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

Background and Aims: Many species of lichen-forming fungi contain yellow or orangeextracellular pigments belonging to the dibenzofurans (usnicacid), anthraquinones (e.g. parietin) or pulvinic acid group.These pigments are all equally efficient light screens, leadingus to question the potential ecological and evolutionary significanceof diversity in yellow and orange lichen substances. Here thehypothesis is tested that the different pigments differ in metal-bindingcharacteristics, which suggest that they may contribute to adaptationto sites differing in pH and metal availability. Methods: UV spectroscopy was used to study the dissociation and the pHdependence of the metal-binding behaviour of seven isolatedlichen substances in methanol. Metals applied were selectedmacro- and micro-nutrients (Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Mn²⁺ andZn²⁺). Key Results: All the pigments studied are strong to moderate acids with pK_a1values between 2·8 and 4·5. Metal complexationis common in the lichen substances studied. Complexation takesplace under acidic conditions with usnic acid, but under alkalineconditions with parietin and most compounds of the pulvinicacid group. The pulvinic acid derivative rhizocarpic acid formsmetal complexes both in the acidic and the alkaline range. Conclusions: Metal complexation by lichen substances could be a prerequisitefor lichen substance-mediated control of metal uptake. Assumingsuch an effect at pH values where the affinity of the metalfor the lichen substance is intermediate would explain the strongpreference of lichens with usnic or rhizocarpic acids to acidicsubstrata. Moreover, it would explain the preference of lichenswith parietin and some lichens with compounds of the pulvinicacid group either for nutrient-rich substrata at low pH or forcalcareous substrata.