Impact of Zn excess on biomineralization processes in Juncus acutus grown in mine polluted sites

Juncus acutus, an halophite plant pioneer in extremely polluted mine areas, was harvested in three different locations of Sardinia (Italy), having Zn soil concentration up to 80g/kg, and Zn water concentration ranging between 10^-3g/L and 10^-1g/L. Rhizosphere and plant samples were investigated combining X-ray microscopy (XM)/spectroscopy (XAFS) and infrared microspectroscopy (FTIR) to elucidate the chemical composition, (bio)mineralogy and Zn coordinative environment. The multi-technique approach allowed recognizing different biomineralization processes, and Zn complexes in the plant tissues. The Zn chemical environment in root biominerals is multi-phase and, depending on the sampling site, can comprise amorphous Zn silicate, Zn apatite, hydrozincite, and Zn sulphate. Zn cysteine and Zn histidine, complexes quoted as part of a detoxification strategy, were found mainly in plants from the site where the Zn water concentration has the highest values. This different site-specific mode of Zn biomineralization has relevant implications for phytoremediation techniques and for further biotechnology development, which can be better designed and developed after knowledge of site-specific-molecular processes ruling mineral evolution and biomineralization. Carboxylic groups and organic compounds (lignin, cellulose, hemicellulose, pectin and esters) were identified by FTIR analysis, thought the Zn speciation is not apparently linked to these carboxylic group rich biopolymers.