Growth suppression of the marine phytoplankton Alexandrium catenella and Tetraselmis sp caused by ultrafiltrated seawater collected from coastal areaon the mouth of Funka Bay

Filtrable pathogens infecting the phytoplankton Alexandrium catenella and T etraselmis sp. were screened from coastal seawater on the mouth of Funka fl ay, Hokkaido, Japan from 1993 to 1994. Growth suppression against these phy toplankton species was observed in the seawater samples collected during Se ptember 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-inoc ulation of the culture filtrate of growth-suppressed phytoplankton after pa ssing the culture through a 0.22-mu m filter also caused growth suppression of the fresh culture. However, these effects disappeared after several tre atments including heating at 50 degrees C for 30 min, exposure to acidic co nditions below pH 5.0, passing through a 0.05-mu m filter, and Proteinase K and RNase treatment. Cell free extracts of the growth-suppressed phytoplan kton caused the same extent of growth suppression. Electron microscopic obs ervation of A. catenella cells that were lead to the growth suppression rev ealed that the cells were severely damaged, whereas no virus-like particles or bacterial cells were observed. Growth suppression was observed in a fre sh culture of A. catenella and an axenic culture of Gymnodinium mikimotoi b y the growth suppressed Tetraselmis sp. culture filtrate, and the A. catene lla culture filtrate affected the growth of Tetraselmis sp. and an axenic c ulture of G. mikimotoi. However, the growth suppression or inhibition was n ot observed in fresh cultures of Prorocentrum micans, P. minimum, A. tamare nse, G. mikimotoi, Chattonella antiqua, C. marina, and Heterosigma akashiwo . These results suggested that unique filterable pathogens might be found i n the seawater samples.