent methods, we collected an extensive dataset of 16S rRNA gene and 16S-23S ITS rRNA region sequences. This dataset included the type locality of Me. tranquilla, from which we successfully recovered morphologically authentic isolates and established an epitype. Subsequent phylogenetic analyses of Merismopedia- and Microcrocis-like taxa revealed the placement of Merismopedia sensu stricto within the Geminocystaceae, herein reinstated to Merismopediaceae and further confirmed extensive polyphyly. Based on these findings, we propose two new genera (Neomerismopedia and Planktopedia), and a new species, Me. sepulta. Additionally, we transfer Microcrocis gigas to Merismopedia. We proposed the provisional transfer of Me. hyalina to Synechocystis; however, further revision is needed. These taxonomic revisions provide a foundation for a comprehensive reevaluation of this widespread and morphologically diverse group of Cyanobacteria.