Iron and phosphate uptake in epiphytic and saxicolous lichens differing in their pH requirements

The hypothesis is tested that pH-dependent Fe and P uptake influence the preference of epiphytic and saxicolous lichens for certain ranges of ambient pH. Five species from acidic substrata (Hypogymnia physodes, Parmeliopsis ambigua, and Platismatia glauca) or covering the range from weakly acidic to alkaline substrata (Lecanora muralis and Phaeophyscia orbicularis) were exposed to solutions of FeCl2, FeCl3, or KH2PO4 at pH 3 and 8 in the laboratory. Avoidance of alkaline substrata is explainable by low Fe3+ uptake at pH 8 in the case of H. physodes and the inability for net P uptake and membrane damage in P. ambigua at this pH. Preference for acidic substrata in Pl. glauca, however, is neither related to Fe nor P uptake. Efficient Fe3+ and P uptake at pH 8 explains the tolerance of L. muralis and Ph. orbicularis to alkaline conditions. Intracellular accumulation of Fe2+ in probably toxic amounts at pH 3 in Ph. orbicularis is correlated with the absence of this lichen from strongly acidic substrata. Avoidance of acidic sites by L. muralis is not attributable to Fe or P uptake. In summary, the results suggest that pH-dependent Fe and P uptake characteristics are involved in the determination of pH preferences of epiphytic and saxicolous lichens, but are not the only relevant factor.