Morphology and Molecular Phylogeny of the Genus Stigeoclonium (Chaetophorales, Chlorophyta) from China, Including Descriptions of the Pseudostigeoclonium gen. nov

Stigeoclonium is a genus of green algae that is widely distributed in freshwater habitats around the world. The genus comprises species with variously developed prostrates and erect systems of uniseriate branched filaments and grows attached to a wide range of different surfaces. It holds significant promise for applications in water quality indicators, sewage treatment, and the development of high-value-added products. Nevertheless, our comprehension of Stigeoclonium remains unclear and perplexing, particularly regarding its fundamental systematic taxonomy. Recent molecular analyses have revealed that the morphologically well-defined genus Stigeoclonium is polyphyletic and requires taxonomic revision. Phylogenetic analysis based on a single molecular marker and limited samples is insufficient to address the polyphyletic nature of Stigeoclonium. In the present study, 34 out of 45 strains of Stigeoclonium were newly acquired from China. Alongside the morphological data, a concatenated dataset of three markers (18S rDNA + ITS2 + tufA) was utilized to determine their molecular phylogeny. The phylogenetic analysis successfully resolved the broadly defined Stigeoclonium into three robustly supported clades (Stigeoclonim tenue clade, S. farctum clade, and S. helveticum clade). The morphological characteristics assessment results showed that the cell type of the main axis-producing branch, considered a crucial morphological characteristic of the Stigeoclonium taxonomy, did not accurately reflect the real phylogeny of the genus. A new taxonomical classification of the genus Stigeoclonium was proposed based on zoospores' germination types, which aligned well with the phylogenetic topologies. Species where zoospores showed erect germination (S. helveticum clade) formed a distinct monophyletic clade, clearly separated from the other two clades, with zoospores showing prostrate germination or pseudo-erect germination. Consequently, a new genus, Pseudostigeoclonium gen. nov., is suggested to include all species in the broadly defined Stigeoclonium with zoospores with erect germination. The taxonomic diversity is supported by distinctive morphological differences and phylogenetic divergence within the broadly defined Stigeoclonium identified in this study. Further evaluation of the genus Stigeoclonium is necessary, especially via examining additional specimens and re-evaluating morphological characters under precisely defined laboratory conditions.