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 16S ribosomal 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-killing bacteria (HAKB) were analyzed by means of restriction fragment length polymorphism (RFLP) and partial sequences of 16S ribosomal RNA genes (16S rDNA) PCR (polymerase chain reaction) amplified from HAKB strains. The RFLPs were generated by separate digestion with 5 restriction enzymes, Eco RI, Rsa I, Mbo I, Bst UI and Hha I. Seventeen ribotypes were observed among 85 strains of HAKB isolated in 1994. Bacterial strains of 3 ribotypes, 2B, 2C and 2D, were dominant in the HAKB populations during the termination period of the H, akashiwo bloom concurrent with the increase in the number of HAKB. Partial sequences, almost 500 bp 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 and 2D were repeatedly isolated from seawater collected at the end of a H. akashiwo bloom in 1995. These results suggest that 3 species of HAKB may play a role in the rapid termination of the H, akashiwo bloom in Hiroshima Bay.