Forest Filter Effect Revisited: First Evidence That Polycyclic Aromatic Hydrocarbon Metabolites Are Produced on Leaves by Biodegradation and Photodegradation

Plants are considered "nature-based solutions" for the improvement of air quality. Through the so-called forest filter effect (FFE), they can remove both gas and particulate matter (PM) associated organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs) from air, reducing their concentrations. In this work, new aspects of FFE were investigated through several laboratory experiments of increasing complexity. More specifically, the study was designed to evaluate for the first time the contribution of light and phyllosphere microbial communities of Quercus ilex leaves to the degradation of deuterated PAHs (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, pyrene-d10, chrysene-d12, and perylene-d12). Such degradation was investigated by observing the production of deuterated PAH hydroxy metabolites (OH-PAHs). OH-PAHs were produced in both light and dark conditions, while their appearance in sterile controls was negligible, highlighting the importance of leaf-associated microorganisms in the degradation of parent PAH. The number of PAH metabolites and their isomers increased when entire leaves were used rather than the inoculated phyllosphere microorganisms only, confirming the degradation ability of native phyllosphere microbial communities. Although preliminary, these results showed that FFE can be enhanced by photo- and biodegradation processes. The continuous removal of PAHs through degradation could result in an enhanced PAH concentration gradient and flux of deposition to leaves.