Presence Of Desferrioxamine B Research Articles

Chelating Effect of Siderophore Desferrioxamine-B on Uranyl Biomineralization Mediated by Shewanella putrefaciens.

In contaminated water and soil, little is known about the role and mechanism of the biometabolic molecule siderophore desferrioxamine-B (DFO) in the biogeochemical cycle of uranium due to complicated coordination and reaction networks. Here, a joint experimental and quantum chemical investigation is carried out to probe the biomineralization of uranyl (UO22+, referred to as U(VI) hereafter) induced by Shewanella putrefaciens (abbreviated as S. putrefaciens) in the presence of DFO and Fe3+ ion. The results show that the production of mineralized solids {hydrogen-uranium mica [H2(UO2)2(PO4)2·8H2O]} via S. putrefaciens binding with UO22+ is inhibited by DFO, which can both chelate preferentially UO22+ to form a U(VI)-DFO complex in solution and seize it from U(VI)-biominerals upon solvation. However, with Fe3+ ion introduced, the strong specificity of DFO binding with Fe3+ causes re-emergence of biomineralization of UO22+ {bassetite [Fe(UO2)2(PO4)2·8(H2O)]} by S. putrefaciens, owing to competitive complexation between Fe3+ and UO22+ for DFO. As DFO possesses three hydroxamic functional groups, it forms hexadentate coordination with Fe3+ and UO22+ ions via these functional groups. The stability of the Fe3+-DFO complex is much higher than that of U(VI)-DFO, resulting in some DFO-released UO22+ to be remobilized by S. putrefaciens. Our finding not only adds to the understanding of the fate of toxic U(VI)-containing substances in the environment and biogeochemical cycles in the future but also suggests the promising potential of utilizing functionalized DFO ligands for uranium processing.

Binding of AraC-Type Activator DesR to the Promoter Region of Vibrio vulnificus Ferrioxamine B Receptor Gene.

Vibrio vulnificus can utilize the xenosiderophore desferrioxamine B (DFOB) as an iron source under iron-restricted conditions. We previously identified in V. vulnificus that transcription of the desA gene encoding the outer membrane receptor for ferrioxamine B (FOXB) is activated by the AraC-type transcriptional regulator encoded by desR together with DFOB. In this study, we overexpressed and purified DesR as a glutathione S-transferase-fused protein and examined interaction between the promoter region of desA and DesR. Electrophoretic mobility shift assay (EMSA) revealed that DesR directly binds to the regulatory region of desA, and this binding was enhanced by the presence of DFOB in a concentration-dependent manner, while the presence of FOXB did not affect the potentiation of their binding. Moreover, EMSA identified that DNA fragments lacking a probable DesR binding sequence were unable to form complexes with DesR. Finally, deoxyribonuclease I footprinting assay demonstrated that the DNA binding sequence of DesR is located between -27 and -50 nucleotides upstream of the desA transcription start site. These results strongly indicate that DesR can directly activate the transcription of desA in cooperation with DFOB, which acts as a coactivator for DesR.

Particulate trace metal dynamics in response to increased CO<sub>2</sub> and iron availability in a coastal mesocosm experiment

Abstract. Rising concentrations of atmospheric carbon dioxide are causing ocean acidification and will influence marine processes and trace metal biogeochemistry. In June 2012, in the Raunefjord (Bergen, Norway), we performed a mesocosm experiment, comprised of a fully factorial design of ambient and elevated pCO2 and/or an addition of the siderophore desferrioxamine B (DFB). In addition, the macronutrient concentrations were manipulated to enhance a bloom of the coccolithophore Emiliania huxleyi. We report the changes in particulate trace metal concentrations during this experiment. Our results show that particulate Ti and Fe were dominated by lithogenic material, while particulate Cu, Co, Mn, Zn, Mo and Cd had a strong biogenic component. Furthermore, significant correlations were found between particulate concentrations of Cu, Co, Zn, Cd, Mn, Mo and P in seawater and phytoplankton biomass (µgC L−1), supporting a significant influence of the bloom in the distribution of these particulate elements. The concentrations of these biogenic metals in the E. huxleyi bloom were ranked as follows: Zn < Cu ≈ Mn < Mo < Co < Cd. Changes in CO2 affected total particulate concentrations and biogenic metal ratios (Me : P) for some metals, while the addition of DFB only significantly affected the concentrations of some particulate metals (mol L−1). Variations in CO2 had the most clear and significant effect on particulate Fe concentrations, decreasing its concentration under high CO2. Indeed, high CO2 and/or DFB promoted the dissolution of particulate Fe, and the presence of this siderophore helped in maintaining high dissolved Fe. This shift between particulate and dissolved Fe concentrations in the presence of DFB, promoted a massive bloom of E. huxleyi in the treatments with ambient CO2. Furthermore, high CO2 decreased the Me : P ratios of Co, Zn and Mn while increasing the Cu : P ratios. These findings support theoretical predictions that the molar ratios of metal to phosphorous (Me : P ratios) of metals whose seawater dissolved speciation is dominated by free ions (e.g., Co, Zn and Mn) will likely decrease or stay constant under ocean acidification. In contrast, high CO2 is predicted to shift the speciation of dissolved metals associated with carbonates such as Cu, increasing their bioavailability and resulting in higher Me : P ratios.

