POPULATION-STRUCTURE OF ALGICIDAL MARINE-BACTERIA TARGETING THE RED TIDE FORMING ALGA HETEROSIGMA-AKASHIWO (RAPHIDOPHYCEAE), DETERMINED BY RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISM ANALYSIS OF THE BACTERIAL 16SRIBOSOMAL-RNA GENES

A total of 233 marine bacterial strains which killed a noxious marine microalga, Heterosigma akashiwo, were isolated from Hiroshima Bay, the Seto Inland Sea, Japan, during blooms of H. akashiwo in 1994 and 1995 . Population structure and genetic diversity of the H, akashiwo-killin g bacteria (HAKB) were analyzed by means of restriction fragment lengt h polymorphism (RFLP) and partial sequences of 16S ribosomal RNA genes (16S rDNA) PCR (polymerase chain reaction) amplified from HAKB strain s. The RFLPs were generated by separate digestion with 5 restriction e nzymes, Eco RI, Rsa I, Mbo I, Bst UI and Hha I. Seventeen ribotypes we re observed among 85 strains of HAKB isolated in 1994. Bacterial strai ns of 3 ribotypes, 2B, 2C and 2D, were dominant in the HAKB population s during the termination period of the H, akashiwo bloom concurrent wi th the increase in the number of HAKB. Partial sequences, almost 500 b p of nucleotides, and RFLP patterns of 16S rDNA from some HAKB strains revealed that the HAKB of 2B, 2C and 2D ribotypes are closely related to the gamma-proteobacteria group. The HAKB strains belonging to 2C a nd 2D were repeatedly isolated from seawater collected at the end of a H. akashiwo bloom in 1995. These results suggest that 3 species of HA KB may play a role in the rapid termination of the H, akashiwo bloom i n Hiroshima Bay.