Bacteria associated with Karenia mikimotoi in modulating its ichthyotoxicity

Harmful algae are often associated with bacteria but whether their toxicity is affected by bacteria is unclear. This study elucidated roles of bacteria associated with Karenia mikimotoi (KMHK) in modulating its ichthyotoxicity. Results revealed KMHK ichthyotoxicity increased in the order of xenic (with algal-associated bacteria) < axenic (free from associated bacteria) < “re-xenic” cultures (with bacterial consortium re-inoculated back to axenic KMHK), indicating the modulation of ichthyotoxicity depended on bacterial composition and types. Ichthyotoxicity of “re-xenic” KMHK culture with white or yellow pigmented isolate was higher than the red one, and white isolate “re-xenic” culture also exhibited higher haemolytic effect than the red isolate and xenic culture. However, “re-xenic” NIES2411, a very toxic strain isolated from Japan, with white bacterial isolate had lower ichthyotoxicity than the respective xenic culture, while the non-toxic strain (CAWD133) from New Zealand was unaffected by associated bacteria, implying bacterial modulation of ichthyotoxicity was algal strain-specific.