Browsing by Subject "Mercury"

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Ecological and molecular characteristics of arbuscular mycorrhizal fungi (AMF) on mercury phytoremediation
(2023) Guo, Yaqin; Frank, Rasche
Environmental pollution caused by harmful chemicals represents a major challenge worldwide. Among these, heavy metals (HM) in soils are of particular concern due to their persistence, toxicity, and bioaccumulation which can significantly threaten human health, plant growth, and ecosystem integrity. Phytoremediation, which uses plants to extract pollutants from soils, has been recognized as a promising approach to remediate HM-contaminated soils. Arbuscular mycorrhizal fungi (AMF)-assisted phytoremediation has shown great potential to enhance plant growth and metal uptake by forming a mutual association between plant roots and AMF, which can improve nutrient uptake and tolerance to environmental stress. Despite its potential, however, the effectiveness of this approach can be limited by various factors, such as environmental and geographic factors, soil properties, and plant-microbe interactions. An advanced fundamental understanding of both ecological and molecular characteristics of this technology is thus crucial to improve its effectiveness and application potential. Therefore, the impetus of this doctoral thesis was to investigate the potential of AMF in phytoremediation of soils contaminated with HM, with a particular focus on mercury (Hg) remediation. The first study (Chapter 2) contributes to the ecological understanding of AMF in a degraded ecosystem. In this study, two geographically distinct, abandoned gold mining locations in Ghana were selected and the genetic diversity and composition of AMF communities both in the rhizosphere and roots of the pioneer plant Pueraria phaseoloides (Roxb.) Benth. (tropical kudzu) were analyzed using a metagenomic sequencing approach. To determine the primary factor shaping AMF communities, both biotic (plant identity) and abiotic factors (geographic locations and soil conditions) were examined. In total, 195 amplicon sequence variants (ASV) affiliated to eight genera of the phylum Glomeromycota were identified. The root compartment showed a lower diversity than the rhizosphere soils and a difference of AMF compositions between the two compartments was detected irrespective of geographical location. Moreover, co-occurrence network analysis revealed two different keystone species in the two compartments, i.e., Acaulospora in rhizosphere soil and Rhizophagus in roots. The high abundance of Rhizophagus in the roots of P. phaseoloides was the result of a good match of functions between plant and AMF. Collectively, the results indicated that plant compartment (root versus rhizosphere) is the main factor shaping AMF communities associated with P. phaseoloides. The second study (Chapter 3) comprises a research synthesis of the role of AMF in zinc (Zn), cadmium (Cd), and Hg bioremediation. The study assumed that mycorrhization plays a role in modulating the uptake of Hg, facilitated by Zn and/or Cd transporters. The synthesis demonstrated that AMF have the ability to regulate the transporters responsible for Zn and Cd uptake and transport, such as ZIP (zinc-iron permease or ZRT-IRT-like protein), CDF (cation diffusion facilitator), NRAMP (natural resistance-associated macrophage proteins), and HMA (heavy metal ATPase). This regulation can either enhance or inhibit the uptake and transport of Zn or Cd. The extent of this regulation is influenced by multiple factors, such as the plant species, the species of AMF involved, and soil conditions, including pH and elements such as phosphorus (P). It was concluded that future research is needed to investigate the optimal environmental conditions under which AMF are effective in Hg remediation for appropriate application. The third study (Chapter 4) offers essential insights into the distinct functions of AMF symbiosis in Hg partitioning in plants. This relationship was assessed in the context of Zn uptake mechanisms and the expression of two Zn transporter genes (ZIP2 and ZIP6). Zn is crucial for plants and has a similar outer electronic configuration as Hg, which implies a potential competition for the same transporters. In a greenhouse experiment, plants of Medicago truncatula were exposed to different Hg concentrations with and without inoculation of the AMF species Rhizophagus irregularis. This study demonstrated that mycorrhizal symbiosis improved plant Hg tolerance under Hg exposure, but the specific roles of mycorrhizal symbiosis in Hg partitioning depended on Hg concentrations in the substrate. A negative relationship between Hg and Zn concentrations in roots was observed, although the expression of Zn transporters (ZIP2 and ZIP6) by mycorrhizal inoculation was upregulated irrespective of Hg concentrations in the substrate. More importantly, mycorrhizal colonization reduced Hg concentrations in leaves compared to controls, regardless of Hg concentrations in the substrate. This study demonstrated that mycorrhizal symbiosis influences Hg uptake in M. truncatula and highlights the importance of AMF in phytoremediation. Overall, this doctoral thesis extends the understanding of AMF in phytoremediation with insights from both ecological and molecular perspectives and provides a knowledge basis to realize the potential and implementation of this technology.