Some zoosporic fungi can grow and survive within a wide pH range

Zoosporic fungi (often called chytrids) have been observed frequently on substrata in habitats with extremely low pH but never with extremely high pH. In the present study, growth, zoospore release and survival of some zoosporic fungi (Phyla Blastocladiomycota and Chytridiomycota) isolated from soil in Australia were examined in the laboratory to assess tolerance of extremes in pH. All 21 isolates grew rapidly in both solid and liquid PYG growth media (peptone, yeast extract, glucose) with pH values near neutrality. Most of the sixteen isolates tested could be maintained in culture on solid growth media at pH 4.7 and pH 8.9. One isolate grew down to pH 2.9, four isolates down to pH 3.3 and four isolates up to pH 11.2. In liquid PYG growth media all of the eight isolates tested grew (increased biomass) at pH 5.5 and pH 7.6, most isolates grew rapidly down to pH 4.5, some grew up to pH 11.2 but none of the isolates grew rapidly, if at all, at pH 2.9. The patterns of release of zoospores broadly reflect the patterns of growth at different pH values. Twenty one isolates survived for 7 d at 20 °C in liquid PYG growth media adjusted to pH 4.7, nine isolates survived down to pH 2.9, twelve up to pH 9.3, eight up to pH 11.2, and three even survived at both pH 2.9 and pH 11.2. The ecological significance of these data remains unclear. Patterns of survival, patterns of growth on solid and in liquid media and relative rates of zoospore release suggest ecotypes which prefer acidic, neutral or alkaline habitats, but these physiological properties were not highly correlated with either the pH of the soils from which these fungi were isolated nor with the taxonomic group in which they are placed. In general, many zoosporic fungi appear to be well adapted to a wider range of pH values than those found in the environments from which these fungi were isolated, and they quickly recover after brief exposure to extremes of pH.