Do urban air pollutants induce changes in the thallus anatomy and affect the photosynthetic efficiency of the nitrophilous lichen Physcia adscendens?

Lichens are symbiotic organisms that are generally sensitive to air pollution due to their specific biological and physiological features. Physcia adscendens is a nitrophilous lichen well-known for being resistant to air pollution associated with progressive anthropopressure. The aim of this study was to investigate the effect of nitrogen oxides and suspended particulate matter (PM10 and PM2.5) on anatomical structure of the thallus and photobiont's photosynthetic efficiency in P. adscendens inhabiting sites that differ in terms of air pollution level and thereby to determine the relevance of these pollutants for shaping the structure of the thallus and the physiological condition of the photosynthetic partner. We found that P. adscendens from polluted sites had increased thickness of the algal layer and the larger size of the algae cells, but a much lower ratio of the algal layer to the whole thallus. Lichens from highly polluted sites had also higher photosynthetic efficiency, which indicates a relatively good physiological condition of the photobiont. This indicates that the photobiont of P. adscendens is well-adapted to function under air pollution stress which may contribute to its success in colonizing polluted sites. Both changes in the anatomy of the lichen thallus and the efficiency of photosynthesis may be related to the enrichment of the environment with nitrogen. The increased photosynthetic efficiency as well as investment in the size of photobiont cells and growth mycobiont hyphae confirms that P. adscendens is well-adapted to urban conditions; however, the mechanism behind those adaptations needs more focus in the context of global environmental changes.

Ammonium and nitrate tolerance in lichens

Since lichens lack roots and take up water, solutes and gases over the entire thallus surface, these organisms respond more sensitively to changes in atmospheric purity than vascular plants. After centuries where effects of sulphur dioxide and acidity were in the focus of research on atmospheric chemistry and lichens, recently the globally increased levels of ammonia and nitrate increasingly affect lichen vegetation and gave rise to intense research on the tolerance of lichens to nitrogen pollution. The present paper discusses the main findings on the uptake of ammonia and nitrate in the lichen symbiosis and to the tolerance of lichens to eutrophication. Ammonia and nitrate are both efficiently taken up under ambient conditions. The tolerance to high nitrogen levels depends, among others, on the capability of the photobiont to provide sufficient amounts of carbon skeletons for ammonia assimilation. Lowly productive lichens are apparently predisposed to be sensitive to excess nitrogen.