The relative impact of lichen symbiotic partners to repeated copper uptake

Abstract

The relative impact of lichen photobiont and mycobiont was evaluated by submitting nine lichen species with: (i) different photobiont types; (ii) different lichen growth forms; and (iii) different nutrients, pH, humidity preferences; to a range of Cu concentrations (μM) supplied in repeated cycles to simulate the natural process of uptake under field conditions. The physiological performance of the photosystem II photochemical reactions was measured using F v/ F m and the metabolic activity of the mycobiont was evaluated using ergosterol and intracellular K-loss as indicators. Lichens with higher cation exchange capacity showed higher intracellular Cu uptake and their ecology seemed to be associated with low-nutrient environments. Thus the wall and external matrix, mainly characteristic of the mycobiont partner, cannot be ignored as the first site of interaction of metals with lichens. No common intracellular Cu concentration threshold was found for the physiological impacts observed in the different species. Most physiological effects of Cu uptake in sensitive lichens occurred for intracellular Cu below 200 μg/g dw whereas more tolerant species were able to cope with intracellular Cu at least 3 times higher. Cyanobacterial lichens showed to be more sensitive to Cu uptake than green-algal lichens. Within the Trebouxia lichens, different species showed different sensitivities to Cu uptake, suggesting that the mycobiont may change the microenvironment close to the photobiont partner providing different degrees of protection. Despite the fact that the photobiont is the productive partner, the metabolic activity of the mycobiont of lichen species adapted to environments rich in nutrients, showed to be more sensitive to Cu uptake than the photochemical performance of the photobiont.