Lethal effects of the harmful dinoflagellate Karenia selliformis (Gymnodiniales, Dinophyceae) on two juvenile kelp sporophytes Saccharina japonica and S. sculpera (Laminariales, Phaeophyceae)

A huge red tide caused mainly by Karenia selliformis in the eastern Hokkaido Pacific Ocean during autumn of 2021 was associated with mass mortalities of sea urchins, salmon, and other marine organisms. Bleaching of seaweeds, including kelps, was reported in the areas with red tide. Thus, K. selliformis is suspected to be harmful to kelp; however, the harmful effects are not yet known. This study aimed to verify the harmfulness of K. selliformis to cultured juvenile sporophytes of two commercially valuable kelp species, Saccharina japonica and Saccharina sculpera. Juvenile kelp sporophytes were exposed to seawater with four different densities of red tide-causing K. selliformis (0, 200, 1000, 2000 cells mL−1), and changes in thallus color over time were examined. Some thalli of both kelps were visibly bleached within 12 h of K. selliformis exposure; no thallus of the negative control was bleached. The ratio of bleached thalli differed significantly depending on the density of K. selliformis and kelp species; S. japonica was more sensitive than S. sculpera. During the follow-up period, most bleached thalli, especially those exposed to high-density K. selliformis seawater, finally withered. The median lethal density (LD50) of K. selliformis within 144 h after 48 h exposure was estimated as 218 cells mL−1 and 2285 cells mL−1 for juvenile thalli of S. japonica and S. sculpera, respectively. These results suggested the lethal effects of K. selliformis to kelp leaves, suggesting the potential risk posed to cultured and natural kelp communities by K. selliformis red tides.