<i>Discostella lacuskarluki</i> (Manguin ex Kociolek &amp; Reviers) comb. nov.: a common nanoplanktonic diatom of Arctic and boreal lakes

<i>Cyclotella lacuskarluki</i> Manguin ex Kociolek &amp; Reviers is a small diatom known only from the original drawing that provides little information on the morphology of this taxon. As Manguin’s original materials are lost, recently collected samples from Lake Karluk, Alaska, the type locality of the species, were studied with light and scanning electron microscopy to determine its identity. As a result of this investigation, <i>C. lacuskarluki</i> is transferred here to the genus <i>Discostella</i>. <i>Discostella lacuskarluki</i> is 2.5–7 µm in diameter, has a shallow mantle, a stellar pattern of alveoli in the valve centre that may be reduced or completely absent, bifurcating costae and 4–5 marginal fultoportulae opening externally by short thickened tubes with round openings. We observed this species in numerous lakes across Alaska, in Lake Tahoe, Nevada, in several lakes across Canada, and in tundra and tree-line lakes of Northeastern Siberia. Although direct comparison with other <i>Discostella</i> species is difficult because the small size of these diatoms requires high-resolution imagery to reveal ultrastructural details, it appears that <i>D. lacuskarluki</i> is conspecific with <i>D. tatrica</i> Procházková et al. recently described from lakes of Central Europe and also reported from the European part of Russia. It is also conspecific with <i>D. nipponica</i> (Skvotzov) Tuji &amp; D. M. Williams known from Japan. The apparent wide distributional range of this species indicates that it has been overlooked and commonly reported as <i>D. pseudostelligera</i> or <i>D. stelligera</i> in arctic, boreal and alpine lakes.卡尔努克湖小环藻（Cyclotella lacuskarluki）Manguin ex Kociolek & Reviers是一种仅通过原始绘图被记录的小型硅藻（diatom），该绘图未能为该分类单元（taxon）的形态学研究提供充足信息。由于曼金的原始标本已遗失，研究人员对该物种的模式产地（type locality）——阿拉斯加卡尔努克湖的新近采集样本开展了光学显微镜与扫描电子显微镜观察，以明确其分类地位。经此番研究，卡尔努克湖小环藻被归入盘状硅藻属（Discostella），即卡尔努克湖盘状硅藻（Discostella lacuskarluki）。该物种壳面（valve）直径为2.5~7微米，具浅壳套（mantle）；壳面中心区域的泡囊（alveoli）呈星状排列，该结构可退化甚至完全消失；肋纹（costae）呈二叉状分枝；边缘具4~5个唇形突（fultoportulae），其外部开口为带有圆形开孔的短粗管。研究人员在阿拉斯加境内的众多湖泊、内华达州塔霍湖、加拿大境内的多处湖泊以及西伯利亚东北部的苔原与林线湖泊中均发现了该物种。尽管由于该类硅藻体型微小，需借助高分辨率成像技术才能展现其超微结构细节（ultrastructural details），因此难以与其他盘状硅藻属物种进行直接比对，但综合来看，卡尔努克湖盘状硅藻与近期从中欧湖泊中发表、且在俄罗斯欧洲部分亦有记录的塔特拉盘状硅藻（Discostella tatrica Procházková et al.）为同一物种；该物种同时与日本记录的日本盘状硅藻（Discostella nipponica (Skvotzov) Tuji & D. M. Williams）为同一物种。该物种看似分布范围广泛，这表明其此前长期被学界忽视，常被误鉴定为假星盘状硅藻（Discostella pseudostelligera）或星盘状硅藻（Discostella stelligera），并被记录于北极、北方及高山湖泊中。