Desmidiales (Conjugatophyceae, Charophyta) are commonly found in freshwater ecosystems and exhibit high species diversity, particularly in acidic wetlands, lakes, swamps, and peat bogs. They possess a distinctive morphology characterized by symmetrical semicells, and their wide variation in cell shape and size presents challenges in species identification due to high morphological plasticity. Although 832 species of Desmidiales have been reported in Korea, phylogenetic studies have been limited to only a few taxonomic groups. This study focused on investigating species-level relationships among Desmidiales using strains from the Freshwater Bioresources Culture Collection (FBCC), integrating morphological characteristics, ecological data, and original species descriptions. A total of 352 new plastid gene sequences were generated for phylogenetic analyses, including accD (30), atpA (42), atpB (22), ndhH (37), petA (37), psaA (32), psbA (44), psbC (1), psbD (39), rbcL (40), rpl2 (19), and rpoB (9). Among the 12 plastid genes analyzed, psbA showed the highest proportion of conserved sites (83.9%), while petA exhibited the highest proportion of variable sites (38.7%). Based on the combined phylogenetic analysis, Desmidiales were grouped into five major clades: Cosmarium Clade-1: Cosmarium punctulatum, Cosmarium sp. 1, Cosmarium Clade-2: C. blyttii, C. botrytis, C. costatum, C. ochthodes, C. pachydermum, C. subcostatum, C. subcrenatum, C. subprotumidum, C. trilobulatum, Cosmarium Clade-3: C. angulosum, C. formosulum, C. granatum, C. impressulum, C. norimbergense, C. regnellii, C. subtumidum, Cosmarium sp. 2, Staurastrum Clade-1: Staurastrum avicula var. lunatum, Staurastrum Clade-2: S. boreale, S. dispar, S. kouwetsii, S. margaritaceum, S. punctulatum. The newly generated sequence data from FBCC strains will serve as a valuable resource for accurate species identification and for exploring the molecular ecology of Desmidiales in freshwater ecosystems. This phylogenetic framework improves our understanding of Desmidiales species diversity in Korea and aids in achieving a more comprehensive taxonomic resolution within this algal order.

The genus Desmodesmus (Chodat) S.S. An, T. Friedl & E. Hegewald is ubiquitous in freshwater ecosystems, such as rivers, ponds, and wetlands. The actual species diversity and distribution of the genus is unknown because of morphological plasticity affected by habitats. Currently, 38 Desmodesmus species have been reported in Korea most of which transferred from the genus Scenedesmus recently, however, no phylogenetic relationships have been studied yet. Despite the challenges in analyzing relationships among Desmodesmus species through the morphology, ecology, and original description, this study focused on examining species-level relationships using the FBCC culture strains isolated from Korea. A total of 299 sequences (66 of 18S rRNA, 47 of atpB, 67 of petA, 52 of rbcL, and 67 of tufA) were newly determined and used for phylogenetic analysis. Four plastid genes tend to have higher variation than 18S rRNA in the variable sites and P-distance. From the combined phylogeny, the Desmodesmus included six clades such as Clade-1: D. pseudoserratus and D. serratus, Clade-2: D. communis, D. dispar, D. maximus, D. pannonicus, unidentified Desmodesmus sp., Clade-3: D. bicaudatus and D. intermedius, Clade-4: D. microspina, D. multivariablis, D. pleiomorphus, D. subspicatus, Clade-5: D. abundans, D. kissii, and D. spinosus, and Clade-6: D. armatus, D. armatus var. longispina, D. opoliensis, unidentified Desmodesmus spp. The new sequence data from FBCC strains will be used to identify species and study the molecular ecology of scenedesmacean green algae in freshwater ecosystems. The phylogenetic information from this study will expand our understanding of Desmodesmus species diversity in Korea.