INFLUENCE OF SIDEROPHORE PRODUCING BACTERIA AND ORGANIC LIGANDS ON PHASE DISTRIBUTION OF CADMIUM AND ITS UPTAKE BY BRASSICA NAPUS IN THE PRESENCE OF GOETHITE
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Cadmium is a non essential trace element, which is toxic at very low concentrations. While Cd can occur at high concentrations as a result of the weathering of Cd-rich minerals, most high concentrations are of human origin. Brassica have been shown to be hyper-accumulators of cadmium and are therefore good candidates for the application of phytoremediation to remediate soils with Cd contamination. The processes leading to successful phytoremediation are not fully understood, and may be dependant upon the bacterial community present in the rhizosphere. Bacteria in the vicinity of plant roots may degrade plant root exudates that would otherwise act to bind metals and bacteria can produce metal binding ligands such as siderophores. This study was conducted to determine if the presence of siderophore producing bacteria will affect cadmium uptake by Brassica napus (canola) in a model system.
An apparatus was designed that included the dominant components of a phytoextraction system, a solid phase (modeled using ?-FeOOH, goethite), a liquid matrix (chemically defined MMS-2 medium in which metal speciation may be calculated), a plant (B. napus) and a siderophore producing bacterium (Burkholderia cepacia). Physical separation of the plant from the Fe was needed to discern plant uptake of Cd and was accomplished by utilization of dialysis membranes. Components of the system were studied independently before combining for short term plant uptake experiments.
The system was kept at a pH of 6.4 to model a typical soil pH. Studies of Cd adsorption on ?-FeOOH in MMS-2 revealed that adsorption was fast, and that desorption was biphasic and incomplete within the time frame of the experiment. A linear equilibrium adsorption isotherm for Cd on ?-FeOOH was developed with a sorption constant, Kd, of 2600 mL/g. A linear sorption relationship was also developed for Cd sorption by the walls of the glass reactor vessel, with a volumetric sorption constant, Kg (unitless), which is proportional to the surface area to volume ratio at a given volume.
Bacterial growth was measured at two initial substrate concentrations, 68 and 272 mg/L dextrose added to the MMS-2 media. Stationary phase was achieved in approximately 40 hours after inoculation. Siderophore production by B. cepacia was measured by the Chrome Azurol Sulfonate (CAS) technique, and yielded a maximum concentration of 39 ?M for the experiments. Cd speciation, phase distribution, and uptake by B. napus were perturbed in the experimental system by the addition of salicylate, citrate, ethylenediamine tetraacetic acid (EDTA), a suspension of B. cepacia and its associated siderophore, or the supernatant containing the B. cepacia siderophore. Changes in phase distribution of Cd upon addition of ligands with defined Cd binding constants were not consistent with the values predicted by MINEQL+, where CdEDTA>CdCitrate>CdSailcylate. Salicylate desorbed more Cd than citrate, and was chosen along with EDTA for plant uptake experiments. Cell suspensions and siderophore containing supernatant were shown to enhance the adsorption of Cd under the experimental conditions. Addition of plant roots to a ?-FeOOH suspension resulted in a rapid capture of goethite onto the roots, as a result it was impossible to distinguish Cd in the plant root from Cd sorbed to the FeOOH. A dialysis membrane was utilized to separate roots from the Fe solid phase. The presence of the dialysis membrane did not create a detectable change in adsorption of Cd. Plant uptake experiments were conducted over a short time scale. An eight hour equilibration step was utilized after perturbations of Cd-?FeOOH adsorption equilibrium (by addition of ligand) and followed by six hours of plant uptake. Both salicylate and EDTA were effective in increasing the phytotextraction of Cd. Salicylate may have more potential of these two ligands for field application because of the concerns associated with toxicity of and the mobilization of the Cd-EDTA complex. B. cepacia and its supernatant with dissolved siderophores did not improve uptake of Cd by canola, but did desorb more Cd.