Adsorption of Cadmium and Zinc onto Micaceous Minerals: Effect of Siderophore Desferrioxamine B

The mobility, bioavailability, and environmental fate of heavy metals in soil are controlled by their adsorption onto soil minerals and solid organic matter. The adsorption is strongly affected by the presence of various low-molecular-weight organic acids. In this study, effect of hydroxamate siderophore desferrioxamine B (DFOB) on cadmium (Cd) and zinc (Zn) adsorption onto two micaceous clay minerals, muscovite and phlogopite, was evaluated in batch experiments. Results showed that the presence of DFOB diminished the adsorption of Cd and Zn onto both minerals, particularly under neutral to alkaline pH conditions. For instance, at pH 8.2, the presence of DFOB caused a decrease in the adsorption of Zn onto phlogopite by nearly 50%. The equilibrium adsorption of Cd and Zn was satisfactorily described using Freundlich isotherm. The adsorption isotherms showed that the affinity of Cd and Zn onto the minerals decreased in the presence of DFOB. For example, at pH 8.0, the presence of siderophore caused a decrease in the Freundlich adsorption isotherm coefficient KF for Zn adsorption onto muscovite and phlogopite from 4.60 to 0.07 L g−1 and from 3.56 to 0.36 L g−1, respectively. These findings confirm the potential influence of siderophore on the fate of Cd and Zn in arid soils containing substantial contributions of micaceous silicate minerals.

Mobility and transport of copper(II) influenced by the microbial siderophore DFOB: Column experiment and modelling

Acid Cu leaching from the European Kupferschiefer ore deposits is a challenge e.g. due to its high carbonate content. In this study, we investigated the transport behaviour of Cu under conditions related to a biohydrometallurgical leaching approach using neutrophil microorganisms in neutral to slightly alkaline solutions. We studied the effect of the microbial siderophore desferrioxamineB (DFOB) as a model leaching organic ligand on Cu mobility in column experiments with kaolinite. The results revealed that DFOB strongly enhances Cu mobility. The breakthrough of Cu occurs considerably earlier in the presence of DFOB than in the absence of the organic ligand. Furthermore, complete elution of Cu was observed at 5 pore volume exchanges faster compared to elution with deionized water. The established geochemical transport model shows good agreement with the experimental data and suggests a maximum efficiency at a Cu to DFOB molar ratio of 1:1. In addition, results of modelling revealed that in the absence of the ligand, a pH increase from 6.5 to 8.5 significantly retarded Cu breakthrough, whereas in the presence of DFOB, Cu breakthrough curves were nearly insensitive to pH changes and close to the breakthrough curve of a non-reactive tracer.

Effect of Natural Organic Matter on Plutonium Sorption to Goethite.

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mgC·L-1 and 50 mgC·L-1 natural organic matter (NOM), 10-9-10-10 M 238Pu, and 0.1 g·L-1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions.

Identification and characterization of Aeromonas hydrophila genes encoding the outer membrane receptor of ferrioxamine B and an AraC-type transcriptional regulator.

