KILLING OF MARINE-PHYTOPLANKTON BY A GLIDING BACTERIUM CYTOPHAGA SP, ISOLATED FROM THE COASTAL SEA OF JAPAN

A marine gliding bacterium Cytophaga sp. (strain J18/M01) was isolated from Harima-Nada, eastern Seto Inland Sea, Japan in 1990. This bacter ium preys upon various species of marine phytoplankton. All of the fiv e raphidophycean flagellates, all of the four diatoms, and one of the two dinoflagellates examined were killed within a few days when cultur ed with the bacterium. The bacterium presumably achieves this by direc t attack, because the culture filtrate in which host organisms were to tally destroyed had no significant effects on the growth of the same h ost organism (Chattonella antiqua). If one or a few bacterial cells we re inoculated into C. antiqua culture, all of the host organisms were killed. The bacterium proliferated in filter-sterilized seawater, sugg esting its ubiquitous existence in the coastal sea. The killing of phy toplankton by bacteria such as Cytophaga sp. J18/M01 may be a signific ant factor influencing the population dynamics of phytoplankton in nat ure and may contribute to the sudden disappearance of red tides in the coastal sea. Bacterial destruction of phytoplankton may also be a fac tor that regulates primary productivity in marine ecosystems.