Macroalgal samples were collected from coastal waters in subboreal to subtropical zones in Japan (< 3–30 m depths) and 32 clonal strains of non-motile dinoflagellate-like protists were established. Molecular phylogenetic analyses of the LSU rDNA D1/D2, SSU rDNA, ITS region, and concatenated SSU rDNA + LSU rDNA D1/D2 sequences revealed that the strains nested within the genus Alexandrium. They were separated into three novel phylotypes: Alexandrium spp. type 1, type 2, and type 3. Analysis of the concatenated sequences revealed that the most closely related species for the three phylotypes was A. ostenfeldii. Most cells from strains of the three phylotypes were non-motile and hemispherical to spherical in shape. The average diameters of the non-motile cells were between 35 and 39 µm. Type 1 and type 2 were widely distributed in Japan from the temperate to subtropical zones, whereas type 3 was restricted to the temperate zone. Furthermore, type 2 was widespread from shallow to deep waters, whereas type 1 and type 3 were restricted to deep waters. Growth experiments in strains belonging to the three phylotypes revealed that the occurrence ratios of motile cells were very low (≤ 1.1% of the total cells). The production of paralytic shellfish poisoning toxins, tetrodotoxin, and cyclic imines was assessed in strains belonging to the three phylotypes by LC/MS/MS analysis. The strains did not produce any of the toxins tested. The strains of the three phylotypes showed lethal toxicity to mice by intraperitoneal administration. To the best of our knowledge, this is the first study to report the existence of Alexandrium associated with marine macroalgae from Japan.

从日本亚热带到亚热带地区（<3–30 m深度）的沿海水域收集大型藻类样本，并建立了32种非活动性鞭毛类鞭毛原生生物的品系。对LSU rDNA D1 / D2，SSU rDNA，ITS区以及串联的SSU rDNA + LSU rDNA D1 / D2序列进行分子系统发育分析，结果表明该菌株嵌套在Alexandrium属中。它们被分为三种新的系统型：Alexandrium spp。1型，2型和3型。对连接序列的分析表明，三种系统型最密切相关的物种是奥斯滕费尔德氏菌。。来自这三种系统型的菌株的大多数细胞是不运动的，并且是半球形到球形的。非活动细胞的平均直径在35至39 µm之间。1型和2型在日本从温带到亚热带地区广泛分布，而3型仅限于温带。此外，类型2从浅水到深水很普遍，而类型1和类型3仅限于深水。在属于三种系统型的菌株中的生长实验表明，运动细胞的发生率非常低（≤总细胞的1.1％）。通过LC / MS / MS分析评估了属于三种系统型的菌株中麻痹性贝类中毒毒素，河豚毒素和环状亚胺的产生。菌株不产生任何测试的毒素。三种系统型的菌株通过腹膜内给药对小鼠显示出致命的毒性。据我们所知，这是第一个报告存在以下情况的研究：与来自日本的海洋大型藻类相关的亚历山大藻。