Uptake and toxicity of manganese in epiphytic cyanolichens

Mn uptake from MnCl2 solution and chlorophyll fluorescence (as a selected vitality parameter) were studied in the epiphytic lichens Lobaria pulmonaria (tripartite, heteromerous lichen with the green alga Dictyochloropsis as primary photobiont and Nostoc in cephalodia), Nephroma helveticum (bipartite, heteromerous lichen with Nostoc photobiont) and Leptogium saturninum (bipartite, homoiomerous lichen with Nostoc photobiont). Extracellular adsorption and intracellular uptake of Mn increased in the order L. pulmonaria < N. helveticum < L. saturninum. Mn increasingly reduced the effective quantum yield of photosystem 2 (Φ2) in the same order. CaCl2 and MgCl2 alleviated the Mn-induced reduction of Φ2. Moist thalli of all species transferred significant amounts of extracellular Mn into the cells during a recovery day subsequent to incubation with metal solution. This suggests that even short exposures to Mn in the field, e.g. via stemflow, can affect the physiology of the lichen species studied. The experimental results support the hypothesis that cyanolichens are sensitive to excess Mn. Data also suggest that the tripartite L. pulmonaria is less Mn-sensitive than the bipartite cyanolichens. This agrees with published field observations from Montana, where bipartite cyanolichens (including L. saturninum and N. helveticum) occurred on conifer bark with the lowest Mn concentration, while L. pulmonaria was also found on bark with higher Mn concentrations.