Abstract
Environmental contamination due to human activities is a worldwide problem that has led to the development of different remediation techniques, including biotechnological approaches such as phytoextraction and phytostabilization. These techniques take advantage of pioneer plants that naturally develop tolerance mechanisms to survive in extreme environments. A multi-technique and multi-disciplinary approach was applied for the investigation of Helichrysum microphyllum subsp. tyrrhenicum samples, bulk soil, and rhizospheres collected from a metal-extreme environment (Zn-Pb mine of Campo Pisano, SW Sardinia, Italy). Zinc, Pb, and Cd are the most abundant metals, with Zn attaining 3 w/w% in the rhizosphere solid materials, inducing oxidative stress in the roots as revealed by infrared microspectroscopy (IR). X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis coupled with synchrotron radiation-based (SR) techniques demonstrate that quartz, dolomite, and weddellite biominerals precipitate in roots, stems, and leaves, likely as a response to environmental stress. In the rhizosphere, Zn chemical speciation is mainly related to the Zn ore minerals (smithsonite and hydrozincite) whereas, in plant tissues, Zn is primarily bound to organic compounds such as malate, cysteine, and histidine molecules that act as metal binders and, eventually, detoxification agents for the Zn excess. These findings suggest that H. microphyllum subsp. tyrrhenicum has developed its own adaptation strategy to survive in polluted substrates, making it a potential candidate for phytostabilization aimed at mitigating the dispersion of metals in the surrounding areas.
Highlights
Anthropogenic activities, such as the production of municipal wastes, burning of fossil fuels, mining activities, use of pesticides, and fertilizers in agricultural practice, etc., [1], are a source of contamination by harmful elements that can disperse into the atmospheric, terrestrial, and aquatic ecosystems [2,3,4]
The chemical analysis of bulk soils and rhizosphere materials (Table 2) from the mine dump (CP samples) showed that metal contents (Zn 24,900–26,300 mg/kg, Pb 5000–5030 mg/kg, Cd 100–170 mg/kg) are extremely well above the threshold limits imposed by Italian laws (D.lgs. 152/2006 [90])
Zn and Pb concentrations in these area (Zn 27,300–32,700 mg/kg, Pb 1240–1600 mg/kg) are higher than median values estimated on stream sediments of the Iglesiente mining district by Boni et al [92], that can be assumed as the local post-mining geochemical-baseline-values
Summary
Anthropogenic activities, such as the production of municipal wastes, burning of fossil fuels, mining activities, use of pesticides, and fertilizers in agricultural practice, etc., [1], are a source of contamination by harmful elements that can disperse into the atmospheric, terrestrial, and aquatic ecosystems [2,3,4]. This environmental threat has resulted in the institution of different legislation controls and in the development of several types of remediation actions that include chemical, physical, and biological methods [5,6,7,8]. To support their resilient behavior, pioneer plants could be able to: (i) sequester metals in organs or subcellular compartments (i.e., vacuoles) with little or no sensitive metabolic activity; (ii) chelate metals with exudates, phytochelatins, and peptides; (iii) mediate biomineral formation [11,12,13,14]
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