EFFECT OF VARYING SEA-WATER SALINITY ON GROWTH-KINETICS OF PHYTOPHTHORA-POLYMORPHICA

The growth kinetics of the estaurine fungus Phytophthora polymorphica were examined in static liquid culture and on agar at four sea-water s alinities, 3, 15, 35 and 60 g sea salt l-1. In liquid culture accumula tion of dry weight followed sigmoidal characteristics and an attempt w as nude to fit logistic equations to the growth curves. A highly signi ficant fit of calculated curves was obtained for the measured growth d ata at 15, 35 and 60 g l-1, but not for 3 g l-1 salinity. The maximum rate of dry weight production (0-37 mg h-1) and the highest specific g rowth rate (0.18 h-1) of mycelium were obtained at 15 g l-1 salinity w hile both these growth parameters were significantly depressed at 3 g l-1 (0.15 mg h-1; 0.11 h-1) and 60 g l-1 salinity (0.16 mg h-1; 0.11 h -1). Possible reasons for this growth response are discussed. A calciu m supplement was found to significantly reduce the growth inhibition c aused by the lowest salinity treatment of 3 g sea salt l-1. Maximum ra dial growth rates occurred at 35 g l-1 salinity (138 mum h-1) while ra dial growth rates were lowest at 3 and 15 g l-1 salinity (42 and 63 mu m h-1). P. polymorphica thus tolerates a wide range of sea-water salin ities but biomass production and hyphal extension may be favoured by d ifferent salinity regimes. It is speculated that this may be an adapta tion to the estuarine habitat of the fungus.