Manganese uptake in the epiphytic lichens Hypogymnia physodes and Lecanora conizaeoides

Exposure of the foliose epiphytic lichen Hypogymnia physodes to 100 mM MnCl2 led to rapid absorption to extracellular cation exchange sites; saturation was reached within about 50 min. Intracellular uptake was constant in samples exposed to 100 mM MnCl2 for 10-120 min and analyzed for their Mn concentration after 1 day of recovery in the growth chamber. About 15 times as much Mn was bound extracellularly than was taken up intracellularly. Both 50 mM CaCl2 and 50 mM MgCl2 applied together with 100 mM MnCl2 significantly reduced Mn uptake intracellularly and binding in the extracellular exchange sites. CaCl2 was more effective than MgCl2 at reducing Mn uptake intracellularly; extracellularly, CaO2 and MgCl2 reduced Mn absorption equally. In the assays with MnCl2, alone or in combination with CaCl2, the amount of extracellularly bound Mg decreased, as did the content of Ca in MnCl2 alone or in combination with MgCl2. The results support the hypothesis that the alleviating effects of Ca and Mg on Mn toxicity, documented in previous culture experiments with H. physodes, were, at least in part, due to reduced Mn uptake. H. physodes bound significantly more Mn from 1 to 100 mM MnCl2 solutions on its extracellular exchange sites than did the crustose Lecanora conizaeoides. Further, H. physodes, but not L. conizaeoides released significant amounts of Ca and Mg from the extracellular exchange sites during Mn uptake. Intracellular Mn concentrations increased and the Ca/Mn and the Mg/Mn ratios decreased with increasing Mn supply in either species. The intracellular Mg/Mn ratio was higher in L. conizaeoides than in H. physodes. Its lower extracellular Mn uptake combined with lower losses of Ca and Mg as well as the higher intracellular Mg/Mn ratio may contribute to higher Mn tolerance of L. conizaeoides in the field, which was observed in spruce forests of the Harz Mountains, northern Germany. Neither H. physodes nor L. conizaeoides had significant amounts of extracellularly bound Mn3+ or Mn4+ when compared with the concentration of Mn2+. Thus, immobilization of Mn2+ by oxidizing, as known from non-lichenized green algae, is unlikely as tolerance mechanism in L. conizaeoides.