Rhizoclonium volgense sp. nov., a new species of Cladophoraceae (Ulvophyceae, Chlorophyta) from the Volga River

Sequence alignments (SSU, LSU and ITS rDNA) used for phylogenetic analyses. Abstract: Unusual large masses of green filamentous algae were recently discovered in the Volga River, European Russia. Macroscopically, the thick, curled , bright-green filaments were distinct from the commonly occurring Cladophora glomerata. Microscopic examination showed that the filaments were entirely unbranched, and composed of relatively large cells containing up to 170 nuclei, features characteristic of Chaetomorpha. However, molecular phylogenetic analyses based on nuclear rDNA sequences placed these algae within the genus Rhizoclonium, leading to the description as a new species, Rhizoclonium volgense sp. nov. Unlike many other Rhizoclonium species, R. volgense lacks lateral rhizoids, is completely unbranched, and becomes unattached in later growth stages. It was consistently found forming free-floating mats, although the presence of primary rhizoids in sporelings indicates an initially attached developmental phase. Molecular analyses indicates a close relationship with taxa from saline environments. Currently, the new species appears to be restricted to the regulated reservoirs of the upper Volga River, where fluctuating hydrological conditions may influence its distribution. Its absence from earlier studies suggests either historical oversight or a recent introduction.用于系统发育分析的序列比对（SSU、LSU及ITS核糖体DNA（rDNA））。 摘要：近日，在欧洲俄罗斯的伏尔加河中发现了大量不同寻常的绿色丝状藻类聚集体。肉眼观察下，这些厚实、卷曲的亮绿色丝状体与常见的簇生刚毛藻（Cladophora glomerata）截然不同。显微镜观察显示，该丝状体完全无分枝，由相对较大的细胞构成，每个细胞最多可含170个细胞核，这些均为乱丝藻属（Chaetomorpha）的典型特征。然而，基于细胞核核糖体DNA（rDNA）序列的分子系统发育分析显示，该藻类隶属于根枝藻属（Rhizoclonium），因此被描述为新物种——伏尔加根枝藻（Rhizoclonium volgense sp. nov.）。与多数其他根枝藻属物种不同，伏尔加根枝藻不具有侧生假根，完全无分枝，且在生长后期会脱离附着状态。尽管其幼孢子体存在初生假根，表明其发育初期为附着生长阶段，但该物种始终以自由漂浮的藻垫形式存在。分子分析结果显示，该物种与来自盐生环境的类群亲缘关系密切。目前，该新物种似乎仅分布于伏尔加河上游的调控水库中，水位波动的水文条件可能会影响其分布范围。早期研究中未发现该物种，这提示要么是此前的研究存在疏漏，要么该物种是近期才被引入的。