Modulation of Karenia mikimotoi's ichthyotoxicity by two algal-associated bacterial strains of Alteromonas oceani under biotic and abiotic conditions

The ichthyotoxicity of harmful algal bloom species can be modulated by their closely associated bacteria, but how such modulation is affected by biotic and abiotic conditions has not been sufficiently evaluated. This study elucidates roles of two strains of Alteromonas oceani, R2 and Y8, isolated from a red tide alga Karenia mikimotoi (KMHK), in modulating its ichthyotoxicity at different microbial growth phases, salinities and direct/indirect contact mode. Results reveal two bacterial strains exhibited prominent but opposite effects on the ichthyotoxicity of KMHK, with significant increases in toxicity by Y8 but decreases by R2. The greatest difference between these strains in modulating algal ichthyotoxicity was observed when both algae and bacteria are at log growth phase. The effect of salinity on the bacterial modulation varied between strains, with Y8 enhanced algal toxicity especially at salinity of 30 while R2 decreased the toxicity. The modulatory effect of Y8 was not affected by contact mode, both direct and indirect contact increased KMHK toxicity to the same degree. The bacterial modulatory effects on ichthyotoxicity were related to the haemolytic activity of algae but independent of their reactive oxygen species. These findings reveal the modulation of algal-associated bacteria on ichthyotoxicity of KMHK was complex, varied between bacterial strains of the same species, microbial growth phase, salinities, and contact mode between bacterial and algal cells. © 2026 Elsevier Ltd