Molecular Ecological Studies on the Microorganisms which Kill or Destroy the Harmful and Toxic Microalgae During their Blooms

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We enumerated the microorganisms which killed the noxious red tide phytoplankton Heterosigma ak ashiwo (raphidophyceae) and Gymnodinium mikimotoi, (dinophyceae)Ha during their blooms which occurred in Hiroshima Bay and Uranouchi Bay or Tanabe Bay, Japan, in 1995 and 1996, using the MPN method.At every sampling site in Hiroshima Bay in both years, the rapid in crease of the algicidal microorganisms in the seawater fractions less than 0.8mm was observed at the termination period of the blooms. Because the seawater fraction less than 0.2mm fluctuated poorly reloted to the extinction of H.ak ashiwo blooms, the algicidal bacteria may play the leading part in the temination of the bloom. We could isolate many strains of H.ak ashiwo-killing bacteria (HAKB) during the blooms. The number of algicidal bacteria against Chattonella antiqua were one or two order of magnitude lower than that of HAKB.Therefore it is suggested that the population dynamics of algicidal bacteria may have a close relatio … More nship to the blooms of the phytoplankton. In marine ecosystem, algicidal bacteria against specific phytoplankton seem to regulate the change of structure of phytoplankton communities.Several strains of the marine bacteria which killed G.mikimotoiwere isolated during the occurrence of the huge red tide of G.mikimotoi in Tanabe Bay. Among them the strains belonged to the genus Vibrio, Flavobacterium, Acinetobacter and Pseudomonas-Alteromonas. The fact that all of them grew in filtered natural seawater without any nutrient supplements suggested that the killing bacteria are ubiquitous in seawter and are related to the occurrence of G.mikimotoi opportunistically. Most of the killing bacteria did not affect the growth of three marine diatoms, Skeletonema costatum, Ditylum brightwelli and Thalassiosira sp., and some of them did not affect the growth of the Alexandrium catenella.A bacterium, Flavobacterium sp.5N-3, obtained from sawater after the disappearance of a noxious G.mikimotoired tide in Uranouchi Bay showed a drastic algicidal effect on this species in particular, while it had no effects on other red tide plankton of Skeletonem costatum, Heterosigma akashiwo, and Chattonella anitiqua.These results suggest that bacteria with species-specific inhibitory effects play an importnat role on the succession of phytoplankton community and algalblooming could be prevented by using such microbes as a "microbial pesticide". Less