Fluctuations of the red tide flagellates Chattonella spp. (Raphidophyceae)and the algicidal bacterium Cytophaga sp in the Seto Inland Sea, Japan

A marine algicidal gliding bacterium Cytophaga sp. strain J18/M01 was isola ted in 1990 from a station in northern Harima-Nada, the Seto Inland Sea, Ja pan, using the harmful red tide alga Chattanella anti-qua (Hada) Ono as a s usceptible organism. The bacterium can prey upon various species of microal gae. Temporal fluctuations of this bacterium and Chattonella spp. [C. antiq ua and C. marina (Subrahmanyan) Kara et Chihara] were investigated weekly a t the above station in the summer of 1997 and 1998, using immunofluorescenc e assay employing highly specific polyclonal antibodies for the bacterium. In the summer of 1997, the cell density of Chattonella spp. showed a maximu m value (70 cells ml(-1)) on 8 July, and decreased thereafter. The bacteriu m Cytophaga sp. J18/M01 was commonly detected around a few hundreds of cell s per milliliter or less. The number of Cytophaga sp. J18/M01 increased aft er the peak of Chattonella spp., and the maximum cell number of the bacteri um was 1350 ml(-1). This algicidal bacterium also followed the changes of t otal amounts of microalgal biomass (chlorophyll a + pheophytin) when Chatto nella spp. were absent. In the summer of 1998, Chattonella spp. were relati vely less abundant (maximum 21 cells ml(-1)), and the algicidal bacterium C ytophaga sp. J18/M01 showed a close relationship with the change of total m icroalgal biomass. The present study suggests that the algicidal bacterium Cytophaga sp. J18/M01 preyed upon, not only harmful red tide microalgae, bu t also other common microalgae such as diatoms, and the bacterium presumabl y plays an important role in regulating microalgal biomass in natural marin e environments.