Biomonitoring of environmental pollutants with lichens: Data interpretation, methodological aspects and applications

This research focuses on the methodological standardization of bioaccumulation techniques by lichens. The leitmotif of this work is a biomonitor of airborne trace elements and other contaminants, the lichen Pseudevernia furfuracea (L.) Zopf, that was here repeatedly selected as test species. Firstly, the background element concentration values (BECs) for such species were assessed, since these are a prerequisite for the assessment of trace element pollution, by a large sampling effort over the Italian territory. In a first contribution, the occurrence of BEC differences of the two varieties of P. furfuracea (var. furfuracea and var. ceratea) was assessed using samples from remote areas. After having proved the absence of significant element content differences ascribable to taxonomic traits, in a second study, it was demonstrated that environmental descriptors were predictive of BECs, and these were provided for environmentally homogeneous Italian macro-regions. In this phase, the analytical outcomes of two acid mixtures for sample mineralization (i.e., a total digestion with hydrofluoric acid and a partial, aqua regia-based one) were also compared. The total digestion showed a better performance for the majority of elements. Two sets of digestion-specific BECs were provided, to be used as references in biomonitoring applications, depending on the selected mineralization procedure. Other approaches can be used to interpret bioaccumulation data. Case in point are the so-called “interpretative scales”. New scales for native lichens and transplants were respectively built up by analyzing the distribution of ratios between element concentration data and species-specific background concentration references (B ratio), contextually provided for two foliose lichens (Flavoparmelia caperata and Xanthoria parietina) and that of ratios between element concentration in exposed and unexposed samples (EU ratio) of two fruticose lichens (Evernia prunastri and Pseudevernia furfuracea). The new scales overcome critical points affecting previous ones. The second part of this research focuses on aspects related to the functioning of the lichen symbiosis, still using P. furfuracea as test species. Firstly, the effects of ozone (O3) on the physiology of previously field-stressed P. furfuracea samples were assessed. Samples were exposed for 6 weeks at sites with different levels of particulate and polycyclic aromatic hydrocarbon (PAH) emissions. Afterwards, samples were transferred to fumigation chambers, where they were O3-treated for 2 weeks. Physiological markers, elemental and PAH concentrations were measured in matched replicate sets at each experimental step. The O3-tolerance of the test species was proved, and interestingly, the content of some PAHs in samples with the highest enrichment levels significantly lowered after the ozonation, suggesting a role of O3 in degrading accumulated PAHs, with potential interpretative repercussions in transplant-based surveys aimed at evaluating PAH depositions in case of high O3 ground levels. A second contribution addresses a frequently investigated aspect for mosses, but not for lichens. Indeed, devitalization procedures are standardly carried out on moss samples, since these enhance the elemental capture by passive uptake processes. In this study, the accumulation performance of living and dead P. furfuracea samples is assessed through a high sample-sized transplant. Paired living-dead samples were exposed for 8 weeks at 40 sites in a study area of NE Italy. The two sample sets, consistently described low deposition levels over the study area, however, higher accumulation signals were revealed in dead lichens. When sites were classified according to the new bioaccumulation scale, some interpretational discrepancies arose, therefore the possibility of sample devitalization should be seriously considered, also to contribute to the methodological harmonization of biomonitoring techniques.