We found that, under iron-limiting conditions, Aeromonas hydrophila ATCC 7966(T) could utilize the xenosiderophore desferrioxamine B (DFOB) for growth by inducing the expression of its own outer membrane receptor. Two consecutive genes, desR and desA, were selected as candidates involved in DFOB utilization. The presence of the ferric-uptake regulator boxes in their promoters suggested that these genes are under iron-dependent regulation. Mutation of desA, a gene that encodes the outer membrane receptor of ferrioxamine B, disrupted the growth of the amonabactin-deficient mutant in the presence of DFOB. β-Galactosidase reporter assays and reverse transcriptase-quantitative PCR demonstrated that desR, a gene that encodes an AraC-like regulator homolog is required for the induction of desA transcription in the presence of DFOB and under iron-limiting conditions. The functions of desA and desR were analyzed using complementation experiments. Our data provided evidence that DesA is powered primarily by the TonB2 system.

Desferrioxamine-B promoted dissolution of an Oxisol and the effect of low-molecular-weight organic acids

This study was conducted to quantify the adsorption of desferrioxamine-B (DFO-B), a commercial trihydroxamate siderophore, onto an Oxisol and particularly examine the DFO-B-promoted soil dissolution in the presence of two small organic acids (citric and oxalic acids). Adsorption of DFO-B to Oxisol was rapid, and the adsorption isotherms yielded maximum surface concentrations of 9.59 μmol g−1 at pH 5.0, much lower than the soil cation exchange capacity. After reaching the maximum concentration within several minutes, the DFO-B adsorption to the soil subsequently decreased and attained a steady state, followed by a slight increase at the end of the reaction. Release of Fe, Mn, and Al from the soil was greatly enhanced in the presence of DFO-B. No competitive or synergic effect between the DFO-B and citric or oxalic acids was found on dissolution of Al- or Fe-containing minerals. However, a synergic effect between DFO-B and citric acid on dissolution of Mn-containing minerals was found, and a slight competitive effect of oxalic acid on release of Mn promoted by DFO-B was observed. These results can enhance our understanding on the role of siderophores in soils dissolution and weathering as well as the release of nutrient elements in the presence of low-molecular-weight organic acids.

Steady-state dissolution kinetics of mineral ferric phosphate in the presence of desferrioxamine-B and oxalate ligands at pH = 4–6 and T = 24 ± 0.6 °C

Ferric phosphate (FePO4·2H2O) is one of the most common secondary phosphate minerals in the environment. Nevertheless, few studies address the biological dissolution mechanism(s) of FePO4·2H2O. This paper reports steady-state dissolution rates of synthetic FePO4·2H2O at 4≤pH0≤6 by desferrioxamine-B (DFO-B) and oxalate (Ox) ligands. The composition of the influent solution was 10mM NaClO4, 5mM MES buffer. The influent solution was adjusted to 4≤pH0≤6 by adding aliquots of HNO3 or NaOH stock solution. The initial concentrations of DFO-B and Ox, [DFO-B]0 and [Ox]0, ranged from 0 to 135μM, and 0 to 345μM. Geochemical thermodynamic equilibrium modeling was conducted using MINEQL+ (Schecher and McAvoy, 1998). Speciation calculations were based on thermodynamic formation constants at 298.17K, K298 (infinite dilution reference state). Ligand-promoted dissolution rates were determined after steady-state values. Iron concentrations in the effluent solution were quantified (t>500h). Typical effluent-flow rate was maintained at 0.10±0.01mLmin−1. The measured dissolution rate of FePO4·2H2O by DFO-B and Ox, RDFO–OxObs, was compared to the sum of dissolution rates by DFO-B (RDFO-B) or Ox (ROx), RDFO–OxSum (RDFO–OxSum=RDFO‐B+ROx). Results were analyzed using the t student test. Obtained data values with p≤0.05 (⁎) and ≤0.01 (⁎⁎) were considered to differ statistically from control experiments. Dissolution rates by DFO-B (RDFO-B) increased with [DFOB]0, and no evidence of surface masking became apparent. By contrast, dissolution rates by Ox (ROx) varied with [Ox]0 and pH0. The kinetics of dissolution by Ox was not explained by a first-order mineral dissolution behavior. Dissolution rates by DFO-B and Ox (RDFO–OxObs) surpassed RDFO-B or ROx, and increased with proton activity. Reacting FePO4·2H2O with DFO-B and high amounts of Ox resulted in higher values for RDFO–OxObs relative to RDFO-B. Observed (RDFO–OxObs) to calculated (RDFO–OxSum=RDFO‐B+ROx) ratio was found to be highest at [DFOB]0=50μM and [Ox]0=49μM. Increases in the proton activity favors the dissolution of FePO4·2H2O by DFO-B and Ox, explained because the sequestration of Fe(III) at the surface vicinity in the form of adsorbed Fe(III)-oxalate complexes. A direct comparison between the dissolution behavior of FePO4·2H2O by DFO-B and Ox against those for goethite (α-FeOOH) and Al goethite (AlFeOOH) was conducted. The dissolution behavior was found to be a function of the mineral structure. RDFO-B values for FePO4·2H2O by 22.5μM DFO-B surpassed those for α-FeOOH or α-AlFeOOH by 20μM DFO-B, namely, 37, and 11.6 and 3–5μmolkg−1h−1, respectively. ROx values for FePO4·2H2O by 49mM Ox surpassed that for α-FeOOH by 70μM Ox or α-AlFeOOH by 50μM Ox, namely, i.e., 12, and 0.7 and 0.1μmolkg−1h−1. The latter results agree with the idea of the inhibition of Fe release in goethite because its sequestration in the form of adsorbed Fe(III) oxalate complexes. In contrast, a different scenario holds true for dissolution by 50μM DFO-B and 49μM Ox. The dissolution rates for FePO4·2H2O, α-FeOOH, and α-AlFeOOH correspond to 50, and 39–42 and 71–129μmolkg−1h−1, respectively. The high extent of iron release from Al goethite is best explained because high-energy surface sites formed after Al substitution in goethite.

