Algicidal activity of a killer bacterium against the harmful red tide dinoflagellate Heterocapsa circularisquama isolated from Ago Bay, Japan

Algicidal activity of a bacterium strain Cytophaga sp. AA8-2 against the ha rmful red tide causing alga Heterocapsa circularisquama was investigated. P hysiological conditions of host cells, incubation temperature and existence of ambient organic substrates or co-existing bacteria affected the lethal effect of Cytophaga sp. AA8-2. Bacterial lysis of H. circularisquama was ca used more rapidly at higher incubation temperature (20-30 degrees C). Growt h of 6 among 7 H. circularisquama strains tested was inhibited by Cytophaga sp. AA8-2, the levels of which were varied. Apart of H. circularisquama ce lls in a culture formed temporary cysts to survive the bacterial attack. Th e envelope of the temporary cyst of H. circularisquama was composed of a ma rkedly thicker layered structure (209+/-72 nm) than that of the vegetative cell (40+/-15 nm).