Discrimination between lichens according to their exposure to pollutants by NIR spectroscopy

Lichens are symbiotic associations between a fungus and a population of unicellular algae and they take up nutrients over the entire thallus, thus entirely depending on the atmosphere for nutrition. For this reason, lichens are extremely sensitive to the presence of substances that alter the atmospheric composition and they are excellent biomonitors of air pollution (Nimis et al., 2002).
NIR spectroscopy and chemometrics were evaluated as fast and highly reliable methods for generating a ‘fingerprint’ of lichens useful to discriminate samples from polluted and non-polluted areas.
Fifty-one thalli of two different varieties of P. furfuracea were collected far from local sources of air pollution and then twenty-six of these were exposed for one month in the industrial harbour of Genoa characterised by an high level of environmental pollution.
For the NIRS analysis, lichens were pulverized with a ball mill, then the powder was placed in an optical glass Petri dish and analysed in the reflectance mode by a FT Near-Infrared Spectrometer (Buchi NIRFlex N-500), in the 4000–10000 cm−1 range with a 8 cm−1 resolution.
Chemometrics methods were successfully applied to discriminate samples from polluted and non- polluted areas.
Principal Component Analysis (PCA) was applied as a multivariate display method on the NIR spectra to visualise the data structure and a clear separation between the twenty-five control samples (not exposed to pollutants) and the twenty-six harbour samples was observed. The loadings associated with each original variable showed some absorption bands most important in defining the direction that seems to distinguish between control and harbour lichen samples. Many organic acids can produce these absorptions but those responsible for the discrimination between the two classes of lichens can be of two types: endogenous organic acids (i.e. the oxiphysodic acid and other similar organic acids that increase in lichens when they are subjected to stresses such as pollution) (Bialonska and Dayan, 2005) and organic acids that come from environmental pollution (i.e. aromatic (poly-) carboxylic acids that originate from fuel oil combustion) (Rogge et al., 1993).
Finally, Linear Discriminant Analysis (LDA) was performed to discriminate between lichens according to their exposure to pollutants. The distinction between control samples and samples exposed in the industrial harbour of Genoa was evaluated; as average values, 95.2 % of samples were correctly classified, 93.0 % of total internal prediction (5 cross-validation groups) and 100.0 % of external prediction (on the test set) were achieved.

References
Nimis, P.L., Scheidegger, C., Wolseley, P.A., 2002. Monitoring with Lichens – Monitoring Lichens. Kluwer Academic Publisher, Netherlands.
Bialonska, D., Dayan, F.E., 2005. Chemistry of the lichen Hypogymnia physodes transplanted to an industrial region. J. Chem. Ecol. 31, 2975-2991.
Rogge, W. F., Hildemann, L. M.; Mazurek, M. A., Cass, G. R., Simoneit, B. R. T., 1993. Quantification of urban organic aerosols at a molecular level: Identification, abundance and seasonal variation, Atmos. Environ. 27, 1309-1330.