GROWTH AND PRODUCTION OF AQUATIC HYPHOMYCETES IN DECOMPOSING LEAF-LITTER

The acetate-to-ergosterol technique was used to estimate fungal produc tivity of three species of aquatic hyphomycetes growing in decomposing ash leaves in stream microcosms. Following a lag of 20-88 min, incorp oration of acetate into ergosterol was linear for at least 10 h. Subst rate saturation was reached in the mM range, and there was no indicati on of isotope dilution. For one species, Articulospora tetracladia, a conversion factor of 5.5 mg mycelial dry mass produced per mu mol acet ate incorporated was determined. This was similar to the theoretical c onversion factor (6.6 mg mu mol(-1)) deduced from pathways of ergoster ol synthesis in fungi. Thus, the acetate-to-ergosterol assay appears t o be suitable for estimating the productivity of aquatic hyphomycetes growing in leaf litter in streams. Estimated growth rates of A. tetrac ladia in microcosms changed markedly over time, with the maximum being as high as 0.72 d(-1) at an early growth stage. After 23 d when 58% o f the initial leaf mass was degraded, the fungus had produced 89 mg bi omass per g of initial leaf mass. Almost half of this production was a llocated to conidia. Assuming an average growth efficiency of 0.35, th is would be equivalent to a fungal assimilation of 25% of initial leaf mass and account for 44% of the observed leaf mass loss. In an experi ment with leaf litter colonized by fungi in a stream, acetate incorpor ation was linear for 6 h, but the estimated growth rate was only 0.017 d(-1).