6533b871fe1ef96bd12d1846

RESEARCH PRODUCT

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

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Abstract We studied whether the presence of a bacterium can affect the functioning or stability of simple fungal-based decomposer food chains. We constructed microcosms with 60 g washed mineral soil and four different food webs: (1) a fungus (Cladosporium herbarum) alone; (2) a fungus and a fungal-feeding nematode (Aphelenchoides sp.); (3) a fungus and a bacterium (Escherichia coli); and (4) a fungus, a bacterium, a fungal- and a bacterial-feeding nematode (Aphelenchoides sp. and Acrobeloides tricornus). Glucose was supplied as the sole carbon source. One replicate set of microcosms was kept at −2°C for the sixth and seventh week as an experimental disturbance. The microcosms were destructively sampled 10 times for food web effects (non-disturbed controls) and three times for freezing treatment. Fungal biomass growing alone was less efficient (i.e. it had higher respiration rate per unit biomass) than the other food webs. This did not, however, result in the greatest C loss in the long term, because the fungal biomass declined when growing alone. Presence of the bacterium increased the decomposition of dissolved organic carbon as much as the presence of the fungal-feeder, but their effects were not additive. Ammonium nitrogen concentration of the medium was higher in systems containing bacteria. Nematode populations and the bacterial 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 incidentally retarded. In spite of their distinct role in the system functioning, the bacteria did not significantly alter the stability properties of the system. Thus, our results contradict some recent food-web based decomposition models, which predict that the effects of parallel energy channels on decomposition are additive. We discuss nutrient limitation and the dependence 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 through 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 fact that the fungal-feeding nematode induced similar changes in the fungus than the bacterium.