Metal homeostasis in Hypogymnia physodes is controlled by lichen substances

The hypothesis was tested that the lichen substances produced by the epiphytic lichen Hypogymnia physodes control the intracellular uptake of divalent transition metals. Incubating lichen thalli with and without their natural content of lichen substances with metal solutions showed that the lichen substances of H. physodes selectively inhibit the uptake of Cu2+ and Mn2+, but not of Fe2+ and Zn2+. Such behavior is ecologically beneficial, as ambient concentrations of Cu2+ and Mn2+ in precipitation and bark are known to limit the abundance of H. physodes, whereas limiting effects of Fe2+ or Zn2+ have never been found. This suggests that increasing the Cu2+ and Mn2+ tolerance stimulated the evolution of lichen substances in H. physodes. The depsidone physodalic acid is apparently most effective at reducing Cu2+ and Mn2+ uptake among the seven lichen substances produced by H. physodes. Probably lichen substances play a general role in the metal homeostasis of lichens.