Cladophora rigida sp. nov., a new freshwater species within Cladophoraceae (Ulvophyceae, Chlorophyta) from China

Cladophora is one of the most species-rich and taxonomically challenging taxa of green algae. Recently, some samples of freshwater filamentous algae were collected from Hubei, Yunnan and Guizhou Provinces, China. The specimens grew attached to solid substrates by basal rhizoids or holdfasts, and their thalli were bright to dark green, long and stiff. They were typically composed of uniseriate unbranched filaments of approximately equal length produced by intercalary and apical cell divisions. We studied the taxonomy of these algae based on morphological and molecular data. Microscopic observations showed that the filaments had some characteristics that differed from those of other species of Cladophora, such as being unbranched, with a large cell diameter (180–429 µm) and smaller length:diameter (L:D) ratio (0.5–3.3) than other species. Phylogenetic trees constructed on the basis of small (SSU) and large (LSU) subunits of ribosomal DNA (rDNA) and the internal transcribed spacer (ITS) sequences showed that all collected specimens belonged to one species. These specimens formed a new clade in the freshwater Cladophora lineage. On the basis of the morphological and phylogenetic analyses, we propose a new species, Cladophora rigida sp. nov.