Si, Al and Zn biomineralization processes in plant roots of Euphorbia pithyusa

In the abandoned mine of Ingurtosu (SW Sardinia, Italy) a field experimental trial was carried out in the frame of the UMBRELLA project (7FP EU). The idea of this project was to couple microorganisms with plants to favour metals accumulation in plants and reduce the effects of metal pollution in soils of heavy metals contaminated sites. Euphorbia pithyusa was selected as an endemic pioneer plant. About two years after the beginning of the experiment, exploration of the processes occurring at the soil-root interface was carried out by combing X-ray fluorescence mapping (S-XRF) and X-ray absorption spectrometry (XAS) to X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). We observed that Si and Al are mainly concentrated in the epidermis of the roots forming a rim that represent a physico-chemical barrier against organic and inorganic stresses. In addition, Si is able to bind Zn by forming an amorphous Zn-silicate in the epidermis zone. This process leads to the decrease of Zn concentration from the epidermis to the internal part of the root, where Zn is bonded to other Zn atoms. In the case of Fe, the Si-Al rim seems to be more efficient and, as observed in LEXRF maps, Fe is mainly concentrated in the external zone of the roots. EXAFS analysis reveals that the coordination environment of Fe is comparable to that of hematite. The exact cause of Fe precipitation around roots or in the epidermis zone is unknown, but the oxidizing activity of roots, together with the action of microorganisms, is thought to be involved. Si and Al particles of the rim can act as a nucleus for hematite precipitation (Batty et al., 2000) that, in turn, could act as a barrier against metals penetration. Although Si and Al are not regarded as essential nutrients and the later is an important growth-limiting factor (Rout et al., 2001), obtained results suggest that these elements are fundamental for controlling metal migration through the roots, and their beneficial function for plant need to be reassessed. The addition of microorganisms to the soil at the Ingurtosu mine seems to have no effect on biominerals features, and on Zn and Fe speciation. Our results can be useful for environmental science because biomineralization in plants can lead to development of specific phytostabilization techniques, reducing the mobility of the contaminants and prevents their migration to the groundwater or air, thus decreasing bioavailability for entry into the food chain.