Dissociation and metal-binding characteristics of yellow lichen substances suggest a relationship with site preferences of lichens.
OBJECTIVE
Many species of lichen-forming fungi contain yellow or orange extracellular pigments belonging to the dibenzofurans (usnic acid), anthraquinones (e.g. parietin) or pulvinic acid group. These pigments are all equally efficient light screens, leading us to question the potential ecological and evolutionary significance of diversity in yellow and orange lichen substances. Here the hypothesis is tested that the different pigments differ in metal-binding characteristics, which suggest that they may contribute to adaptation to sites differing in pH and metal availability.


METHODS
UV spectroscopy was used to study the dissociation and the pH dependence of the metal-binding behaviour of seven isolated lichen substances in methanol. Metals applied were selected macro- and micro-nutrients (Cu(2+), Fe(2+), Fe(3+), Mg(2+), Mn(2+) and Zn(2+)).


RESULTS
All the pigments studied are strong to moderate acids with pK(a1) values between 2.8 and 4.5. Metal complexation is common in the lichen substances studied. Complexation takes place under acidic conditions with usnic acid, but under alkaline conditions with parietin and most compounds of the pulvinic acid group. The pulvinic acid derivative rhizocarpic acid forms metal complexes both in the acidic and the alkaline range.


CONCLUSIONS
Metal complexation by lichen substances could be a prerequisite for lichen substance-mediated control of metal uptake. Assuming such an effect at pH values where the affinity of the metal for the lichen substance is intermediate would explain the strong preference of lichens with usnic or rhizocarpic acids to acidic substrata. Moreover, it would explain the preference of lichens with parietin and some lichens with compounds of the pulvinic acid group either for nutrient-rich substrata at low pH or for calcareous substrata.
Hauck
Markus
M
Albrecht von Haller Institute of Plant Sciences, Department of Plant Ecology, University of Göttingen, Untere Karspüle 2, Göttingen, Germany. mhauck@gwdg.de
Jürgens
Sascha-René
SR
Willenbruch
Karen
K
Huneck
Siegfried
S
Leuschner
Christoph
C
D000880:Anthraquinones / Q000378:metabolism; D001572:Benzofurans / Q000378:metabolism; D002264:Carboxylic Acids / Q000378:metabolism; D003300:Copper / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism; D007783:Lactones / Q000378:metabolism; D008011:Lichens / Q000254:growth & development / Q000378:metabolism* / Q000502:physiology; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D008670:Metals / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D015032:Zinc / Q000378:metabolism
D000880:Anthraquinones; D001572:Benzofurans; D002264:Carboxylic Acids; D007783:Lactones; D008670:Metals; C073339:usnic acid; D008345:Manganese; D003300:Copper; C023614:dibenzofuran; D007501:Iron; D008274:Magnesium; D015032:Zinc; C492654:pulvinic acid