Symbiotic effects of the fungus Glomus sp. on chromium(III), chromium(VI), and lead(II) uptake by mesquite (Prosopis sp.): A novel method to remediate heavy metals.

摘要

The indiscriminate disposal of lead (Pb) and chromium (Cr) containing wastes has resulted in a detrimental impact on environmental health. Lead and Cr are in the EPA priority list due to their degree of toxicity on humans, animals, plants, and other living organisms. The removal of toxic wastes involves technologies that are expensive, sometimes incomplete, and could represent health hazards. In addition, site restoration after contaminant removal encompasses time and technological resources. Phytoremediation, the use of plants alone or associated with microorganisms, represents a potentially cost-effective option in the long term management of contaminated sites. Various plants known as "hyperaccumulators" have shown the ability to remove toxic elements at high levels that they are being used to restore contaminated sites. In addition, Arbuscular Mycorrhizal (AM) fungi have proven to help plants to survive and increase the adsorption of toxic elements from heavily polluted sites. Previous reports indicated that Mesquite (Prosopis sp.) is a very robust desert plant species with high tolerance and uptake capacity for toxic metals/metalloids. It is also known that this leguminous species grows associated to microorganisms. However, there is no information about the contribution of AM fungi on metals/metalloids uptake capacity of Mesquite. In this investigation the Cr and Pb uptake capability of Mesquite associated with the AM fungus Glomus deserticola was studied. This research was completed in three phases. In phase I, five species of Glomus were treated with Cr(III), Cr(VI), and Pb(II) at 0, 10, 20, 40, 80 and 160 mg L-1 in the agar-based Murashige and Skoog nutrient medium. After 30 days of incubation, the metal tolerance of Glomus species was evaluated through mycelia growth and the metal uptake capacity through inductively coupled plasma-optical emission spectroscopy (ICP-OES). In phase II, Mesquite plants associated with Glomus deserticola (identified in phase I as the most promising species), were treated for 15 days in hydroponics with Pb at 0, 10, 50, or 100 mgL-1, and Cr(III) and (VI) at 0, 20, 40, 75, or 125 mg Cr L-1. Plant stress was evaluated by using total amylase activity (TAA). The metal uptake and metal tissue distribution was determined through ICP-OES, electron scanning microprobe (x-ray mapping) and transmission electron microscopy. In phase III, uninoculated and inoculated Mesquite plants with G. deserticola or amended with EDTA were grown for 30 days in soil containing Cr(III), Cr(VI) or Pb at 0, 40, 80, and 160 mg L-1. Evapotranspiration and TAA were monitored as stress indicators. The fungus experiment showed that G. deserticola had higher capabilities to grow and uptake Pb and Cr, becoming a potential candidate for future research. The hydroponic experiment showed that all Cr ions and the highest Pb concentrations reduced shoot size compared to control samples. Toxic effects (yellowish leaves, leaf decay) were observed after seven days of treatment. However, Pb and Cr(III) treated plants recovered upon conclusion of the experimental period. The TAA in leaves increased upon the addition of Pb and Cr. The ICP-OES results showed that plants treated with Pb at 50 mgL-1 accumulated in roots, stems, and leaves: 61947, 9584, and 478 mg Pb kg-1; whereas plants treated with Cr(III) and Cr(VI) at 125mgL-1 accumulated 28815, 6055, and 647; and 13767, 5010, and 2530 mg Cr kg-1, respectively. TEM micrographs showed the presence of G. deserticola within roots and the x-ray mapping demonstrated higher Cr and Pb deposition in xylem and phloem cells. Results showed that G. deserticola improved metal tolerance/accumulation in mesquite. The soil experiment demonstrated that AM inoculated Cr(VI) and Pb treated plants had in roots 21% and 30% more Cr and 142% and 112% more Pb than uninoculated and EDTA treated roots, respectively, at 80 ppm treatment. In the case of Cr(III), EDTA produced the highest Cr accumulation in roots. TAA was higher in inoculated plants grown with Cr(III) at 80 and 160 ppm and Cr(VI) at 40 and 160 ppm, while evapotranspiration was higher in control plants without/with fungus. TEM micrographs corroborated the presence of G. deserticola in roots and stems and the x-ray mapping showed higher metal concentrations in the vascular system of inoculated plants. These results corroborated (1) the heavy metal phytoremediation capabilities of mesquite, (2) the increase in the uptake and translocation of Cr and Pb by AM fungus G. deserticola, and (3) the differential distribution and accumulation of Cr and Pb within plant tissues.