Abstract
Experiments on the physiological requirements of aquatic hyphomycetes isolated from hard and soft water showed that all grew best at pH values between 4 and 5. When grown on solid media amended with increasing amounts of Ca 2+ and/or HCO 3 − at a constant pH, growth rates were always highest if Ca 2+ or both ions were present, whereas HCO 3 − alone inhibited growth in some species. Some species reached a growth plateau at low Ca 2+ concentrations, while some had a distinct optimum at intermediate concentrations. In liquid culture amended with constant amounts of Ca 2+ and/or HCO 3 − over the pH range of 4–9, best results were again obtained when both ions or at least Ca 2+ were present. Some species responded strongly, in others the response was only minor. Neither of the components studied seems to be the decisive factor for habitat preference. It may be that higher calcium levels benefit the growth of some species at the unfavourable high pH of hardwater streams.
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