Growth and production of aquatic hyphomycetes in decomposing leaf litter

The acetate-to-ergosterol technique was used to estimate fungal productivity of three species of aquatic hyphomycetes growing in decomposing ash leaves in stream microcosms. Following a lag of 20-88 min, incorporation of acetate into ergosterol was linear for at least 10 h. Substrate saturation was reached in the mM range, and there was no indication of isotope dilution. For one species, Articulospora tetracladia, a conversion factor of 5.5 mg mycelial dry mass produced per mu mol acetate incorporated was determined. This was similar to the theoretical conversion factor (6.6 mg mu mol(-1)) deduced from pathways of ergosterol synthesis in fungi. Thus, the acetate-to-ergosterol assay appears to be suitable for estimating the productivity of aquatic hyphomycetes growing in leaf litter in streams. Estimated growth rates of A. tetracladia 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% of the initial leaf mass was degraded, the fungus had produced 89 mg biomass per g of initial leaf mass. Almost half of this production was allocated to conidia. Assuming an average growth efficiency of 0.35, this 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 experiment with leaf litter colonized by fungi in a stream, acetate incorporation was linear for 6 h, but the estimated growth rate was only 0.017 d(-1).