Biological control potential of red-tide marine dinoflagellate blooms by an Amoebophrya parasitic killer.

Despite the widespread prevalence of the genus Karenia, a notorious contributor to harmful algal blooms (HABs) worldwide, there are no reported instances of Amoebophrya spp. infecting Karenia. In 2020, we isolated infected Karenia mikimotoi cells, performed repeated inoculations of these infected cells to uninfected host cultures, and isolated a parasite strain of Amoebophrya from Osaka Bay, Japan. This strain exhibited infectivity toward K. mikimotoi and Karenia papilionacea but not Karenia selliformis or 14 other dinoflagellate species, highlighting its remarkable host specificity. Laboratory coculture experiments revealed pronounced host growth suppression upon introducing the parasite. The prevalence of infection surged rapidly, reaching a peak of 97% at 60 h post addition of the parasite, underscoring its potent infectivity. Our findings suggest that Amoebophrya sp. OSK2020 strain can inhibit K. mikimotoi, making them a promising natural biological control agent for HABs with a minimal impact on marine microbial ecosystems.