Effects of metal speciation on metal plant dynamics in the presence of plant growth promoting bacteria
Adele, Nyekachi Chituru
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Excessive metal deposition in soil is of major concern to the environment due to the toxicity of metals to animals and plants. Since metals do not degrade, reducing risk of exposure relies in either removing the metals from soil, or changing their speciation which leads to changes in bioavailability, mobility and toxicity. Plants have been shown to provide a cheap alternative to chemical methods for both removing and changing metal speciation, particularly when augmented with plant growth promoting bacteria. The focus of this thesis was to investigate whether the form (speciation) in which a metal contaminant is introduced to soil affects both plant health and the efficiency of metal remediation by the plant, using the well-known hyperaccumulator Brassica juncea (L.) Czern and zinc (Zn) as the metal contaminant. This study also examined the role of plant growth promoting bacteria in changing metal speciation, impact on metal toxicity and phytoremediation efficiency. Brassica juncea was grown in pots containing soil spiked with equal amounts (600 mg Zn kg-1) of soluble Zn (ZnSO4) and nanoparticulate ZnS and ZnO. Plant height, number of leaves, root length, plant biomass and chlorophyll content of Brassica juncea were used to assess Zn toxicity. Zn localisation and speciation in soil and plant tissues was studied using transmission electron microscopy (TEM), synchrotron micro-X-ray fluorescence elemental mapping (μXRF) and synchrotron X-ray absorption spectroscopy (XAS). Growth parameters showed that ZnSO4 was the most toxic form of Zn whilst ZnS and ZnO effects were not statistically different. These differences were linked to differences in Zn content in root and shoot biomass, which was higher in ZnSO4 treatments. Inoculation with Rhizobium leguminosarum and Pseudomonas brassicacearum enhanced plant growth, Zn concentration in plant biomass and translocation of Zn in all Zn treatments. XAS analysis showed that Zn speciation was altered in roots of plants inoculated with bacteria, with Zn cysteine as the most dominant form of Zn in all inoculated Zn treatments, suggesting a role for cysteine in ameliorating Zn toxicity. By also assessing Zn speciation changes across the soilrhizosphere- plant interface, this study established that Rhizobium leguminosarum modified Zn speciation at the rhizosphere. Through this thesis work, metal speciation is a major factor in determining the efficiency of metal phytoremediation and plant tolerance. Hence, this research provides useful information on Zn speciation which will contribute to effective implementation of Zn phytoremediation.