Secondary Metabolites in Soil Ecology

Interactions among organisms are central to understanding any ecosystem, perhaps with the exception of a short period when a newly created niche is colonized by its first inhabitants. Soil environment is not an exception, but biotic interactions dominating soil biology differ from those in other systems because of the dominating role of sessile organisms and the lack of autotrophy in soil (chemolithoautotrophs being an interesting but not significant exception). When chemical processes in soil are discussed, the traditional concept of food webs comes first to mind as a framework for the exchange of organic substances and flow of energy. Feeding, predation, degradation of macromolecular substrates and absorption of nutrients have dominated thinking about biogenic chemical processes in soil. The food web approach proved extremely fruitful in generating hypotheses and inspiring experimental approaches concerning the bulk transformation of organic matter, but it did not address phenomena related to chemical interactions which are more specific both on the chemical and on the taxonomical level and which cannot be adequately described in terms of energy flow and biomass transformation. These interactions involve compounds named secondary metabolites, which are not strictly needed for the survival and reproduction of their producers. Secondary metabolites are structurally highly diverse and each of them is produced only by a small number of species. They exert various biological effects, often at very low concentrations, and can be regarded as carriers of chemical communication among soil inhabitants.