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 (Raphidophyceae) occurred in Hiroshima Bay.Japan. in 1994 and 1995.During these blooms we monitored rnicroorganisms which killed H. akashiwo by use of the microplate MPN (most probable number) method using an axenic culture of H. akashiwo as a susceptible host organism.At every sampling site abundance of algicidal microorganisms in seawater samples filtered through 0 8 pm nuclepore filters increased rapidly during the termination period of each bloom.However, the number of algicidal microorganisms In seawater samples filtered through 0 2 pm 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, it is possible that H. akashiwo-killing bacteria (HAKB) played a more dominant role in the terrninatlon of the blooms in 1994 and 1995 in Hiroshima Bay than viruses.The number of algicidal bacteria targeting Chattonella antiqua (Raphidophyceae), which was not detected during the investigation period, was 1 or 2 orders of magnitude lower than that of HAKB.We isolated some HAKB strains capable of causing mortality in H. akashiwo.These results suggest that the population dynamics of algicidal bacteria has a close relationship to the blooms of the phytoplankton, and that, in marine ecosystems, algicidal bacteria targeting specific phytoplankton may be one of the agents which regulate the change of species structure of phytoplankton communities.