The sapro-rhizosphere: Carbon flow from saprotrophic fungi into fungus-feeding bacteria

•Many rhizosphere bacteria have the ability to feed on fungi.•Rhizosphere bacteria that feed on fungi occupy a distinct niche, the sapro-rhizosphere.•Bacterial feeding on fungi has implications for below-ground carbon flow and should be incorporated in food-web models.

Root-derived, labile organic compounds are thought to enter the rhizosphere food web mainly via consumption by mycorrhizal fungi and bacteria. Studies tracking the fate of root derived carbon via stable isotope probing (SIP), however, indicate an important role for saprotrophic fungi as consumers of root exudates. In addition, it was shown that a diverse group of rhizosphere bacteria have the ability to withdraw carbon from saprotrophic fungal hyphae. Based on the fast carbon-flow into saprotrophic rhizosphere fungi and the wide-spread occurrence of rhizosphere bacteria with mycophagous abilities, we argue that the current concept of rhizosphere carbon flow should be revised to account for more complex microbial food web interactions. We propose to split the bacteria thriving in the rhizosphere niche into “plant-feeders” and “fungus-feeders”. The latter would include rhizosphere bacteria feeding on all functional groups of fungi, i.e. including mycorrhiza. To indicate the importance of saprotrophic fungi as a food source for rhizosphere bacteria, we introduce a new niche, the “sapro-rhizosphere niche”. We want to stimulate a discussion on rhizosphere carbon flow by challenging the established division into a mere bacterial- and fungal channel consisting of primary consumers, and to point at research directions based on the sapro-rhizosphere concept.