SUMMARY: All seven species of aquatic hyphomycetes tested produced both polygalacturonases and pectin lyases. The polygalacturonases were constitutive, whereas the pectin lyases were induced on pectic substrates at pH 6.5 and above. Some species could grow on pectic substrates at both pH 5 and pH 7; other species at pH 7 only. grew as well on a polypectate substrate as it did on glucose. This fungus elaborated three endo-polygalacturonase isoenzymes, as did secreted a presumed exo-pectin lyase and a pectin esterase, and an endo-pectin lyase and a pectin esterase. The four species macerated alder-leaf strips totally within 9 to 12 d at stream pH 7, utilizing predominantly pectin lyases and pectin esterases. These enzymes are stimulated by Ca, and this may explain why plant decay proceeds most rapidly in calcium-rich streams of pH 6.5 and above. The frequency with which the four species of aquatic hyphomycetes were observed on experimental leaf packs placed in a stream does not appear to be related to their pectolytic capability.


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