Contrasting physico-chemical and oxidative relationships to thalli nitrogen and metal ion contents in Usnea spp. and Hypotrachyna spp. from Himalayan forests of Nepal.

South Asian nations are facing the challenge of increasing nitrogen pollution with the Indo-Gangetic Plain having some of the highest levels of atmospheric ammonia pollution globally. However, there is a lack of in-country research to evaluate the possible impact of nitrogen-related pollutants on South Asian biodiversity. In the Himalayas, there is an opportunity to utilize lichens from natural habitats to establish field-based references for better future tracking of changes in ecosystem health relevant to the wider South Asian region. In this study, we assessed the natural chemical variability of two lichens (Usnea spp. and Hypotrachyna spp.) based on thallus nitrogen and metal ion contents along with their physico-chemical and oxidative responses in two 1-km long transects from two forests of Nepal representing local gradients. Our results revealed a moderate concentration of total Kjeldalh nitrogen (0.36-0.98 % DM in Chandragiri, KTM and 0.57-2.04 % DM in Ghorepani, ACA), as well as ammonium (40.42-159.84 mg/L in Chandragiri, KTM and 80.60-280.64 mg/L in Ghorepani, ACA) and considerable amount of metal ions in both lichens, though with the highest values for lichens collected from the Ghorepani, ACA (from Western Nepal). A noteworthy background concentration of atmospheric ammonia was also observed at both sites. The highest variation in physico-chemical responses, such as electrical conductivity, chlorophyll content, chlorophyll degradation, chlorophyll fluorescence, and phenolic content was observed in the lichens from the same area, consistent with the higher levels of air pollution. Moreover, there appeared to be associated impacts on oxidative responses such as radical scavenging and catalase activities. Furthermore, the metal ions in the lichen thalli were found to originate from both anthropogenic and natural sources in Chandragiri, KTM and few of the metal ions were deposited from long-range transport mechanisms in Ghorepani, ACA, which signifies the diverse sources of pollution in the study areas. The sampling line-wise variation in thallus chemistry signifies the local pollution gradient in both sites. Further, environmental covariables (slope, elevation, crown settings, wind pattern) were observed to affect the lichen abundance and accumulation of nitrogen and metal ions. In comparison, Hypotrachyna spp. showed greater potential to accumulate pollutants and variability in physico-chemical and oxidative responses. From this study, we conclude that a range of physico-chemical and biochemical responses of the target lichens can be used as proxies for the bioindication of nitrogen and metal ion pollution to assess lichen’s health and ecological functioning. Wider studies covering large spatial extent and cellular mechanisms of lichen response are now recommended to fully understand the functional biology explaining contrasting responses between lichen species in different geographic settings of Nepal and South Asia.

 

Keywords: Lichens; Bioindicators; Pollution; Ecosystem; Reference