Lichen substance concentrations in the lichen Hypogymnia physodes are correlated with heavy metal concentrations in the substratum

Lichen substances (i.e. lichen-specific carbon-based secondary compounds) are known to be involved in the uptake and immobilization of metal ions, though the biochemical mechanisms of this interaction are largely unexplained. Previous research on potential effects of lichen substances on heavy metal uptake and tolerance mostly focused on lichens in heavily polluted areas with exceptionally high metal concentrations. In the present study, we aimed at gathering information as to whether lichen substances might be involved in the fine-tuning of metal uptake even at not or low-polluted sites. Therefore, we studied lichen substance concentrations in the epiphytic lichen Hypogymnia physodes and metal concentrations in its substratum in a montane spruce forest of Germany. H. physodes produces two depsides and five depsidones, which had been shown to be involved in metal homeostasis, namely in Cu and Mn uptake, in previous laboratory experiments. The amount of lichen substances increased with increasing heavy metal concentration in the substratum, though the latter varied only in the range of a few μmol g−1 between the sample trees. Variability of lichen substance concentrations in H. physodes within the individual trees was low. Among the different lichen substances of H. physodes, the amount of the depsidone physodalic acid relative to the total of lichen substances was most closely correlated to the concentrations of Cu and Mn in the substratum, whereas the amount of the depsidone 3-hydroxyphysodic acid decreased both with increasing concentrations of these two metals and physodalic acid. Thus, our data suggest that lichen substances contribute to metal homeostasis not only in heavy metal-rich habitats, but also at not or low-polluted sites where the lichen substances apparently help to maintain constant intracellular metal concentrations despite of spatially varying availabilities of metal ions.

► Lichen substances are known to be involved in metal homeostasis. ► The lichen Hypogymnia physodes produces several depsides and depsidones. ► Their concentration varies with the heavy metal concentrations in the substratum. ► Such correlation was substantiated for the first time for a non-metal polluted site. ► A role of lichen substances in the metal homeostasis of this lichen is probable.