SUMMARY Twelve strains of thin filamentous cyanobacteria belonging to the genus Drouetiella were isolated from freshwater and subaerophytic environments in the Republic of Korea and examined using a polyphasic approach integrating morphological, ultrastructural, and molecular data. Of these, five strains (ACKU959–ACKU963) were clearly distinct from previously described Drouetiella species based on combined morphological and molecular evidence. Strain ACKU959 was characterized by wider cells than previously described Drouetiella species, frequent solitary or paired false branching, and rounded, swollen terminal cells. In the 16S rRNA phylogeny, strain ACKU959 was positioned within a clade comprising D. fasciculata , D. epilithica , and D. elegans ; however, sequence similarity values (97.79–98.32%) to these taxa were below the 98.65% threshold generally accepted for species delimitation, supporting its recognition as a distinct species. Strains ACKU960–ACKU963 were morphologically indistinguishable, sharing olive‐brownish cell coloration, wider cells than previously described Drouetiella species, and necridic cells appear in rows. These strains showed 100% 16S rRNA gene sequence similarity among themselves and formed a well‐supported monophyletic clade in phylogenetic analyses. They were closely related to the Chinese strain Drouetiella sp. FACHB‐3567, with a sequence similarity of 99.37%. Transmission electron microscopy corroborated observations from light microscopy, including the presence of a sheath and constrictions at the cross‐walls, and revealed ultrastructural characteristics such as a parietal arrangement of thylakoids. The secondary structures of the 16S–23S ITS region (D1–D1′, Box‐B, and V3 helices), together with ITS percent dissimilarity (3.36–16.02%), further supported their distinction from previously described Drouetiella species. Accordingly, two novel species, Drouetiella lacustris sp. nov. and Drouetiella olivacea sp. nov., are described in accordance with the International Code of Nomenclature for algae, fungi, and plants.
Kim et al. (Mon,) studied this question.