# Quantifying Moss Response to Metal Contaminant Exposure Using Laser-Induced Fluorescence
## 4 Discussion
Unfortunately, since all Cu levels tested resulted in clear quantifiable separation from the control, it is not currently possible to define the lowest detection threshold for this method. More work needs to be carried out to explore minimum detection limits with this particular design, and we can currently only say that the methodology of using LIF can identify our lowest applied dose of Cu at 0.034 mg/cm\({}^{2}\). While the presented results show the viability of LIF to detect a physiological change in moss in response to Cu treatment, its applicability in the confirmed Cu ranges to environmental contamination is limited but may be relevant to industrially contaminated areas as is discussed next. The lowest Cu dose administered in this work, 0.034 mg/cm\({}^{2}\) or 340 mg/m\({}^{2}\), is much higher than wet deposition values reported by published environmental levels for urban environments. Such measurements in the US vary from as low as 0.49 to 2.2 mg/m\({}^{2}\)/yr for five cities reported in New Jersey [38], and 0.70 mg/m\({}^{2}\)/yr to 1.06 mg/m2/yr for the more suburban areas of Reston, Virginia [39], and Chicago, Illinois [40]. Global studies reported similar Cu values of 0.8 \(\pm\) 0.7 mg/m\({}^{2}\)/yr in urban areas for an average wet deposition [41]. However, higher levels of Cu were recorded in areas of Asia at 4.7 mg/m\({}^{2}\)/yr for Hong Kong, 14.6 mg/m\({}^{2}\)/yr for Singapore, and 10.5 mg/m\({}^{2}\)/yr in the North Sea [41]. When considering dry weight, the lowest measured Cu concentrations in moss fronds from the current experiment were 1300 \(\pm\) 700 \(\mathrm{\SIUnitSymbolMicro g}\) (equivalent to ppm). Correspondingly, examples of Cu levels observed in moss, e.g., from Chengdu in South-west China, range from 19.5 \(\pm\) 3.6 to 139 \(\pm\) 27 \(\mathrm{\SIUnitSymbolMicro g}\)[42]. A survey in Portugal used two mosses (_Scleropodium touretii_ (Brid.) [PERSON]) and _Hyprum cupressforme_ Hedw.) across 178 sites, resulting in a documented Cu range of 0.4-52 \(\mathrm{\SIUnitSymbolMicro g}\)[43]. A total of 79 samples of _Orthotrichum lyelli_ Hook. & Taylor were collected from 18 sites in Seattle, Washington, and showed varying levels of Cu ranging from 15.6 to 114.6 mg/kg [44]. Likewise, 346 moss samples of the same moss species were collected in Portland, Oregon, over a 3-week period, and, though the work showed a mean of 20 \(\mathrm{\SIUnitSymbolMicro g}\)/g, the maximum observed value was 357.25 \(\mathrm{\SIUnitSymbolMicro g}\)/g [45]. Clearly, the demonstrated sensitivity range of this LIF method does not extend to urban environments. However, while urban areas are of great interest to immediate human health, higher Cu levels of concern and sustained environmental impacts are predominately from mining and industrial activities. Introduction of PTEs into the environment comes mostly from anthropogenic sources, mainly due to the processing or production of metals through mining, smelting, waste incineration, and fossil fuel industries [46]. When observing industrial areas such as the Serbian thermoelectric Nikola Tesla power plant, examples of Cu levels 100\(\times\) above urban areas are reported [46], which would fall closer to the currently detectable levels found while using LIF in this study. Sampling of moss around the Nikola Tesla plant revealed a range of 14.27 to 37.23 \(\mathrm{\SIUnitSymbolMicro g}\)/g in comparison to moss measured near Canadian thermal power plants, which had an average of 19.8 \(\mathrm{\SIUnitSymbolMicro g}\)/g [47]. In Turkey, the Murgul copper mine had 9-13 \(\mathrm{\SIUnitSymbolMicro g}\)/g/day of copper deposited via dry deposition [48]. However, 13 moss species collected at 23 sites around the plant revealed a range from 105 to 1916 \(\mathrm{\SIUnitSymbolMicro g}\)/g [48]. Comparison of soil samples at the same sites showed Cu ranges of 81-1266 \(\mathrm{\SIUnitSymbolMicro g}\)/g [48].