Baiting of rhizosphere bacteria with hyphae of common soil fungi reveals a diverse group of potentially mycophagous secondary consumers

Fungi and bacteria are primary consumers of plant-derived organic compounds and therefore considered as basal members of soil food webs. Trophic interactions among these microorganisms could, however, induce shifts in food web energy flows. Given increasing evidence for a prominent role of saprotrophic fungi as primary consumers of root-derived carbon, we propose that fungus-derived carbon may be an important resource for rhizosphere bacteria. To test this assumption, two common saprotrophic, rhizosphere-inhabiting fungi, Trichoderma harzianum and Mucor hiemalis, were confronted in a microcosm system with bacterial communities extracted from the rhizospheres of a grass and sedge species, Carex arenaria and Festuca rubra. This showed a widespread ability of rhizosphere bacteria to attach to and feed on living hyphae of saprotrophic fungi. The identity of the fungi had a strong effect on the composition of these potentially mycophagous bacteria, whereas plant species identity was less important. Based on our results, we suggest that food web models should account for bacterial secondary consumption since this has important consequences for carbon fluxes with more carbon dioxide released by microbes and less microbial carbon available for the soil animal food web.