Siderophore-Mediated Dissolution of Hematite (α-Fe2O3): Effects of Nanoparticle Size

This study focused on the effects of particle size (3.6, 8.6, and 40 nm) on hematite dissolution by the siderophore desferrioxa- mine-B (DFOB) at pH 3, 5, and 7 and 22°C, to 48 hours. In the presence of DFOB, batch dissolution was enhanced and consisted of an initial fast release of “labile” Fe followed by “steady state” dissolution. Dissolution rates decreased with increasing pH and were up to an order of magnitude faster for particles <10 nm. The <10 nm particles have a more hydroxyl-rich (near) surface structure and likely contain a higher density of surface active sites, resulting in faster dissolution kinetics. These results suggest that hematite nanoparticles <10 nm in size are likely to have enhanced bioavailability to aerobic microorganisms.

Synergistic Dissolution of Manganese Oxides as Promoted by Siderophores and Small Organic Acids

Recent studies have revealed that siderophores, biogenic chelating agents that strongly complex Fe(III) and other hard metals, may function in concert with low‐molecular‐mass organic acids (LMMOAs) to facilitate Fe (hydr)oxide dissolution via synergistic reactions. The siderophore desferrioxamine B (DFOB) may also participate in a number of chemical reactions at Mn (hydr)oxide surfaces. The goal of this study was to determine the rates and products of δ‐MnO2, a layer type Mn(IV) oxide, dissolution as promoted by LMMOA–DFOB mixtures. As with Fe (hydr)oxides, the rate of DFOB‐promoted dissolution of δ‐MnO2 is strongly influenced by the presence of the LMMOAs citrate and oxalate. In the presence of DFOB, citrate increases the dissolution rate relative to the sum of dissolution rates from corresponding single‐ligand systems by promoting both Mn(III)HDFOB+ complex formation and Mn(II) production in a pH‐dependent mechanism. In contrast, oxalate–DFOB mixtures produce predominantly Mn(II), with rates enhanced up to threefold from the sum of dissolution rates in single‐ligand systems at acidic pH values. We investigated possible mechanisms to describe this synergistic dissolution processes by building on observations of single‐ligand systems. These results suggest that biological exudation of LMMOAs in conjunction with siderophores may allow the selection of dissolution products, including regulation of the formation of potentially reactive aqueous Mn(III) complexes.

