Effects of metal speciation on metal plant dynamics in the presence of plant growth promoting bacteria

Excessive metal deposition in soil is of major concern to the environment due to the \ntoxicity of metals to animals and plants. Since metals do not degrade, reducing risk \nof exposure relies in either removing the metals from soil, or changing their \nspeciation which leads to changes in bioavailability, mobility and toxicity. Plants have \nbeen shown to provide a cheap alternative to chemical methods for both removing \nand changing metal speciation, particularly when augmented with plant growth \npromoting bacteria. The focus of this thesis was to investigate whether the form \n(speciation) in which a metal contaminant is introduced to soil affects both plant \nhealth and the efficiency of metal remediation by the plant, using the well-known \nhyperaccumulator Brassica juncea (L.) Czern and zinc (Zn) as the metal contaminant. \nThis study also examined the role of plant growth promoting bacteria in changing \nmetal speciation, impact on metal toxicity and phytoremediation efficiency. Brassica \njuncea was grown in pots containing soil spiked with equal amounts (600 mg Zn kg-1) \nof soluble Zn (ZnSO4) and nanoparticulate ZnS and ZnO. Plant height, number of \nleaves, root length, plant biomass and chlorophyll content of Brassica juncea were \nused to assess Zn toxicity. Zn localisation and speciation in soil and plant tissues was \nstudied using transmission electron microscopy (TEM), synchrotron micro-X-ray \nfluorescence elemental mapping (μXRF) and synchrotron X-ray absorption \nspectroscopy (XAS). Growth parameters showed that ZnSO4 was the most toxic form \nof Zn whilst ZnS and ZnO effects were not statistically different. These differences \nwere linked to differences in Zn content in root and shoot biomass, which was higher \nin ZnSO4 treatments. Inoculation with Rhizobium leguminosarum and Pseudomonas \nbrassicacearum enhanced plant growth, Zn concentration in plant biomass and \ntranslocation of Zn in all Zn treatments. XAS analysis showed that Zn speciation was \naltered in roots of plants inoculated with bacteria, with Zn cysteine as the most \ndominant form of Zn in all inoculated Zn treatments, suggesting a role for cysteine in \nameliorating Zn toxicity. By also assessing Zn speciation changes across the soilrhizosphere- \nplant interface, this study established that Rhizobium leguminosarum \nmodified Zn speciation at the rhizosphere. Through this thesis work, metal speciation \nis a major factor in determining the efficiency of metal phytoremediation and plant \ntolerance. Hence, this research provides useful information on Zn speciation which \nwill contribute to effective implementation of Zn phytoremediation.