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Evaluating the atmospheric trace element contamination by lichens
Figure 1 | Sample of Xanthoria parietina (L.) Th. Fr.
Trace elements (As, Cd, Cu, Pb, Sb, Zn…) constitute chemicals with low concentrations in the environment. Some of them are considered as nutrients, but all are toxic for living organisms in high concentrations. Trace elements are transported in the atmosphere over long distances that impact the environment [1]. The evaluation of atmospheric deposition on land surfaces, however, is missing and complex due to the relative low concentration of these elements.

Lichens (Figure 1), symbiosis between a fungus and an alga, are more sensitive to atmospheric pollution than tree leaves thanks to their biological features (lack of root system, lack of protective cuticle, almost permanent biological activity) [2]. Trace metal atmospheric deposition was monitored in remote French forested sites on the national scale through two different biomonitoring approaches: bioaccumulation and bioindication.
Trace element accumulation in lichen samples showed a strong regional influence of the lithology (particularly for As, Co, Cr, Ni, and Ti; Figure 2): this is the geochemical background [3]. Rare earth elements, geochemical tracers of sources and processes, allowed highlighting the influence of particles derived from ocal rock weathering on lichen monitoring [4]. Additional anthropogenic activities were recorded as regional sources for other chemical elements (e. g., Cd, Cu, Sb, and Zn) [3].

The comparison between these lichen samples and historical specimens stored in university herbaria allowed investigating the evolution of atmospheric deposition for 150 years. Thus, it seemed that the use of fossil coal was the dominant source of metal contamination during the late 19th and early 20th century (especially for As, Cd and Pb) [5]. In contrast, the current context showed more local sources compared to previous periods (e.g., Sb and Sn) [6].
Figure 2 | Geochemical signatures of lichen samples depending on the region
Figure 3 | Principal component analysis on trace metal concentrations in lichen, moss, bulk deposition, and throughfall samples
In parallel, the factors influencing the metal bioaccumulation by lichens were tested. The considered species plays a role through the thallus morphology and, therefore, the entrapment of airborne particles. Bark, however, showed no major influence. Considering the atmospheric deposition signature, vegetation via throughfall was found to be more influent for terricolous mosses than for corticolous lichens (Figure 3) [7].

Additional experiments of adsorption/desorption showed a quick accumulation process (about one week) for Pb and Cd, but neither metal competition, nor leaching influence on bioaccumulation were highlighted.
Lichen bioindication scales required major changes over time related to the atmospheric contamination changes. To evaluate the susceptibility of different lichen species to metals, we combined bioindication and bioaccumultion approaches [8]. Thus, each studied species was placed on a scale of susceptibility (Figure 4), which will subsequently update the pre-existing scales taking into account metal pollution.
Figure 4 | Canonical analysis evaluating the lichen susceptibility to metal pollution
[1] Pacyna J. M. – Atmospheric trace elements from natural and anthrogenic sources. In Nriagu J. O. & Davidson C. L. (ed.), Toxic metals in the atmosphere, pp. 33–52. John Wiley and Sons. New York (1986)
[2] van Haluwyn C., Lerond M. – Guide des lichens. Lechevalier. Paris. 344 p. (1993)
[3] Agnan Y. et al.Large-scale atmospheric contribution of trace elements registered in foliose lichens in remote French areas. Proceeding of 16th International Conference on Heavy Metals in the Environment, Rome, 23–27 Sept. 2012, volume 1 (2013)
[4] Agnan Y. et al.Origin and distribution of rare earth elements in various lichen and moss species over the last century in France. Science of the Total Environment 487: 1–12 (2014)
[5] Agnan Y. et al.Investigation of spatial and temporal metal atmospheric deposition in France through lichen and moss bioaccumulation over one century. Science of the Total Environment 529: 285–296 (2015)
[6] Agnan Y. et al.Comparing early twentieth century and present-day atmospheric pollution in S-W France: a story of lichens. Environmental Pollution 172: 139–148 (2013)
[7] Gandois L. et al.Use of geochemical signatures, including rare earth elements, in mosses and lichens to assess spatial integration and the influence of forest environment. Atmospheric Environment 95: 96–104 (2014)
[8] Agnan Y. et al. Evaluation of lichen species resistance to atmospheric metal pollution by coupling diversity and bioaccumulation approaches: a new bioindication scale for French forested areas. Ecological Indicators 72: 99–110 (2017)