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2.  Small increase in sub-stratum pH causes the dieback of one of Europe's most common lichens, Lecanora conizaeoides 
Annals of Botany  2011;108(2):359-366.
Backgrounds and Aims
Lecanora conizaeoides was until recently western and central Europe's most abundant epiphytic lichen species or at least one of the most common epiphytes. The species is adapted to very acidic conditions at pH values around 3 and high concentrations of SO2 and its derivatives formed in aqueous solution, and thus spread with increasing SO2 deposition during the 19th and 20th centuries. With the recent decrease of SO2 emissions to nearly pre-industrial levels within 20 years, L. conizaeoides declined from most of its former range. If still present, the species is no longer the dominant epiphyte, but is occurring in small densities only. The rapid spread of the L. conizaeoides in Europe from an extremely rare species to the probably most frequent epiphytic lichen and the subsequent rapid dieback are unprecedented by any other organism. The present study aimed at identifying the magnitude of deacidification needed to cause the dieback of the lichen.
Methods
The epiphytic lichen diversity and bark chemistry of montane spruce forests in the Harz Mountains, northern Germany, were studied and the results were compared with data recorded with the same methods 13–15 years ago.
Key Results
Lecanora conizaeoides, which was the dominant epiphyte of the study area until 15 years ago, is still found on most trees, but only with small cover values of ≤1 %. The bark pH increased by only 0·4 pH units.
Conclusions
The data suggest that only slight deacidification of the substratum causes the breakdown of the L. conizaeoides populations. Neither competitors nor parasites of L. conizaeoides that may have profited from reduced SO2 concentrations are likely causes of the rapid dieback of the species.
doi:10.1093/aob/mcr136
PMCID: PMC3143046  PMID: 21788378
Acidity; air pollution; bark chemistry; epiphytes; Lecanora conizaeoides; lichen-forming fungi; sub-stratum pH; sulfur dioxide
3.  Dissociation and metal-binding characteristics of yellow lichen substances suggest a relationship with site preferences of lichens 
Annals of Botany  2008;103(1):13-22.
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.
doi:10.1093/aob/mcn202
PMCID: PMC2707280  PMID: 18977765
Anthraquinones; dibenzofurans; pulvinic acid derivatives; usnic acid; parietin; dissociation constant (pKa1); metal complexation; lichenized Ascomycetes; lichen ecology; nutrients; UV spectroscopy
4.  Surface Hydrophobicity Causes SO2 Tolerance in Lichens 
Annals of Botany  2007;101(4):531-539.
Background and Aims
The superhydrophobicity of the thallus surface in one of the most SO2-tolerant lichen species, Lecanora conizaeoides, suggests that surface hydrophobicity could be a general feature of lichen symbioses controlling their tolerance to SO2. The study described here tests this hypothesis.
Methods
Water droplets of the size of a raindrop were placed on the surface of air-dry thalli in 50 lichen species of known SO2 tolerance and contact angles were measured to quantify hydrophobicity.
Key Results
The wettability of lichen thalli ranges from strongly hydrophobic to strongly hydrophilic. SO2 tolerance of the studied lichen species increased with increasing hydrophobicity of the thallus surface. Extraction of extracellular lichen secondary metabolites with acetone reduced, but did not abolish the hydrophobicity of lichen thalli.
Conclusions
Surface hydrophobicity is the main factor controlling SO2 tolerance in lichens. It presumably originally evolved as an adaptation to wet habitats preventing the depression of net photosynthesis due to supersaturation of the thallus with water. Hydrophilicity of lichen thalli is an adaptation to dry or humid, but not directly rain-exposed habitats. The crucial role of surface hydrophobicity in SO2 also explains why many markedly SO2-tolerant species are additionally tolerant to other (chemically unrelated) toxic substances including heavy metals.
doi:10.1093/aob/mcm306
PMCID: PMC2710189  PMID: 18077467
Contact angle; hydrophilicity; hydrophobicity; lotus effect; cortex; sulphur dioxide; air pollution; water uptake; lichens

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