Contribution of fungi and bacteria to leaf litter decomposition in a polluted river

Abstract

The contribution of fungi and bacteria to decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (Northwest Portugal). Leaf mass loss, microbial production (from incorporation rates of radiolabeled compounds into biomolecules), fungal biomass (from ergosterol concentration), sporulation rates and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. Concentrations in organic nutrients and inorganic nitrogen and phosphorous in the stream water were elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 d-1 < k < 0.042 d-1) and the highest value was estimated at the most downstream polluted site, where maximum values of microbial biomass and production and sporulation rates of aquatic hyphomycetes were found. The slowest decomposition occurred at the other polluted site where, along with the nutrient enrichment, the lowest values of current velocity and dissolved oxygen in water were observed. Although bacterial production was high at this site, fungal biomass, sporulation and production were depressed; suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Bacterial production was higher at polluted sites, where fungi accounted for 94.4 and 96.6% of the total microbial net production. Fungal yield coefficient varied from 10.2 to 13.6%, while those of bacteria were less than 1%. Fungal contribution to overall leaf carbon loss (29.0-38.8%) exceeded that of bacteria (4.2-13.9%). Results indicate that although the importance of bacteria might not be neglected, fungi were the main microbial contributors to leaf decomposition in this polluted river.