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 isolated in 1990 from a station in northern Harima-Nada, the Seto Inland Sea, Japan, using the harmful red tide alga Chattonella antiqua (Hada) Ono as a susceptible organism. The bacterium can prey upon various species of microalgae. Temporal fluctuations of this bacterium and Chattonella spp. C. antiqua and C. marina (Subrahmanyan) Hara et Chihara] were investigated weekly at the above station in the summer of 1997 and 1998, using immunofluorescence assay employing highly specific polyclonal antibodies for the bacterium. In the summer of 1997, the cell density of Chattonella spp. showed a maximum value (70 cells ml⁻¹) on 8 July, and decreased thereafter. The bacterium Cytophaga sp. J18/M01 was commonly detected around a few hundreds of cells per milliliter or less. The number of Cytophaga sp. J18/M01 increased after the peak of Chattonella spp., and the maximum cell number of the bacterium was 1350 ml⁻¹. This algicidal bacterium also followed the changes of total amounts of microalgal biomass (chlorophyll a+pheophytin) when Chattonella spp. were absent. In the summer of 1998, Chattonella spp. were relatively less abundant (maximum 21 cells ml⁻¹), and the algicidal bacterium Cytophaga sp. J18/M01 showed a close relationship with the change of total microalgal biomass. The present study suggests that the algicidal bacterium Cytophaga sp. J18/M01 preyed upon, not only harmful red tide microalgae, but also other common microalgae such as diatoms, and the bacterium presumably plays an important role in regulating microalgal biomass in natural marine environments. Received: 20 April 2000 / Accepted: 1 December 2000