Cell-free filtrate from algicidal bacteria drives concentration-dependent damage and recovery in Karenia mikimotoi: Insights into harmful bloom persistence

Harmful algal blooms (HABs) threaten coastal ecosystems and aquaculture, yet how bloom-forming species recover from algicidal bacterial stress remains unclear. We examined responses of the harmful dinoflagellate Karenia mikimotoi to extracellular metabolites from Gaetbulibacter jejuensis DYW7 using physiological assays, cell-cycle analysis, metabolomics, and transcriptomics. The bacterial cell-free filtrate (BFF) induced clear concentration-dependent effects: high concentration (10%, v/v) caused severe damage and cell death, moderate concentration (5%, v/v) led to initial inhibition followed by partial recovery, and low concentration (0.1%, v/v) promoted population growth. Exposure to BFF triggered concentration-dependent redox perturbations, as evidenced by intracellular DCF fluorescence that increased under 5% and 10% BFF exposure but decreased under 0.1% exposure, whereas malondialdehyde (MDA) content and superoxide dismutase (SOD) and catalase (CAT) activities increased in all BFF-treated groups. Under recoverable exposure, transcriptomic analyses revealed coordinated upregulation of genes involved in antioxidant defense, energy metabolism, protein synthesis, lipid metabolism, and cell cycle regulation, consistent with stress adaptation and renewed cell division. Notably, BFF exposure induced the release of micrometer-scale extracellular particles (EPs). Fluorescence imaging and metabolomic profiling indicated that these EPs were associated with membrane-derived signals and redox- or lipid-related metabolites, suggesting a potential extracellular route for bioactive component release. These findings reveal a damage-recovery continuum in K. mikimotoi , suggesting that recovery capacity and extracellular particle (EP) release may contribute to HAB persistence under algicidal bacterial pressure.