Mobilisation of Nickel by hyperaccumulating plants

In areas, where nickel-containing serpentine minerals occur close to the surface, Nickel-rich soils have developed. Due to the high nickel concentration in these soils a specific plant species community adapted to the local conditions can be typically found. A certain group of plant species has developed a very specific strategy, i.e. accumulating nickel in the leav es. These plants are called hyperaccumulators. Whereas some processes involved in uptake and storage of nickel are already to some extent clarified, processes of nickel mobilization in root-near soil are still largely unknown. Since only a small fraction of nickel in soil is soluble (and thus bioavailable), it is likely that root-released compounds contribute to the solubilization of nickel. The characteristics of these compounds, so-called root exudates, and their role in nickel mobilization are not known yet. Thus, the aim of this project is to collect and characterize the root exudates of nickel hyperacumulators grown in specific growth containers, so-called rhizoboxes. For clarifying their effect on nickel mobilsation in soil, a novel technology of isotope fractionation will be applied. Isotopes are elemental varieties with the same chemical characteristics, but different masses. Since the reaction speed in chemical or physical processes can differ between isotopes, small but detectable changes in the isotopic composition may occur. By analyzing the isotopic composition of nickel and iron it will be tried to identify the source, i.e. the soil chemical fraction of nickel, mobilized by the plants. A further aspect investigated in this project is the contribution of soil microorganisms to nickel mobilization and plant uptake. Whereas the contribution of soil bacteria on plant growth and nickel uptake is already known, the role of fungi in soil, on the root surface or in root tissues is still unknown and will therefore be investigated in this project. The project is an interdisciplinary collaboration between two Austrian Universities (Universität für Bodenkultur Wien, Montanuniversität Leoben), the CNRS in Pau (Frankreich) and the Jagiellonian University in Krakw (Poland), funded by the FWF (P34719).

Plant Ni uptake in aboveground biomass exceeding concentrations of 1000 g g1 in dry weight is defined as Ni hyperaccumulation. Whether hyperaccumulators are capable of mobilizing larger Ni pools than non-accumulators is still debated and rhizosphere processes are still largely unknown. The aim of this project was to investigate rhizosphere processes and possible Ni mobilization by the Ni hyperaccumulator Odontarrhena chalcidica and to test Ni uptake in relation to a soil Ni gradient. We could show that Ni and Fe concentrations, pH as well as DOC concentrations in pore water were significantly increased by O. chalcidica compared to unplanted soils. A positive correlation between Ni in shoots and pseudo-total concentrations and pH in soil was observed, although plant Ni concentrations did not clearly show the same linear pattern with soil available Ni. This implicates a clear root-induced Ni and Fe mobilization in the rhizosphere of O. chalcidica and suggests a rhizosphere mechanism based on soil alkalinization and exudation of organic ligands. Furthermore, it was demonstrated that soil pH and pseudo-total Ni are better predictors of Ni plant uptake in O. chalcidica than labile soil Ni. We further investigated the processes controlling the acquisition of soil-borne Ni by hyperaccumulators, particularly in relation to root-induced changes of rhizosphere chemistry. Using in situ, high-resolution chemical imaging via planar optodes and diffusive gradients in thin-films (DGT) combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we investigated localized changes in the spatial distribution of pH along with Ni, iron (Fe), zinc (Zn), and phosphorus (P) availability in the rhizosphere of the Ni hyperaccumulator Odontarrhena chalcidica grown on two ultramafic soils differing in pH as well as total and extractable Ni. Significant rhizosphere alkalinization of up to 1.5pH units was observed at the immediate root surface in both soils, indicating root exudation of alkaline compounds. While Ni, Fe, and P fluxes were generally depleted around roots, increased Ni fluxes were observed only at root tips in the lower-Ni soil, highlighting a distinct biogeochemical niche for Ni mobilization. In contrast, increased Zn fluxes were observed consistently, irrespective of the soil or root type, revealing a previously unrecognized process for enhanced Zn availability in the rhizosphere. These findings suggest that O. chalcidica employs a highly selective rhizosphere modification strategy, combining pH shifts with element-specific mobilization mechanisms to control trace element availability and plant uptake.