Bacteria and microbial-feeders modify the performance of a decomposer fungus

We studied whether the presence of a bacterium can affect the functioning o r stability of simple fungal-based decomposer food chains. We constructed m icrocosms with 60 g washed mineral soil and four different food webs: (1) a fungus (Cladosporium herbarum) alone; (2) a fungus and a fungal-feeding ne matode (Aphelenchoides sp.); (3) a fungus and a bacterium (Escherichia coli ); and (4) a fungus, a bacterium, a fungal- and a bacterial-feeding nematod e (Aphelenchoides sp. and Acrobeloides tricornus). Glucose was supplied as the sole carbon source. One replicate set of microcosms was kept at -2 degr eesC for the sixth and seventh week as an experimental disturbance. The mic rocosms were destructively sampled 10 times for food web effects (non-distu rbed controls) and three times for freezing treatment. Fungal biomass growi ng alone was less efficient (i.e. it had higher respiration rate per unit b iomass) than the other food webs. This did not, however, result in the grea test C loss in the long term, because the fungal biomass declined when grow ing alone. Presence of the bacterium increased the decomposition of dissolv ed organic carbon as much as the presence of the fungal-feeder, but their e ffects were not additive, Ammonium nitrogen concentration of the medium was higher in systems containing bacteria. Nematode populations and the bacter ial biomass were not sensitive to the disturbance of freezing. Although the freezing disturbance decreased total fungal biomass in the absence of the bacterial energy channel, dissolved organic carbon decomposition was only i ncidentally retarded. In spite of their distinct role in the system functio ning, the bacteria did not significantly alter the stability properties of the system. Thus, our results contradict some recent food-web based decompo sition models, which predict that the effects of parallel energy channels o n decomposition are additive. We discuss nutrient limitation and the depend ence of fungal properties on the food-web configuration as explanations for the contradiction of theory and data. We did not find evidence supporting a hypothesis that an additional energy channel should stabilize flow throug h a food web, and hence decomposition. This is explained by high resilience of the reference system with a fungus alone, and by redundancy, i.e. the f act that the fungal-feeding nematode induced similar changes in the fungus than the bacterium. (C) 2001 Elsevier Science Ltd. All rights reserved.