A comprehensive systematic analysis of Coelastrum (Sphaeropleales, Chlorophyceae) from China, including the characterization of the chloroplast genome of Coelastrum rugosum

The genus Coelastrum, a type of green algae, is among the most common phytoplankton groups found in freshwater environments. However, our knowledge of this taxon is largely based on morphological studies and the available molecular data are very limited. To explore the diversity of Coelastrum and perform a systematic revision using molecular evidence, we collected 112 strains from across China and gathered molecular markers, including sequences from 18S rDNA, ITS, and the chloroplast tufA gene. A thorough systematic analysis of Coelastrum was conducted for the first time by combining morphological observations with multi-locus phylogenetic analyses. Two species were newly identified in China: Coelastrum pulchrum Schmidle and Coelastrum rugosum (Rich) P. M. Tsarenko. Phylogenetic analysis indicated that: (1) Coelastrum microporum is most closely related to Coelastrum astroideum; (2) Coelastrum. pseudomicroporum is closely related to both Coelastrum indicum and C. pulchrum; (3) Coelastrum morus and Coelastrum sphaericum form a clade and are closely related. Notably, C. rugosum displayed significant intraspecific divergence, forming two distinct subclades with considerable molecular diversity, indicating a need for further taxonomic revision. Analysis of the ITS2 secondary structure showed that variable sites were mainly found in Helix I, with interspecific variations consisting of 1–6 compensatory base changes (CBCs) and 3–9 hemi-compensatory base changes (hCBCs). Furthermore, we successfully assembled the complete chloroplast genome of C. rugosum (164 981 bp), which has a relatively compact genome within the Scenedesmaceae. These results greatly enhance the known diversity of Coelastrum species in China and provide essential foundational data for future taxonomic revisions of this genus.