Information on the diversity and distribution of harmful microalgae in the Gulf of Thailand is very limited and mainly based on microscopic observations. Here, we collected 44 water samples from the Gulf of Thailand and its adjacent water (Perhentian Island, Malaysia) for comparison in 2018. DNA metabarcoding was performed targeting the partial large subunit ribosomal RNA gene (LSU rDNA D1–D3) and the internal transcribed spacers (ITS1 and ITS2). A total of 50 dinoflagellate genera (made up of 72 species) were identified based on the LSU rDNA dataset, while the results of ITS1 and ITS2 datasets revealed 33 and 32 dinoflagellate genera comprising 69 and 64 species, respectively. Five potentially toxic Pseudo-nitzschia (Bacillariophyceae) species were detected, with four as newly recorded species in the water (Pseudo-nitzschia americana/brasilliana, Pseudo-nitzschia simulans/delicatissima, P. galaxiae and P. multistriata). The highest relative abundances of P. galaxiae and P. multistriata were found in Trat Bay and Chumphon (accounting for 0.20% and 0.06% of total ASVs abundance, respectively). Three paralytic shellfish toxin producing dinoflagellate species were detected: Alexandrium tamiyavanichii, Alexandrium fragae, and Gymnodinium catenatum. The highest abundance of A. tamiyavanichii was found in the surface sample of Chumphon (CHO7 station), accounting for 1.95% of total ASVs abundance. Two azaspiracid producing dinoflagellate species, Azadinium poporum ribotype B, Azadinium spinosum ribotype A, and a pinnatoxin producing dinoflagellate species Vulcanodinium rugosum, with two ribotypes B and C, were revealed from the datasets although with very low abundances. Six fish killing dinoflagellate species, including Margalefidinium polykrikoides group IV, Margalefidinium fulvescens, Karenia mikimotoi, Karenia selliformis ribotype B, Karlodinium australe, and Karlodinium digitatum were detected and all representing new records in this area. The findings of numerous harmful microalgal species in the Gulf of Thailand highlight the potential risk of human intoxication and fish killing events.

关于泰国湾有害微藻多样性和分布的信息非常有限，主要基于显微镜观察。在这里，我们在 2018 年从泰国湾及其邻近水域（马来西亚停泊岛）收集了 44 个水样进行比较。 DNA 宏条形码针对部分大亚基核糖体 RNA 基因（LSU rDNA D1-D3）和内部转录垫片（ITS1 和 ITS2）。基于 LSU rDNA 数据集共鉴定了 50 个甲藻属（由 72 个物种组成），而 ITS1 和 ITS2 数据集的结果分别显示了 33 个和 32 个甲藻属，包括 69 个和 64 个物种。检测到五种具有潜在毒性的假尼茨奇亚(Bacillariophyceae) 物种，其中四种是水中新记录的物种。Pseudo- nitzschia americana / brasilliana, Pseudo-nitzschia simulans / delicatissima, P. galaxiae和P. multistriata )。在达叻湾和春蓬发现了P. galaxiae和P. multistriata的最高相对丰度（分别占 ASV 总丰度的 0.20% 和 0.06%）。检测到三种产生麻痹性贝类毒素的甲藻：Alexandrium tamiyavanichii、Alexandrium fragae和Gymnodinium catenatum。A. tamiyavanichii丰度最高在春蓬（CHO7 站）地表样本中发现，占 ASV 总丰度的 1.95%。两个azaspiracid鞭毛藻物种，Azadinium poporum核糖型B，Azadinium棘核糖型A，和一个pinnatoxin鞭毛藻物种Vulcanodinium rugosum，用两个核糖型B和C，从该数据集尽管以非常低的丰度表明。六种杀死甲藻的鱼类，包括Margalefidinium polykrikoides group IV、Margalefidinium fulvescens、Karenia mikimotoi、Karenia selliformis ribotype B、Karlodinium australe和Karlodinium digitatum被检测到，并且都代表了该区域的新记录。泰国湾许多有害微藻物种的发现突出了人类中毒和鱼类死亡事件的潜在风险。