Usnic acid controls the acidity tolerance of lichens.
The hypotheses were tested that, firstly, lichens producing the dibenzofuran usnic acid colonize substrates characterized by specific pH ranges, secondly, this preferred pH is in a range where soluble usnic acid and its corresponding anion occur in similar concentrations, and thirdly, usnic acid makes lichens vulnerable to acidity. Lichens with usnic acid prefer an ambient pH range between 3.5 and 5.5 with an optimum between 4.0 and 4.5. This optimum is close to the pK(a1) value of usnic acid of 4.4. Below this optimum pH, dissolved SO(2) reduces the chlorophyll fluorescence yield more in lichens with than without their natural content of usnic acid. This suggests that usnic acid influences the acidity tolerance of lichens. The putative mechanism of the limited acidity tolerance of usnic acid-containing lichens is the acidification of the cytosol by molecules of protonated usnic acid shuttling protons through the plasma membrane at an apoplastic pH<pK(a1).
Hauck
Markus
M
Department of Plant Ecology, Albrecht von Haller Institute of Plant Sciences, University of Göttingen, Untere Karspüle 2, D-37073 Göttingen, Germany. mhauck@gwdg.de
Jürgens
Sascha-René
SR
D000393:Air Pollutants / Q000633:toxicity; D001572:Benzofurans / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008011:Lichens / Q000145:classification / Q000187:drug effects / Q000378:metabolism*; D008954:Models, Biological; D013045:Species Specificity; D013458:Sulfur Dioxide / Q000633:toxicity
D000393:Air Pollutants; D001572:Benzofurans; D013458:Sulfur Dioxide; C073339:usnic acid