The effects ofpH and water chemistry on the growth and activity of aquatic hyphomycetes colonizing leaves

Aquatic hyphomycetes degrade leaf Utter in freshwater streams. During growth on leaves, they secrete an array of extracellular enzymes that degrade plant structural polymers such as pectic polysaccharides. These enzymes are differentially affected by pH and, consequently, by the water chemistry of the stream water in which they are produced. Sufficient evidence suggests that among the enzymes secreted by aquatic hyphomycetes, pectin lyase activity is most closely associated with leaf breakdown.; The purpose of the present study was to examine the effects of pH on the growth and activity of aquatic hyphomycetes species in both field and laboratory studies. In field studies, yellow poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.) leaves were placed in two softwater streams (pH 6 and above) and two hardwater streams (pH 8 and above). Faster breakdown rates and softening rates as well as higher pectin lyase activities were associated with the fungal communities colonizing both leaf types in Hendrick Mill, the hardwater stream with the highest nutrient concentrations. Breakdown rates for yellow poplar decomposing in Hendrick Mill (k = 0.026) were two times greater than rates in Basin Creek (k = 0.013), the softwater stream with the lowest nitrate-N concentrations. Additionally, Tetracladum marchalianum, the dominant species colonizing yellow poplar leaves in Hendrick Mill, degraded sterile yellow poplar leaves at a fast rate in laboratory studies. It is suggested that differences in nutrient concentrations as well as species composition of the colonizing fungi are responsible for differences in the pectin lyase activities among the streams.; In laboratory studies, aquatic hyphomycetes species exhibited greater growth at acidic pH in broth cultures despite their stream preference. In contrast, these fungi exhibited greater degradative capabilities when grown on leaf material that was close to the pH of streams that they typically inhabit. The pH of artificial media can modify medium constituents and lead to results that may be only indirectly related to pH. Since leaf disks are the natural substrata, it is suggested that the effects of pH on aquatic hyphomycetes in leaf cultures are similar to those occurring in streams...