<i>Desmodesmus pseudoperforatus</i> sp. nov., a giant in <i>Desmodesmus</i>, series <i>Maximi</i>

Series <i>Maximi</i> is a group within <i>Desmodesmus</i> (Chlorophyta) which contains the three species <i>D. maximus, D. perforatus</i> and <i>D. tropicus</i>. Besides their large cell sizes, the most striking morphological trait unique to the members of this genus is the presence of large perforations between the coenobial cells. The perforations are either present (<i>D. perforatus</i> and <i>D. tropicus</i>) or the space between the cells is completely sealed (<i>D. maximus</i>). We report here a fourth species of the series based on two strains (AICB 1155 and AICB 1572) from freshwater environments of the Danube Delta, Romania. The ultrastructure of the cell walls revealed a partial closure of the space between the cells which creates slightly visible perforations in light microscopy. Analyses of DNA sequences and secondary structure of the ITS2 fragment by coalescent (PTP, GMYC), distance (ASAP), and evolutionary modelbased methods (ML, BI) indicated a fourth candidate species which included both the AICB strains investigated and other strains with similar morphological traits from reference literature. Despite its poor representation in public databases, analysis of the SSU rDNA gene for the series <i>Maximi</i> strengthened the ITS2 results. Therefore, based on its larger cell sizes relative to the other members of the series, cell morphology, the presence of partial perforations and other peculiarities of the cell wall ultrastructure, the independent phylogenetic position, CBCs analysis, and a different ‘DNA signature’ within the ITS2 fragment we suggest that this clade should be described as a new taxon of the series <i>Maximi, Desmodesmus pseudoperforatus</i> sp. nov. Morphological differences and DNA-based lines of evidence indicate new species.Microperforations of the space between the coenobial cells.Strains have the largest cell sizes in the series <i>Maximi.</i> Morphological differences and DNA-based lines of evidence indicate new species. Microperforations of the space between the coenobial cells. Strains have the largest cell sizes in the series <i>Maximi.</i>*Maximi*系隶属于绿藻门（Chlorophyta）被刺藻属（*Desmodesmus*），包含<i>D. maximus</i>、<i>D. perforatus</i>与<i>D. tropicus</i>3个物种。除细胞体积较大外，该类群最显著的独有形态特征为定形群体细胞间存在大型穿孔：<i>D. perforatus</i>与<i>D. tropicus</i>的细胞间隙带有穿孔，而<i>D. maximus</i>的细胞间隙则完全闭合。本研究报道了该系的第4个物种，研究材料为采自罗马尼亚多瑙河三角洲淡水生境的2株菌株：AICB 1155与AICB 1572。细胞壁超微结构观察显示，其细胞间隙呈部分闭合状态，在光学显微镜下可见微弱的穿孔结构。通过联合溯祖分析法（PTP、GMYC）、距离法（ASAP）以及基于进化模型的最大似然法（ML）、贝叶斯推断法（BI）对ITS2（内转录间隔区2）片段的DNA序列及二级结构进行分析，结果表明该类群为第4个候选物种，涵盖本次研究的AICB菌株以及参考文献中具有相似形态特征的其他菌株。尽管该类群在公共数据库中的收录信息较少，但针对*Maximi*系的SSU rDNA基因分析进一步验证了ITS2的分析结果。综上，结合其相较于该系其他类群更大的细胞体积、细胞形态特征、部分闭合的细胞间隙穿孔、独特的细胞壁超微结构、独立的系统发育位置、补偿性碱基变化（CBC）分析结果，以及ITS2片段内独特的“DNA特征序列”，我们建议将该演化支描述为*Maximi*系的新分类单元：<i>Desmodesmus pseudoperforatus</i> sp. nov.。该新种兼具形态差异与基于DNA的多项证据支持，其定形群体细胞间隙存在微穿孔，且为*Maximi*系中细胞体积最大的类群。