1Faculty of Bioresources, Mie University, Tsu, Mie, 514-8507, Japan
A bacterial strain MA10 which showed algicidal activity against Karenia (=Gymnodinium) mikimotoi was isolated from Ise Bay, Mie pref., Japan. The bacterial strain MA10 was identified as Flavobacterium sp. Strain MA10 had algicidal activity against K. mikimotoi, but did not inhibit the growth of Chattonella antiqua, Chattonella marina, Heterosigma akashiwo or Skeletonema costatum. When strain MA10 was inoculated into an axenic culture of K. mikimotoi at the initial bacterial number of 4.5×101 cfu/ml, the bacteria increased rapidly and reached 6.2×106 cfu/ml after incubation for 3 days. Simultaneously, the density of K. mikimotoi decreased and no K. mikimotoi cells were observed after 6 days. On the other hand, strain MA10 could not inhibit the growth of K. mikimotoi when the bacteria were inoculated into K. mikimotoi in the presence of the natural bacterial community at the initial bacterial number of 1.1×101 cfu/ml, but the algicidal effect was shown when the initial bacterial number was more than 1.1×102 cfu/ml. These results suggested that strain MA10 has algicidal activity against K. mikimotoi at an inoculum size of more than 1.1×102 cfu/ml in the field.