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

Abstract

Heavy metal accumulation in mesquite trees (<i>Prosopis laevigata</i>) 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 <i>P. laevigata</i> 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 <i>P. laevigata</i> associated bacteria was studied. Eighty-eight isolates from <i>P. laevigata</i> nodules were obtained; all isolates tolerated high concentrations of Al, Fe, Zn and Cr <i>in vitro</i>. The top-six chromium tolerant strains were identified by 16S rRNA sequence analysis as belonging to genus <i>Bacillus</i>. <i>Bacillus</i> sp. MH778713, close to <i>Bacillus cereus</i> 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, <i>Bacillus</i> 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 <i>Bacillus</i> sp. MH778713 ability to assist <i>Prosopis laevigata</i> phytoremediation; twenty plants were inoculated or non-inoculated with <i>Bacillus</i> 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 <i>Bacillus</i> sp. grew in the presence of chromium showing the ability of this strain to assist chromium phytoremediation. <i>P. laevigata</i> and <i>Bacillus</i> spp. may be considered as good candidates for soil restoration of arid and semiarid sites contaminated with heavy metals.