A spectroscopic study of the effects of a microbial siderophore on Pb adsorption to kaolinite

Batch adsorption experiments were combined with X-ray Absorption Spectroscopy (XAS) analysis to determine the mechanism(s) whereby the microbial trihydroxamate siderophore ligand desferrioxamine-B (DFO-B) affects Pb sorption to kaolinite at pH 4, 6, and 7.5 (in 0.1M NaClO4, 22°C; Pb:DFO-B ratio 120:240μM). In the absence of DFO-B, Pb adsorbs only slightly to kaolinite at pH 4, by a combination of inner- and outer-sphere complexation. Adsorption increases at pH 6, and sorption (adsorption/surface precipitation) further increases at pH 7.5. At pH 4, DFO-B does not bind Pb in solution appreciably, and the Pb adsorption mechanism(s) is unchanged by the presence of DFO-B. At pH 6, DFO-B slightly enhances Pb adsorption, due at least in part to formation of a DFO-B–Pb–kaolinite type A ternary surface complex. At pH 7.5, DFO-B decreases Pb sorption and Pb adsorption is dominated by a DFO-B–Pb–kaolinite type A ternary surface complex. Although XAS and thermodynamic speciation modeling indicate that Pb is bound by multiple DFO–B functional groups in solution at pH 7.5, the DFO-B–Pb–kaolinite surface complex appears to involve only a single hydroxamate group. This study thus demonstrates that the detailed structure of a ternary surface complex cannot necessarily be predicted from the structure of the solution organic-metal complex.

Wavelength-Dependence of Photoreductive Dissolution of Lepidocrocite (γ-FeOOH) in the Absence and Presence of the Siderophore DFOB

Photoreductive dissolution of lepidocrocite (gamma-FeOOH) in the presence/absence of the siderophore desferrioxamine B (DFOB) was investigated at different wavelengths. At pH 3 in the absence of DFOB, Fe(II) formation rates normalized to the photon flux increased with decreasing wavelengths below 515 nm, consistent with enhanced Fe(II) formation at lower wavelengths by photolysis of surface Fe(III)-hydroxo groups or by surface scavenging of photoelectrons generated in the semiconducting bulk. In the presence of DFOB at pH 3, photoreductive dissolution rates, normalized to the photon flux, increased more strongly with decreasing wavelengths below 440 nm. We hypothesize that acid-catalyzed hydrolysis of DFOB generates degradation products that form photoreactive surface complexes leading to an increase in photodissolution rates at low pH. At pH 8 in the presence of DFOB, normalized photodissolution rates had a maximum in the spectral window 395-435 nm and were significantly smaller at lower wavelengths, suggesting that adsorbed DFOB is directly involved in the reduction of surface Fe(III) by a light-induced ligand-to-metal charge-transfer reaction within the surface Fe(III)-DFOB complex. The strong response in the visible light suggests that photoreductive dissolution of iron (hydr)oxides promoted by siderophores with hydroxamic acid groups may occur deep into in the euphotic zone of oceans.

SIDEROPHORE PROMOTED DISSOLUTION OF PYROMORPHITE

Apatite amendment to lead (Pb)-contaminated soils has been investigated in the last decade as a method of Pb remediation. Phosphate released by apatite dissolution combines with Pb to form highly insoluble and supposedly nonbioavailable pyromorphite (PY). As a first approach to determining whether Pb present in PY might be susceptible to remobilization by microorganisms, we investigated the dissolution of PY in the presence of the trihydroxamate microbial siderophore desferrioxamine-B (DFO-B) as a function of pH (from 2 to 11), temperature, DFO-B concentration, and different solution/solid ratios in batch experiments. Desferrioxamine-B strongly complexes Pb in solution approximately above pH 6, and we observed enhancement of the net release of Pb from PY at pH 6 to 11. Dissolution in the presence of DFO-B was stoichiometric. Modeling using PHREEQC indicated that the increase of PY solubility can be accounted for by an increase in concentration of Pb-DFO-B complexes. Based on the enthalpy of dissolution reaction (ΔHr = −199.35 kJ/mol), dissolution of PY is exothermic, and PY solubility decreases with temperature. However, in the presence of DFO-B, the equilibrium-dissolved concentrations of Pb and P, along with the rate of net Pb and P release, increase with increasing temperature from 4°C to 50°C, suggesting that Pb-DFO-B complexation constants likely increase with temperature. Our results indicate that Pb may be mobilized from PY by microbial siderophores over the pH range of many soils, and that the potential effects of microorganisms on Pb remobilization thus need to be considered in Pb remediation strategies that rely on PY formation.

