Abstract
Filtrable pathogens infecting the phytoplankton Alexandrium catenella and Tetraselmis sp. were screened from coastal seawater on the mouth of Funka Bay, Hokkaido, Japan from 1993 to 1994. Growth suppression against these phytoplankton species was observed in the seawater samples collected during September and October 1993. The growth of A. catenella was suppressed from 40 to 45%, and that of Tetraselmis sp. was suppressed from 20 to 30%. Re-inoculation of the culture filtrate of growth-suppressed phytoplankton after passing the culture through a 0.22-μm filter also caused growth suppression of the fresh culture. However, these effects disappeared after several treatments including heating at 50°C for 30min, exposure to acidic conditions below pH 5.0, passing through a 0.05-μm filter, and Proteinase K and RNase treatment. Cell free extracts of the growth-suppressed phytoplankton caused the same extent of growth suppression. Electron microscopic observation of A. catenella cells that were lead to the growth suppression revealed that the cells were severely damaged, whereas no virus-like particles or bacterial cells were observed. Growth suppression was observed in a fresh culture ofA. catenella and an axenic culture of Gymnodinium mikimotoi by the growth suppressed Tetraselmis sp. culture filtrate, and theA. catenella culture filtrate affected the growth of Tetraselmis sp. and an axenic culture of G. mikimotoi. However, the growth suppression or inhibition was not observed in fresh cultures of Prorocentrum micans, P. minimum, A. tamarense, G. mikimotoi, Chattonella antiqua, C. marina, and Heterosigma akashiwo. These results suggested that unique filterable pathogens might be found in the seawater samples.
