A novel parasitoid, Pseudopirsonia chaetoceri n. sp. (Imbricatea, Cercozoa), of marine diatoms discovered in Korean coastal waters

The cercozoan parasitoid Pseudopirsonia infects marine diatoms. Currently, only one species, <i>P. mucosa</i>, in the genus has been formally described in the North Sea. In June 2022 and 2023, we found that the diatom <i>Chaetoceros constrictus</i> 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 <i>P. mucosa</i> 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, <i>P. mucosa</i> has median- or submedian-inserted flagella, which disappear soon after attachment to the host cell. The auxosome of <i>P. mucosa</i> 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 <i>Chaetoc eraos</i> species, whereas <i>P. mucosa</i> exhibited a broad host range, infecting species of four diatom genera without the genus <i>Chaetoceros</i>. We, therefore, suggest that this novel parasitoid is a new species of the genus <i>Pseudopirsonia</i>.