Karenia mikimotoi is one of the most deadly ichthyotoxic dinoflagellate species and has caused algal blooms in different places around the world. Algal blooms caused by K. mikimotoi have significant impacts on the fish farming and shellfish industries. Hong Kong have suffered greatly from K. mikimotoi blooms and the species has been regularly detected in Hong Kong waters. However, there is no effective measure for the control of algal blooms. The use of bacteria as an algicidal agent has received substantial attention and could become a promising approach toward algal bloom control.

In 2016, K. mikimotoi blooms occurred in Yim Tin Tsai, Hong Kong caused massive kills of more than 200 tons of fish in different fish farming zones. We collected the water samples during the late stage of the algal bloom and identified a bacterial strain P4 which exhibited strong algicidal activity against the Hong Kong K. mikimotoi strain (KMHK). P4 is a gram-negative and rod-shaped bacteria with specific growth rate of approximately 0.1/h. The strain was identified as Maribacter dokdonensis, which belongs to the family Flavobacteriaceae based on 16S rRNA sequence analysis.

K. mikimotoi cells were completely lysed by both the supernatant and bacterial culture of P4 within 48 hours. No cell death or lysis was observed in the control. Our results indicated that P4 exhibits an efficient indirect algicidal action against K. mikimotoi. Our preliminary data also showed that the algicidal effect of P4 on K. mikimotoi was dose - and growth phase - dependent. The algicidal efficiency of P4 culture added to the K. mikimotoi cells at 10% and 25% (v/v) after 48 hours incubation was around 65% and 100% respectively. P4 cells growing at stationary growth phase exhibited much stronger algicidal efficiency (~ 40-50% higher) than the cells at exponential growth phase. Algicidal effect of P4 on other K. mikimotoi strain was also promising. Results of present study indicated that P4 strain has great potential to be the algicidal agent to control K. mikimotoi blooms. Certainly, more research is required to unravel the mechanism underlying algicidal activity of P4.