Journal Article

Dissociation and metal-binding characteristics of yellow lichen substances suggest a relationship with site preferences of lichens

Markus Hauck, Sascha-René Jürgens, Karen Willenbruch, Siegfried Huneck and Christoph Leuschner

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 103, issue 1, pages 13-22
Published in print January 2009 | ISSN: 0305-7364
Published online October 2008 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcn202
Dissociation and metal-binding characteristics of yellow lichen substances suggest a relationship with site preferences of lichens

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  • Ecology and Conservation
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Background and Aims

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 (Cu2+, Fe2+, Fe3+, Mg2+, Mn2+ and Zn2+).

Key Results

All the pigments studied are strong to moderate acids with pKa1 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.

Keywords: Anthraquinones; dibenzofurans; pulvinic acid derivatives; usnic acid; parietin; dissociation constant (pKa1); metal complexation; lichenized Ascomycetes; lichen ecology; nutrients; UV spectroscopy

Journal Article.  5924 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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