Impact of heavy metals on physiological health of lichens growing in differently polluted areas of central Assam, North East India

Accumulation of heavy metals on naturally grown epiphytic lichens and its impact on their physiological integrity was examined. Lichen species collected from differently polluted areas (oil refinery, roadside) were compared with the species collected from relatively lesser polluted areas. Heavy metal buildup in lichens were found to have significant damaging effect on their physiological health. Pb, Cd, Zn, Cu, Co, Ni and Cr concentrations were found to be higher in the lichen species collected from both the polluted areas. Metal accumulation in lichen thallus has positive correlation at p ≤ 0.01 (r = 0.417 to 0.772) with the recorded protein concentrations. Whereas, other studied parameters like chlorophyll and carotenoid concentrations, membrane stability and pH documented negative correlation with the accumulated heavy metals. However, abundance of species Dirinaria applanata and Dirinaria sp. at polluted areas with better protein content indicates their tolerance towards heavy metal. Chlorophyll degradation and protein contents in the lichens were found to be efficient physiological parameters to correlate with the air quality of a region. Sensitivity to heavy metals may be the cause of sparsity of species Arthonia cinnabarina, Cryptothecia subnidulans and Lecanora sp. in the polluted areas. However, lesser sensitivity to heavy metals favored the growth of species Dirinaria applanata and Dirinaria sp. even in higher polluted areas.