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

Verónica Ramírez,Ricardo Carreño,José Luis Contreras,José Antonio Munive,Jesús Muñoz-Rojas,Miguel Ángel Villalobos,Primavera López,Rocío Bustillos,Antonino Baez,Javier Martínez,Luis Ernesto Fuentes

doi:10.3389/fmicb.2019.01833

Abstract

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

Highlights

With the onset of fast-developing industries, farming, energy stations and the inappropriate waste disposal practices, soil and water have been deeply contaminated with organic compounds and heavy metals with permanent toxic effects on ecosystems and human health (Kalbitz and Wennrich, 1998; Claret et al, 2011; Mahar et al, 2016)
Prosopis laevigata wildlife trees grown in a heavy metal-polluted landform were analyzed for their heavy-metal bioaccumulation
Since the interaction of metal-tolerant bacteria with hyperaccumulating plants can improve phytoremediation, we looked for Fe, Zn, Cr-tolerant strains by growing each of the 88 nodule-isolated bacteria with increasing concentrations of each metal in Yeast Mannitol Agar (YMA) plates

Summary

Introduction

With the onset of fast-developing industries, farming, energy stations and the inappropriate waste disposal practices, soil and water have been deeply contaminated with organic compounds and heavy metals with permanent toxic effects on ecosystems and human health (Kalbitz and Wennrich, 1998; Claret et al, 2011; Mahar et al, 2016). The vulnerability of ecosystems to contaminants is closely related to water flow (Everts, 1997), so that the low rainfall, characteristic of arid and semiarid zones, make these ecosystems affected by the presence of toxic elements. Heavy metal is the generic name given to the group of elements with an atomic density greater than 6 g/cm, ubiquitous in the Earth’s crust. These metals include lead (Pb), cadmium (Cd), nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn), chromium (Cr), silver (Ag), titanium (Ti) and copper (Cu). The most stable and abundant forms of chromium in nature are chromium (III) and chromium (VI), the latter being the most toxic (Kotas and Stasicka, 2000)

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