The genus Pleurastrum is a coccoid green alga comprising 10 species worldwide. Pleurastrum exhibits simple morphology and high polymorphism, which complicates the understanding of its diversity. We examined the morphological and ultrastructural characteristics of Pleurastrum using light, confocal, and transmission electron microscopy. Additionally, we performed phylogenetic analysis based on multigene sequences (nuclear SSU rDNA, 5.8S, internal transcribed spacer (ITS2) region, and plastid rbcL and tufA genes) from Pleurastrum strains to report two previously unrecorded freshwater species (Pleurastrum insigne and Pleurastrum microstigmatum) in Korea. The vegetative cells were predominantly spherical, with a few being ellipsoidal, and each cell contained a chloroplast with one pyrenoid. The sporangia produced several daughter cells, while the biflagellate zoospores were ellipsoidal and motile. Phylogenetic analysis confirmed that P. insigne and P. microstigmatum form well-supported monophyletic clades. Analysis of ITS2 secondary structures revealed similar patterns, with several differences in nucleotide sequences and insertions between the two species. The findings of this study expand the known distribution of Pleurastrum and enhance our understanding of its species diversity in Korea.

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 Chroomonas is a group of blue-green colored cryptomonads. This study describes two freshwater Chroomonas species for the first time in South Korea: Chroomonas nordstedtii Hansgirg and Chroomonas coerulea (Geitler) Skuja. We examined the morphology and ultrastructure of these species by light microscopy, scanning electron microscopy, and transmission electron microscopy. These two Chroomonas species were blue-green colored and ovate to oval-shaped. Chroomonas nordstedtii was characterized by two Maupas ovals with hexagonal periplast plates, whereas C. coerulea was characterized by one eyespot with rectangular periplast plates. A molecular phylogeny with data from nuclear SSU rRNA and chloroplast rbcL genes revealed that Korean C. nordstedtii formed a distinct clade with NIES-708, NIES-1004 from Japan, and UTEX 2779 from Colorado, USA, while C. coerulea formed a clade with ACOI 1366 from Portugal.

Cyanobium is a genus of picoprokaryotic cyanophytes, which includes species worldwide. The present study investigated the morphology, ultrastructure, and molecular phylogeny of the unrecorded genus Cyanobium Rippka & Cohen-Bazire 1983 and species Cyanobium gracile Rippka & Cohen-Bazire 1983. A C. gracile culture from a freshwater sample collected from the Adongji pond was established by singlecell isolation. Morphological data were analyzed using light and transmission electron microscopy. C. gracile lives as solitary cells without gelatinous envelopes and is ovate, oval, or shortly rod-shaped. Thylakoids are laid along the cell walls, with three thylakoid membranes parallel to each other. Nucleoplasm was observed in the center of the cell. Molecular phylogeny performed with data from 16S small subunit ribosomal DNA gene (SSU rDNA) sequences showed that the three strains of C. gracile, including the type strain (PCC6307) and a newly recorded strain (Adong101619), formed a distinct clade with a high supporting value (maximum-likelihood=100, pp=1.00). Based on morphology and molecular data, we report the newly recorded C. gracile in Korea.

Rhodomonas salina is a phototrophic marine flagellate. We examined the ultrastructure of R. salina with particular attention to the flagellar apparatus by transmission electron microscopy and compared it with that of other cryptomonads reported previously. The major components of the flagellar apparatus in R. salina were a keeled rhizostyle (Rhs), a striated fibrous root (SR), a SR-associated microtubular root (SRm), a mitochondrion-associated lamella (ML), and three types of microtubular roots (9r, 4r, and 2r). The keeled Rhs originated near the proximal end of the dorsal basal body, passed near the nucleus and dissociated at the posterior end of the cell. The SR and SRm originated between two basal bodies and laterally extended to the right side of the cell. The ML originated between two basal bodies and extended to the left side of the cell. The 9r originated between the ventral basal body and the Rhs and extended toward the anterior dorsal lobe of the cell. The 4r originated near the 9r and extended toward the dorsal lobe with the 2r, which originated between two basal bodies. Here, the flagellar apparatus in R. salina is described, and the ultrastructure of the flagellar apparatus is compared among cryptomonad species.