Circumscription of three new taxa of Merismopedia-like Cyanobacteria shows how far convergent evolution can go.

The genus Merismopedia (Merismopediaceae, Chroococcales), characterized by rectangular colonies with cells arranged in regular perpendicular rows and columns, represents a challenging study system due to its limited cultivability. The genus recently underwent taxonomic revision using cultivation-independent techniques and a polyphasic approach. Following the designation of an epitype, its type species, M. tranquilla was placed within a phylogenetic framework. Polyphyletic "Merismopedia" was split, and its species clustered within at least three distinct families. The case represents a clear example of convergent evolution, a phenomenon not uncommon among Cyanobacteria. The present study further explores the convergent evolution of Cyanobacteria with colonies exhibiting regularly perpendicular cell rows. Using predominantly a single-colony sequencing approach, partial 16S rRNA gene and ITS rRNA region sequence data were obtained for four additional Merismopedia morphotypes. Phylogenetic analyses supported the placement of two of these taxa within a recently described genus Eucapsopsis. Accordingly, two new species are proposed: Eucapsopsis complanata, distinguished by its loosely arranged cells, and Eucapsopsis sphagnicola, a new combination for Merismopedia sphagnicola, typically occurring in oligotrophic peat bogs. Furthermore, the description of a morphologically cryptic genus Cryptoplanktopedia is presented. A strain identified as "Merismopedia" cf. angularis NIVA-CYA 199, characterized by its L-shaped colonies, clustered as another distant lineage within Chroococcales although with ambiguous systematic placement. The results demonstrate that Merismopedia-like colony morphology is a widespread trait and has evolved convergently among Cyanobacteria.