The Ciliate Euplotes balteatus Exhibits Removal Capacity upon the Dinoflagellates Karenia mikimotoi and Prorocentrum shikokuense.

The significant threat posed by the ichthyotoxic dinoflagellate Karenia mikimotoi to coastal aquaculture, resulting in substantial economic losses, underscores the need for control and mitigation strategies. Bio-mitigation of algal blooms through grazers presents advantages in sustainability compared to methods relying on chemical or physical procedures. This study explored the inhibitory effect of nine Euplotes spp. (Alveolata, Ciliophora) isolates on simulated blooms, with E. balteatus W413 displaying removal capacity for K. mikimotoi and robust growth in co-cultivation. The unique size plasticity in W413 revealed an efficient predation strategy, as an increase in cellular size enables it to shift prey from bacteria to algal cells. The enlarged cell volume facilitates W413 to accommodate more algal cells, bestowing it with a high ingestion rate and removal capacity upon K. mikimotoi. Furthermore, W413 exhibited considerable inhibition towards co-occurring bloom species, specifically Prorocentrum shikokuense and Karenia spp., implying its potential to mitigate mixed-species blooms. The study enhances our understanding of the prey selectivity of Euplotes species and proposes E. balteatus as a potential bio-mitigation candidate for K. mikimotoi blooms, emphasizing the significance of micro-grazers in marine ecosystems.