Morphological and molecular characterization of novel species and intraspecific taxa in <i>Desmodesmus</i> (Scenedesmaceae)

<i>Desmodesmus</i> (Scenedesmaceae) is a ubiquitous genus of green algae that inhabits freshwater environments. <i>Desmodesmus</i> species are characterized by distinct morphological features such as spines, warts, ribs, and rosettes. These ultrastructures are crucial for morphological characterization and species identification within this genus but are frequently overlooked in light microscopy (LM), which can lead to misidentification. The genus <i>Desmodesmus</i> was established based on both morphological characteristics and phylogenetic analysis using 18S ribosomal RNA (rRNA) and internal transcribed spacer 2 sequences. However, only internal transcribed spacer sequences, which are relatively short and not highly conserved, are widely used in taxonomic studies of this genus. Therefore, characterizing ultrastructures (e.g., cell surfaces) and 18S rRNA sequences in <i>Desmodesmus</i> species is becoming increasingly important, particularly for species identification. This study aims to report two novel species (<i>D. similis</i> and <i>D. complicatus</i>) and four novel intraspecific taxa (<i>D. communis</i> var. <i>columnaris</i>, <i>D. denticulatus</i> var. <i>denticuloides</i>, <i>D. denticulatus</i> var. <i>simplex</i>, and <i>D. denticulatus</i> var. <i>distinctus</i>) within the genus <i>Desmodesmus</i>. These taxa show similar morphological features under LM observation to the coenobial arrangements and spines of <i>D. communis</i> and <i>D. denticulatus</i>, respectively, but their cell surface ultrastructures differ. Comparative analysis of 18S rRNA sequences with other <i>Desmodesmus</i> species confirmed our observations. In addition, we propose transferring <i>Scenedesmus</i> praetervisus to the genus <i>Desmodesmus</i>. Our results will contribute to a better understanding of the morphological features and species identification within the genus <i>Desmodesmus</i>.