Chromium Hyper-Tolerant sp. MH778713 Assists Phytoremediation of Heavy Metals by Mesquite Trees ().

Heavy metal accumulation in mesquite trees () growing in aluminum, titanium, chromium and zirconium-polluted soils of a semi-arid region in Mexico was investigated using wavelength dispersive X-ray fluorescence analysis. The results showed that trees can hyper accumulate up to 4100 mg/kg of Al, 14000 mg/kg of Fe, 1600 mg/kg of Ti, 2500 mg/kg of Zn, but not chromium, regarding high chromium concentrations found in soils (435 mg/kg). Since plant-associated microorganism can modulate phytoremediation efficiency, the biodiversity of associated bacteria was studied. Eighty-eight isolates from nodules were obtained; all isolates tolerated high concentrations of Al, Fe, Zn and Cr . The top-six chromium tolerant strains were identified by 16S rRNA sequence analysis as belonging to genus . sp. MH778713, close to group, showed to be the most resistant strain, tolerating up to 15000 mg/L Cr (VI) and 10000 mg/L of Al. Regarding the bioaccumulation traits, sp. MH778713 accumulated up to 100 mg Cr(VI)/g of cells when it was exposed to 1474 mg/L of Cr VI. To assess sp. MH778713 ability to assist phytoremediation; twenty plants were inoculated or non-inoculated with sp. MH778713 and grown in nitrogen-free Jensen's medium added with 0, 10 and 25 mg/L of Cr(VI). Only plants inoculated with sp. grew in the presence of chromium showing the ability of this strain to assist chromium phytoremediation. and spp. may be considered as good candidates for soil restoration of arid and semiarid sites contaminated with heavy metals.