A CLOSE RELATIONSHIP BETWEEN ALGICIDAL BACTERIA AND TERMINATION OF HETEROSIGMA-AKASHIWO (RAPHIDOPHYCEAE) BLOOMS IN HIROSHIMA BAY, JAPAN

Blooms of the noxious red tide phytoplankton Heterosigma akashiwo (Rap hidophyceae) occurred in Hiroshima Bay, Japan, in 1994 and 1995. Durin g these blooms we monitored microorganisms which killed H, akashiwo by use of the microplate MPN (most probable number) method using an axen ic culture of H, akashiwo as a susceptible host organism. At every sam pling site abundance of algicidal microorganisms in seawater samples f iltered through 0.8 mu m nuclepore filters increased rapidly during th e termination period of each bloom. However, the number of algicidal m icroorganisms in seawater samples filtered through 0.2 mu m nuclepore filters was less abundant and correlated poorly with the extinction of H. akashiwo blooms. The latter samples were assumed to indicate viral activity. Thus, ii is possible that H. akashiwo-killing bacteria (HAK B) played a more dominant role in the termination of the blooms in 199 4 and 1995 in Hiroshima Bay than viruses. The number of algicidal bact eria targeting Chattonella antiqua (Raphidophyceae), which was not det ected during the investigation period, was 1 or 2 orders of magnitude lower than that of HAKB. We isolated some HAKB strains capable of caus ing mortality in H, akashiwo. These results suggest that the populatio n dynamics of algicidal bacteria has a close relationship to the bloom s of the phytoplankton, and that, in marine ecosystems, algicidal bact eria targeting specific phytoplankton may be one of the agents which r egulate the change of species structure of phytoplankton communities.