Browsing by Subject "Heavy metal"

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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.
Einfluss von Phosphatmangel und erhöhter atmosphärischer CO2-Konzentration auf die Wurzelexsudation und ihre Auswirkungen auf Mobilisierung und Aufnahme von Schwermetallen durch verschiedene Lupinenarten und Tomate
(2011) Kawanishi, Ayumi; Römheld, Volker
There is an increasing awareness of a contamination of the food chain by toxic heavy metals as consequence of anthropogenic induced pollution of the environment since the industrialization in the 18. century. In addition the CO2 concentration might promote the biomass formation of plants and thus, via an increased allocation of photo-assimilates into the roots, chemical changes in the rhizosphere. These changes can promote mobility and uptake of various heavy metals by crop plants, too. Therefore it was the main objective of this Ph.D. research, to study the possible consequences of such observed increase in the atmospheric CO2 concentration on the intensification of the rhizosphere chemistry on the uptake of heavy metals by selected plant species in continuation of the research work by Egle (2003) at the University Göttingen. As plant species various lupinus species and tomato were chosen, which differ in principle in their reaction to a low phosphate nutritional status such as root growth characteristics and secretion of protons and carboxylates. As approach two nutrient solution experiments (Chapter 4 and 5) and a soil experiment with heavy metal polluted soils (Chapter 6) were conducted. In both nutrient solution experiments the well-described root-induced changes such as proton and carboxylate release could be confirmed, which were intensified at higher atmospheric CO2 concentrations (Chapter 4 and 5). Surprisingly the detected increase in proton (tomato) and caboxylate release (particularly by white lupin) with a simultaneously increased mobility of Cu and Cd in the soil did not result in an increased concentration of heavy metals in roots and shoots of the growth experimental plants. The unexpected finding in chapter 6 were discussed in the outlook of chapter 6 (6.7) and a repetition of this experiment with consideration of the discussed aspects is urgently recommended.
Heavy metals from phosphate fertilizers in maize-based food-feed energy systems
(2023) Niño Savala, Andrea Giovanna; Fangmeier, Andreas
The problem of polluted agricultural lands with heavy metals due to anthropogenic activities, including applying phosphorous (P) fertilizers polluted with cadmium (Cd) and other metal such as uranium, has been extensively studied. Several reviews, including the one in the present dissertation, have elaborated this issue with often the same results: the application of P fertilizers with high Cd levels is strongly correlated to Cd accumulation in arable soil, which could imply environmental risks as well as health risks for humans and animals through the food chain. Therefore, these reviews have often the same conclusion: the application of low Cd-P fertilizers, either mineral, organic or recycled, is diminishing the risks of Cd pollution at the soil, crop and consumption level. However, globalization, trade politics, economy, dependency on Morocco mineral P fertilizers, and the finite stock in the raw material have challenged this possibility, especially in the European Union. Meanwhile, in China, polluted arable soils are related to other anthropogenic activities and type of fertilizers rather than Cd-polluted phosphate rock and mineral P fertilizers. At the farm level, other options to diminish Cd pollution in soil and crops, besides low Cd-P fertilizers, could consist of different fertilizer and crop management. These options were studied in this dissertation. A different P management, including different rate applications and placements, did not influence the total Cd concentration in silage maize grown in Germany, regardless of the developmental stage of the crop and the Cd levels in P fertilizer. Silage maize might take up Cd derived from P fertilizers under unpolluted soils, without high risks due to its high biomass production. However, significant changes in the labile Cd fraction were already visible after applying Cd-polluted P fertilizers at 150% of the required amount to the soil after only two growing seasons. Further research should be done to understand the correlations between the bioavailable metal fraction and the actual Cd uptake by silage maize, especially in unpolluted soils. This recommendation also follows the meta- analysis results presented in the second publication, which indicated a possible bias as most of the studies are performed under polluted conditions. Considering the results of the third and fourth publication, the Cd uptake by silage maize was strongly correlated to labile Zn in the soil and the Zn uptake at the early development stage after two field seasons. Placed P fertilizer had a significant and negative effect on the Zn uptake by young silage maize. Further research is needed to understand the behavior of Cd and Zn in the uptake process by maize under P fertilization in unpolluted soils. According to three of the four publications presented in this dissertation, the soil pH was the main soil characteristic influencing the bioavailability and the plant uptake of Cd under unpolluted conditions, regardless of the P treatment, the development stage, and the maizes intended use. However, the total Cd concentration in the soil was the dominant variable for the Cd concentration in maize grain when the soil was polluted with high Cd levels, which was the case in several experiments analyzed in the second publication. P fertilizers with average Cd contamination might enhance labile Cd accumulation in arable land and crops when applied to low biomass crops, such as wheat and legume crops. In this regard, crop management such as crop rotation in the central field experiment indicated that the wheat rotation induced a lower Cd accumulation in maize-soil systems, owing to wheat likely accumulating Cd at higher levels than other crops. The results presented in the second publication also indicated high Cd accumulation by the wheat crop: the wheat grain accumulated more Cd than the maize grain. Thus, potential hazards related to Cd accumulation in wheat grain should also be considered in wheat-maize systems. In conclusion, suitable crop rotations considering the crop-specific potential of Cd accumulation, efficient P management including soil P levels and nutrient use efficiency, and low Cd-P fertilizers remain the most viable options and the main challenge to avoid Cd accumulation in arable soils.
The AtIREGs - characterization of a new family of metal transporters in Arabidopsis thaliana
(2009) Kirchner, Silvia; von Wirén, Nicolaus
Essential transition metals are required in all plant cells for the activities of numerous metal-dependent enzymes and proteins, but can become toxic when present in excess. For the detoxification of heavy metals and to adjust to changes in micronutrient concentrations in the environment, plants possess a tightly controlled metal homeostasis network. In this regard, transition metal transporters are of central importance. Many metal transporters have already been identified, but a large number of candidates for heavy metal transport proteins still have to be analyzed at the biochemical level and within the plant metal homeostasis network. Based on the description of the animal IREG1 metal transporter as an iron exporter in vertebrates, a phylogenetic analysis of eukaryote and prokaryote sequences with similarity to IREG1 showed three homologous genes in Arabidopsis, which were named AtIREG1, AtIREG2 and AtIREG3. As these AtIREG family members were candidates for yet uncharacterized metal transporters, the main objective of this thesis was to investigate the physiological function of this newly identified transporter family in plants.