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ORIGINAL ARTICLE

Uptake and release of copper ions in epiphytic lichens Stefano Loppi 1 & Alessio Di Lucia 1 & Andrea Vannini 1 & Stefania Ancora 1 & Fabrizio Monaci 1 & Luca Paoli 2 Received: 26 March 2020 / Accepted: 12 May 2020 # Plant Science and Biodiversity Centre, Slovak Academy of Sciences 2020

Abstract This study investigated the uptake and release of Cu ions in the epiphytic lichen Evernia prunastri, a species widely used to monitor trace metal pollution. A cross transplant experiment from a background area to a polluted one and then back to an unpolluted one was simulated by incubating lichen thalli with 10 and 100 µM Cu2+ solutions and subsequently in deionized water to induce metal release. The working hypothesis was that after the accumulation of Cu ions, a forced release brings the concentration back to the original values. Copper treatment caused a significant uptake, proportional to the concentrations in the solution. Accumulation occurred mostly extracellularly (90% after incubation with Cu 10 µM and 60% with Cu 100 µM). The subsequent induced release was evident only in samples treated with 100 µM, and was determined by the loss from intracellular compartments. Lichen vitality, expressed in terms of photosynthetic efficiency, was not affected by Cu treatment. It is concluded that while ionic uptake is a fast process, metal release requires a much longer time. In addition, it is confirmed that the cell wall is a buffer between the outer environment and the cell interior. Keywords Bioaccumulation . Biomonitoring . Evernia prunastri . Heavy metals . Photosynthetic efficiency

Introduction Lichens colonizing rock surfaces may accumulate heavy metals directly from the substrate, depending on their growth form and the degree of thallus adhesion to the substrate (Rola et al. 2016). On the other hand, epiphytic (tree inhabiting) lichens have a remarkable ability to take up and accumulate persistent pollutants from the atmosphere and are widely used to monitor heavy metal pollution (Loppi 2014). Element accumulation may occur by three mechanisms: deposition of particles, extracellular cation exchange, intracellular complexation, the relative proportions being dependent, besides element characteristics, on environmental conditions, cation exchange capacity (CEC) and physiological interactions (Bačkor and Loppi 2009). Accumulation of elements in ionic form offers an estimate of recent bioavailable environmental inputs and its evaluation is thus of paramount importance in biomonitoring research (Puckett et al. 1973; Paoli et al. 2018a). The concentration of elements taken up as ions basically

* Stefano Loppi [email protected] 1

Department of Life Sciences, University of Siena, Siena, Italy

2

Department of Biology, University of Pisa, Pisa, Italy

depends on environmental availability, CEC as well as mechanisms of accumulation and disaccumulation (Paoli et al. 2018a). Uptake and release of heavy metals by lichen thalli are reversible processes and lichens tend to reach an equilibrium with the surro