Dissolution of arsenopyrite (FeAsS) and galena (PbS) in the presence of desferrioxamine-B at pH 5

Microorganisms and higher plants produce biogenic ligands, such as siderophores, to mobilize Fe that otherwise would be unavailable. In this paper, we study the stability of arsenopyrite (FeAsS), one of the most important natural sources of arsenic on Earth, in the presence of desferrioxamine (DFO-B), a common siderophore ligand, at pH 5. Arsenopyrite specimens from mines in Panasqueira, Portugal (100–149μm) that contained incrustations of Pb, corresponding to elemental Pb as determined by scanning electron microscopy–electron diffraction spectroscopy (SEM–EDX), were used for this study. Batch dissolution experiments of arsenopyrite (1gL−1) in the presence of 200μM DFO-B at initial pH (pH0) 5 were conducted for 110h. In the presence of DFO-B, release of Fe, As, and Pb showed positive trends with time; less dependency was observed for the release of Fe, As, and Pb in the presence of only water under similar experimental conditions. Detected concentrations of soluble Fe, As, and Pb in suspensions containing only water were found to be ca. 0.09±0.004, 0.15±0.003, and 0.01±0.01ppm, respectively. In contrast, concentrations of soluble Fe, As, and Pb in suspensions containing DFO-B were found to be 0.4±0.006, 0.27±0.009, and 0.14±0.005ppm, respectively. Notably, the effectiveness of DFO-B for releasing Pb was ca. 10 times higher than that for releasing Fe. These results cannot be accounted for by thermodynamic considerations, namely, by size-to-charge ratio considerations of metal complexation by DFO-B. As determined by SEM–EDX, elemental sample enrichment analysis supports the idea that the Fe–S subunit bond energy is limiting for Fe release. Likely, the mechanism(s) of dissolution for Pb incrustations is independent and occurs concurrently to that for Fe and As. Our results show that dissolution of arsenopyrite leads to precipitation of elemental sulfur, and is consistent with a non-enzymatic mineral dissolution pathway. Finally, speciation analyses for As indicate variability in the As(III)/As(V) ratio with time, regardless of the presence of DFO-B or water. At reaction times <30h, As(V) concentrations were found to be 50–70%, regardless of the presence of DFO-B. These results are interpreted to indicate that transformations of As are not imposed by ligand-mediated mechanisms. Experiments were also conducted to study the dissolution behavior of galena (PbS) in the presence of 200μM at pH0 5. Results show that, unlike arsenopyrite, the dissolution behavior of galena shows coupled increases in pH with decreases in metal solubility at t>80h. Oxidative dissolution mechanisms conveying sulfur oxidation bring about the production of {H+}. However, dissolution data trends for arsenopyrite and galena indicate {H+} consumption. It is plausible that the formation of Pb species is dependent on {H+} and {OH−}, namely, stable surface hydroxyl complexes of the form Pb4(OH)44+ (pH50 5.8) and Pb6(OH)84+ for pH values 5.8 or above.

Effects of Siderophores on Pb and Cd Adsorption to Kaolinite

AbstractSiderophores are low molecular weight organic ligands synthesized by aerobic microorganisms to acquire Fe. In addition to Fe(III), siderophores may complex other metals such as Pb and Cd. This study compared the effects of the trihydroxamate siderophores desferrioxamine-B (DFO-B), desferrioxamine-D1 (DFO-D1), desferrioxamine-E (DFO-E), and the monohydroxamate siderophore-like ligand acetohydroxamic acid (aHA) on Pb and Cd (except for DFO-E) adsorption to kaolinite (KGa-1b) at pH 4.5 to 9, in 0.1 M NaClO4, at 22°C, in the dark. At pH &gt;6, all of the studied ligands decreased Pb adsorption to kaolinite: aHA by 5–40% and DFO-B, DFO-D1 and DFO-E by 30–75%; the greater effects were at higher pH. The studied ligands decreased Cd adsorption to kaolinite at pH &gt;8: aHA by 5–20% and the trihydroxamates by as much as 80%. We also observed enhancement of Pb adsorption in the presence of DFO-B at pH ≈5–6.0, probably due to adsorption of the doubly positively charged H3Pb (DFO-B)2+ complex, although spectroscopic evidence is needed.

