Cyanobacteria (phylum Cyanophyta/Cyanobacteria, class Cyanophyceae) are among the most widespread organisms and are able to adapt themselves to different extreme environments. These micro‐organisms have an important ecological role, given their ability to perform oxygenic photosynthesis, and are employed in different fields based on their ability to produce several bioactive compounds. Their prokaryotic nature, the presence of many cryptic species, and the coexistence of different nomenclature systems make the taxonomic identification of cyanobacteria particularly difficult. Moreover, for several species, the original reference strains (holotypes) are lacking. Increasingly, authors are using a polyphasic approach to characterize cyanobacteria, while typification is a recent trend that is being used to solve the problem of missing holotypes in other micro‐organisms. Here we focus on a filamentous cyanobacterium, isolated from the Euganean Thermal District (Padova, Italy) and temporarily named strain ETS‐02, using a polyphasic approach that includes morphological, ultrastructural, biochemical (pigment and fatty acid content), physiological (nitrogen fixation), and genetic (16S rRNA, 16S–23S ITS, cpcB‐IGS‐cpcA, rpoC1, gyrB, rbcL, nifD loci) analyses. The description of Phormidium cf. irriguum CCALA 759 as the epitype of Phormidium irriguum was also used to complete the characterization of strain ETS‐02.

A microalga was isolated from the pack-ice of Wood Bay (Antarctica) during austral summer 1993–4. By comparison with an authentic strain obtained from Sammlung von Algenkulturen at the University of Go$ ttingen (SAG 860–3), its ultrastructural details and pigment composition were very similar to those of Pleurochloris meiringensis. However, sequence analysis of the entire 18S rRNA gene revealed substantial genetic divergence (57 nucleotide substitutions and 19 insertions/deletions) between SAG 860–3 and the Antarctic isolate. Phylogenetic analysis of complete 18S rRNA gene sequences from 18 microalgal species, including the two strains studied, rejected the hypothesis that the Antarctic isolate is the sister species to Pleurochloris meiringensis. Molecular evidence indicates that the Antarctic isolate is the sister group of a clade that comprises two other representatives of the Xanthophyceae (synonym Tribophyceae): Pleurochloris meiringensis and Botrydiopsis intercedens. This leads us to conclude that this microalga is a new species belonging to a new genus and the name Pseudopleurochloris antarctica is proposed.