The genus Pseudo-nitzschia is one of the major contributors to harmful algal blooms (HABs) and is known to produce domoic acid (DA), a neurotoxin that can affect marine ecosystems. However, the diversity of potentially toxigenic Pseudo-nitzschia species and their toxin production remain poorly understood in Korean coastal waters, particularly around Jeju Island. In this study, Pseudo-nitzschia species occurring in the coastal waters of Jeju Island were identified through morphological and molecular analyses, species diversity was investigated using NGS analysis, and toxin production was analyzed. Samples were collected from 12 coastal stations around Jeju Island from March 2022 to April 2024, and monoclonal cultures of Pseudo-nitzschia were established. Morphological identification was conducted using light microscopy (LM) and scanning electron microscopy (SEM). Molecular phylogenetic analysis was performed based on ITS rDNA sequences using the maximum likelihood (ML) method. In addition, toxin production was analyzed, and next-generation sequencing (NGS) was used to examine species diversity and spatial and temporal distribution patterns. A total of seven species were identified during the study period, including P. brasiliana, P. calliantha, P. cuspidata, P. delicatissima, P. multiseries, P. multistriata and Pseudo-nitzschia jejuensis sp. nov. Morphological observations indicated that the six identified species, excluding Pseudo-nitzschia jejuensis sp. nov., were consistent with previous reported descriptions. Likewise, phylogenetic analysis showed that these species clustered with previously reported sequences, whereas Pseudo-nitzschia jejuensis sp. nov. formed a distinct clade, suggesting that it is a new species. NGS analysis revealed a total of 15 Pseudo-nitzschia species. Among them, P. americana (7.08%), P. delicatissima (23.26%), P. galaxiae (36.71%) and P. multistriata (6.68%) were considered dominant species, each accounting for more than 5% of occurrence proportion. P. delicatissima was consistently abundant from March to August. Seasonal variation in species composition was observed, with the highest number of species recorded in August (10 species). Toxin analysis revealed that three species produced domoic acid during different growth phases, including P. lundholmiae (1.89 fg cell⁻¹), P. multiseries (6.85 fg cell⁻¹), and Pseudo-nitzschia jejuensis sp. nov. (3.84 fg cell⁻¹). This study provides taxonomic information on the newly discovered Pseudo-nitzschia species in Jeju Island and contributes to the development of future HAB warning and management strategies.

Harmful Algal Blooms (HABs),Pseudo-nitzschia,Domoic acid,New species