Funktion von Flechtenstoffen bei der Metallhomöostase und Säureresistenz von Flechten
Final Report Abstract
Effects of lichen secondary chemistry on the acidity tolerance and metal homeostasis in lichens were studied. Lichens contain more than 800 so-called lichen substances, most of which are only produced by lichen-forming fungi. The role of lichen substances in the tolerance to acidity (combined with SO2 or other compounds) was never studied before. Previous studies to effects of lichen substances on metal uptake and tolerance are limited to a few papers. In addition to the function of lichen substances, the role of surface hydrophobicity in the acidity tolerance of lichens was explored. The tolerance of lichens to acidity and dissolved SO2 was strongly correlated with the hydrophobicity of the thallus surface. Lichen species known to be highly acidity and SO2-tolerant stay virtually dry when a water droplet is placed on their surface. Increasing hydrophilicity is correlated with increasing sensitivity. Lichen substances play an additional role in the acidity tolerance of lichens. Lichen species with identical lichen substances prefer substrates of a similar pH range. This pH range is correlated with the pKa1 value of the lichen substances. The mechanism behind this correlation was exemplarily studied in the dibenzofuran usnic acid. Usnic acid-containing lichens prefer a pH range of 3.5 to 5.5 with an optimum between pH 4.0 and 4.5. Below pH 3.5, lichens are significantly more sensitive to dissolved SO2 with than without their natural content of usnic acid. Since usnic acid is able to shuttle protons through phospholipid membranes, a shift of the equilibrium between usnic acid and its corresponding anion usneate towards the protonated form at pH values far below its pKa1 value in the small water-soluble fraction of usnic acid is thought to result in an acidification of the cytoplasm. The general correlation between the pKa1 value of lichen substances and the substrate pH of lichen species producing these substances indicates a similar effect with other lichen substances. Usnic acid-producing lichen species with a strongly hydrophobic surface, including Lecanora expallens, Parmeliopsis ambigua, and the occasionally usnic acid-producing Lecanora conizaeoides, are not subject to this effect, as acidic solutions can hardly penetrate into such lichens. Metal homeostasis in lichens is for the first time shown to be influenced by the content of lichen substances. Lichen substances control the intracellular uptake and the extracellular adsorption of metals. This effect is specific for certain ions and differs between lichen substances. Some lichen substances reduce the uptake of certain metal ions, while the uptake of other ions is apparently promoted. The mechanisms of the effects of metal ions on metal homeostasis remain to be studied in detail. Nonetheless, some distribution patterns can already be attributed to the content of lichen substances, including the absence of lichens with Fe-adsorbing lichen substances on Fe-rich rock and slag, the specialization of Lecidea inops on cupriferous rock and slag, as its major lichen substances perlatolic acid apparently forms a barrier that inhibits Cu uptake, the widespread occurrence of the Fe3+-adsorbing anthraquinone parietin in calciphytic lichens, or the absence of lichen substances from most lichens specialized on mineral-rich, neutral to slightly acidic substrates, where a mobilization of nutrient ions seem to be unnecessary.
Publications
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(2007): Does secondary chemistry enable lichens to grow on iron-rich substrates? - Flora 202: 471-478
Hauck, M., S. Huneck, J.A. Elix & A. Paul
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(2007): Lichen substances affect metal adsorption in Hypogymnia physodes. - Journal of Chemical Ecology 33: 219-223
Hauck, M. & S. Huneck
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(2007): The putative role of fumarprotocetraric acid in the manganese tolerance of the lichen Lecanora conizaeoides. - Lichenologist 39: 301-304
Hauck, M. & S. Huneck
