Does secondary chemistry enable lichens to grow on iron-rich substrates?

Abstract

Lichen substances are shown to increase or to inhibit the adsorption of Fe at cation exchange sites. The influence on the adsorption strongly differs between individual lichen substances and is different for Fe2+ and Fe3+. These results add a new biological role to the known functions of lichen secondary metabolites. In an experiment with cellulose filters, which were soaked with acetone solutions of lichen substances and were then incubated with micromolar solutions of FeCl2 or FeCl3, many lichen substances were found to increase Fe3+ adsorption, whereas others had no effect. Most lichen substances had no effect on Fe2+ adsorption, but two were found to reduce and one to increase the level of adsorption. Lichens of Fe-poor and -rich sites contain lichen substances with different adsorption behavior towards Fe2+ and Fe3+. All the studied lichen substances, which only occur in lichens of Fe-poor sites, turned out to be effective Fe3+ adsorbents. Lichens of Fe-bearing rock and slag, however, were found to lack lichen substances, or to contain substances that did not adsorb Fe3+ and had no effect on Fe2+ adsorption, or thirdly, to contain substances that increased Fe3+ adsorption, but decreased Fe2+ adsorption. These results suggest that lichen substances do play a significant role in Fe adsorption in lichens and determine their tolerance to excess concentrations of Fe. Notwithstanding the strong correlation between the secondary chemistry of lichen species and their preference for Fe-rich or Fe-poor substrates, the postulated mechanism of temporary Fe adsorption by lichen substances has to be subject of future biochemical research.