Metabolic responses of terrestrial macrolichens to nickel

Short-term (24 h) responses of Cladonia arbuscula subsp. mitis (formerly known as Cladina and this name is used to distinguish the tested species) and Cladonia furcata to nickel (Ni²⁺) excess (10 or 100 μM) were compared. Cladonia accumulated more Ni at higher Ni dose (1.717 mg total Ni/g DW), K amount was unaffected and Ca amount decreased in Cladina only. Fluorescence microscopy detection of total/general ROS and hydrogen peroxide showed Ni-stimulated increase in both species being more pronounced in Cladonia and in mycobiont partner mainly. Nitric oxide visualization (diaminonaphthalene staining) also revealed elevation in response to Ni that could contribute to synthesis of protective metabolites: they may include ascorbic acid or reduced glutathione which increased in Ni-exposed Cladina or Cladonia, respectively. Only low content of phytochelatin 2 was detected in Ni-treated Cladonia and the role in Ni chelation is not apparent. Among aliphatic organic acids, content of citric or succinic acid was not or slightly affected by Ni, production of malic acid dropped by ca. 50% in both species and α-ketoglutaric acid showed the opposite behavior in the tested species. Data indicate that even short-term Ni treatments induce metabolic changes and symptoms of oxidative stress in lichens, confirming that nickel is not non-toxic metal as frequently visible from standard biochemical assays of basic physiology. Ascorbic acid and GSH rather than aliphatic organic acids seem to contribute to Ni tolerance.