Molybdenum: Antioxidant or Pro-oxidant Role in Lichens?

Most of the trace elements toxicity data on ecophysiological status of lichen species were the result of laboratory experiments where thalli were totally immersed in solutions at different concentrations of heavy metals to test their relative stressing effect. This made it possible, when using same/comparable concentrations and time of exposure, to rank their toxicity and, at the same time, the sensitivity of lichens versus the same element. However, such data hardly reproduce the environmental toxicity due to wet depositions where the mode of exposure (drops) and the contact time (seconds – minutes) are much shorter. In addition, many works studied the effect of a restricted group of elements often ignoring unreasonably others. Molybdenum is one of the least studied elements in lichens concerning its effect on ecophysiological status and interaction with other micronutrients. Based on its involvement in several metabolic pathways, and multiplicity in valency values, we carried out exposure of thalli of the lichen Pseudevernia furfuracea (L.) Zopf to sprays at different concentrations of molybdenum (0.5, 1, 2, 4 ppm), copper (6, 12 mM) and mixed treatments of the two elements to evaluate the response in cell peroxidation, photopigment amounts, chlorophyll a – chlorophyll b ratio and chlorophyll degradation. Results showed that copper promoted a strong increase in thiobarbituric acid reactive species levels, whereas molybdenum reduced them below the limits of detection of the analytical procedure. No significant variations in photobiont parameters were detected versus control. Interestingly, when the lichens were co-exposed to the combinations of the four molybdenum concentrations with the highest one of copper oxidative stress dropped, on average, 20% compared to only copper exposure, whereas in case of the lower Cu²⁺ concentrations malondialdehyde levels increased 80%, most time showing a statistically significant variation compared to controls. Algal physiology was not affected by metals exposure. Together these data suggest that molybdenum in itself plays, in Pseudevernia furfuracea, a potential antioxidant role. It can be overwhelmed or, paradoxically, translated in a pro-oxidant one by the presence of elements like copper, affecting the formation of oxidative damage. This is the first work where such an outcome is showed in lichens.