Active transplantation study of nickel uptake by <i>Pyxine cocoes</i> (Sw.) Nyl.: prospection for atmospheric nickel biomonitoring

The prevalence of nickel pollution is anticipated to rise due to the advent of novel low-carbon technologies and electric vehicles. Biomonitoring, which is increasingly overlooked in favour of technology-driven methods, remains a cost-effective approach and enables the monitoring of extensive spatial areas. In the present study, <i>Pyxine cocoes</i> (<i>P. cocoes</i>), a symbiotic lichen, was examined for the first time for its capacity to uptake sprayed nickel (Ni) <i>in vivo</i> and the subsequent effects on its physicochemical parameters. Transplanted <i>P. cocoes</i> was treated with different concentrations of Ni solutions (5 µM, 50 µM, 100 µM, 150 µM, 200 µM). The lichen, <i>P. cocoes,</i> had the capacity to accumulate Ni linearly in a dose- and time-dependent manner. The effect of Ni on photosynthetic parameters, cell membrane integrity, antioxidants and protein content was quantified corresponding to concentrations and durations of treatment. At a low dose (5 µM), a beneficial effect was observed on chlorophyll-a, chlorophyll-b, total chlorophyll and protein content in <i>P. cocoes</i>. At higher doses of Ni (150 µM, 200 µM), it exhibited an inhibitory effect as observed by reduced photosynthetic parameters and antioxidant activity. Cell membrane integrity (CMI) deteriorated in response to increasing Ni exposure, as indicated by increased electrolyte conductivity. Using the linear regression coefficient, it was determined that at lower Ni concentrations, the adsorption kinetics followed pseudo-second-order (chemisorption) and, at higher concentrations, it followed pseudo-first-order kinetics (physisorption). This active (transplant) monitoring method is a novel endeavour in monitoring Ni stress and utilising the physicochemical parameters as a bioindicator for Ni pollution.