Effects of manganese on element distribution and structure in thalli of the epiphytic lichens Hypogymnia physodes and Lecanora conizaeoides

Abstract

Thallus pieces of the Mn-sensitive epiphytic lichen Hypogymnia physodes and of the Mn-resistant Lecanora conizaeoides were incubated in 5 mM MnCl 2 for 1 h. Element concentrations and thallus structure were subsequently studied with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray microanalysis. Mn concentrations both in fungal and algal cell walls and cell lumina were much lower in L. conizaeoides than in H. physodes, because the former immobilised Mn in the thallus (e.g. in polyphosphate granules) and in apothecia. Within the apothecia, Mn was deposited with phosphate in the hypothecium and in an unknown form in the asci. Effective immobilisation could cause the high Mn tolerance of L. conizaeoides. H. physodes also immobilised some Mn in extracellular particles in the upper cortex and in intracellular polyphosphate granules in the lower cortex. However, extra- and intra-cellular Mn concentrations in H. physodes increased much more during incubation with Mn compared with L. conizaeoides. The highest Mn concentrations were found in the upper and the lower cortex (i.e. in the cell walls and in the interhyphal polysaccharide matrix). The photobiont of H. physodes took up considerably less Mn than the mycobiont; this suggests that the latter is capable of protecting the photobiont to a certain extent from Mn invasion. Mn uptake released much Ca and Mg from H. physodes, especially from cortical cell walls and polysaccharide matrices. In the medulla, Mn was incorporated in Ca oxalate crystals especially on the surface of young growing hyphae. On a long-term basis, this is suspected to affect the integrity of the crystals, which fulfil important structural and physiological functions. Mn exposure decreased the Fe/Mn ratio more in H. physodes than in L. conizaeoides. As Fe is known to alleviate Mn toxicity in H. physodes, this could be a mechanism causing the higher Mn sensitivity of this species. Si/Mn ratios decreased in all thallus layers of H. physodes, but not of L. conizaeoides. Previous studies with soredia of H. physodes suggested possible alleviating effects of Si on Mn toxicity in lichens. Structural changes were observed in neither the mycobiont nor the photobiont of either lichen species.