Species of the marine dinoflagellate genus Karenia are known to produce various potent biotoxins and can form noxious blooms that cause mass mortalities of fish and shellfish. To date, harmful blooms of the species K. mikimotoi have been reported in Korea, but K. papilionacea was recently recorded off the southern coast of Korea. Here, we developed a quantitative real-time PCR (qRT-PCR) assay with specific primer pairs for the accurate detection and quantification of these two similar-looking unarmored species, K. mikimotoi and K. papilionacea, and investigated their distribution and dynamics in Korean coastal waters. Overall, K. papilionacea had not only a wider distribution, but also higher cell abundances (15–2553 cells L⁻¹) than K. mikimotoi (3–122 cells L⁻¹) in surface waters. Of 18 sampling sites, the two Karenia species were found to coexist at two sites. During monitoring at a fixed station (S5), K. papilionacea was generally predominant over K. mikimotoi; however, the two species exhibited similar dynamics and occasionally co-occurred. Both Karenia species showed similar physiological responses to temperature and salinity, requiring similar conditions for optimum growth. These results suggest that blooms of the two species may co-occur and induce a synergistic adverse effect on marine environments.

The dinophyte family Amphidomataceae, comprising the genera Azadinium and Amphidoma, includes several species known to produce azaspiracids (AZAs), a group of lipophilic marine biotoxins associated with shellfish poisoning. Despite their ecological relevance and the increasing number of newly described species in recent years, their diversity is likely still underestimated, and knowledge of their global distribution and biogeography remains limited. Based on detailed morphological and phylogenetic analyses of strains from Korea and Japan, we describe Azadinium fusiforme sp. nov., a taxon that corresponds to the phylogenetic position of a strain previously reported from Japan. Light and scanning electron microscopy revealed that Az. fusiforme possesses a unique combination of morphological traits that distinguish it from its congeners. Notably, the ventral pore is located on the left side of the first apical plate, the pyrenoid is consistently positioned in the posteriormost part of the cell, the nucleus is elongated and centrally located, and pronounced thickenings occur along the sutures between the lateral and dorsal apical plates. Molecular data place Az. fusiforme in a well-supported, distinct clade within Azadinium, clearly supporting its designation as a new species. Mass spectrometric analyses showed no evidence of AZA production in any of the Az. fusiforme strains. This study contributes to a more fundamental understanding of the species diversity, distribution, and potential toxicity of Amphidomataceae in the Asia-Pacific region.

The cercozoan parasitoid Pseudopirsonia infects marine diatoms. Currently, only one species, P. mucosa, in the genus has been formally described in the North Sea. In June 2022 and 2023, we found that the diatom Chaetoceros constrictus was infected by a novel cercozoan parasitoid nanoflagellate during regular monitoring at Yongho Harbor, Korea. We es tablished two strains of the parasitoid and their hosts in culture and performed morphological, biological, and molecular analyses. Molecular phylogeny inferred from small subunit ribosomal RNA (SSU rRNA) gene sequences demonstrated that the novel parasitoid clustered tightly with P. mucosa with a very low genetic distance. However, substantial morpho logical differences were identified between the two parasitoids. In the zoospore stage, two flagella of the novel parasitoid were subapically inserted and never completely retracted after attachment to the host cell. In contrast, P. mucosa has median- or submedian-inserted flagella, which disappear soon after attachment to the host cell. The auxosome of P. mucosa was surrounded by a mucilaginous envelope, and its division resulted in a morula-shaped accumulation of four flagellate mother cells (FMCs), producing eight zoospores. In contrast, the novel parasitoid auxosome does not cover a mucilaginous coat and produces FMCs via binary fission, which immediately detached from the auxosome and divided to produce two motile zoospores. The novel parasitoid presented a high degree of host specificity, infecting only Chaetoc eraos species, whereas P. mucosa exhibited a broad host range, infecting species of four diatom genera without the genus Chaetoceros. We, therefore, suggest that this novel parasitoid is a new species of the genus Pseudopirsonia.