The phenolic compounds in Cladonia lichens are not antimicrobial in soils.

According to classic text books on lichen biology, the phenolic secondary chemicals in lichens have antibiotic effects on soil microorganisms and mycorrhizal fungi in ecosystems. However, the experimental evidence for this under natural conditions is still relatively scarce. We examined some of the assumptions behind the concept of antimicrobial effects of lichen secondary substances: (1) the secondary substances of Cladonia stellaris, usnic and perlatolic acids, are leached out from the lichens by rainwater; (2) these substances inhibit the microbial activity of soil, and; (3) since they are extremely resistant to microbial decomposition, the soil underneath a continuous lichen mat is enriched in usnic and perlatolic acids. Our results did not support any of these assumptions. The evidence for the antimicrobial activity of lichen secondary substances seems to be weak in comparison to other suggested functions such as light filtering and herbivore protection. We suggest that it is time to re-evaluate the evidence for the antimicrobial ecological role of lichen secondary substances in natural systems.