Steady-state dissolution kinetics of goethite in the presence of desferrioxamine B and oxalate ligands: implications for the microbial acquisition of iron

This paper reports an investigation of the effects of a trihydroxamate siderophore, desferrioxamine B (DFO-B), and a common biological ligand, oxalate, on the steady-state dissolution of goethite at pH 5 and 25 °C. The main goal of our study was to quantify the adsorption of the ligands and the dissolution of goethite they promote in a two-ligand system. In systems with one ligand only, the adsorption of oxalate and DFO-B each followed an L-type isotherm. The surface excess of oxalate was approximately 40 mmol kg −1 at solution concentrations above 80 μM, whereas the surface excess of DFO-B was only 1.2 mmol kg −1 at 80 μM solution concentration. In the two-ligand systems, oxalate decreased DFO-B adsorption quite significantly, but not vice versa. For example, in solutions containing 40 μM DFO-B and 40 μM oxalate, 30% of the DFO-B adsorbed in the absence of oxalate was displaced. The mass-normalized dissolution rate of goethite in the presence of DFO-B alone increased as the surface excess of the ligand increased, suggesting a ligand-promoted dissolution mechanism. In systems containing oxalate only, mass-normalized goethite dissolution rates were very low at concentrations below 200 μM, despite maximal adsorption of the ligand. At higher oxalate concentrations (up to 8 mM), the steady-state dissolution rate continued to increase, even though the surface excess of adsorbed ligand was essentially constant. Chemical affinity calculations and dissolution experiments with variation of the reactor flow rate showed that far-from-equilibrium conditions did not obtain in systems containing oxalate at concentrations below 5 mM. The dissolution rate in the presence of DFO-B at solution concentrations between 1 and 80 μM was approximately doubled when oxalate was also present at 40 μM solution concentration. The dissolution rate in the presence of oxalate at solution concentrations between 0 and 200 μM was increased by more than an order of magnitude when DFO-B was also present at 40 μM solution concentration. Chemical affinity calculations showed that, in systems containing DFO-B, goethite dissolution was always under far-from-equilibrium conditions, irrespective of the presence of oxalate. These results were described quantitatively by a model rate law containing a term proportional to the surface excess of DFO-B and a term proportional to that of oxalate, with both surface excesses being determined in the two-ligand system. The pseudo first-order rate coefficient in the DFO-B term has the same value as measured for goethite dissolution in the presence of DFO-B only, while the rate coefficient in the oxalate term must be measured in the two-ligand system, since it is only in this system that far-from-equilibrium conditions obtain. These latter conditions do not exist in the system containing oxalate only, but they do exist in the DFO-B/oxalate system because the siderophore is able to remove Fe(III) from all Fe–oxalate complexes rapidly, leaving the uncomplexed oxalate ligand in solution free to react again with the goethite surface. This synergy observed in the two-ligand system implies that the production of modest quantities of siderophore in the presence of very low concentrations of oxalate would be an extremely effective mechanism for the microbially induced release of Fe from goethite.

Steady-State Dissolution Kinetics of Aluminum-Goethite in the Presence of Desferrioxamine-B and Oxalate Ligands

This paper reports steady-state dissolution rates of synthetic low-substitution Al-goethites (mol % Al < 10) at pH 5 in the presence of the trihydroxamate siderophore, desferrioxamine B (DFO-B), and the common biological ligand, oxalate. The siderophore-promoted Fe release rate increased both with the level of Al substitution and with DFO-B concentration up to about 100 microM, after which a plateau occurred, suggesting a saturation effect from DFO-B adsorption as a precursor to dissolution. At concentrations above 200 microM, oxalate also enhanced the Fe release rate, which however was not influenced by Al substitution. For Al-goethites with mol % Al < 4, the Fe release rate in the presence of 40 microM DFO-B together with varying concentrations of oxalate was typically greater than the corresponding sum of dissolution rates in the presence of the two ligands alone. This synergism may be the combined result of the ability of oxalate to adsorb strongly at the goethite surface, thus promoting Fe release, and of the high selectivity of DFO for Fe(III). Ferric oxalate complexes formed during dissolution will likely lose Fe3+ by ligand substitution with DFO-B, leading to the production of Fe(HDFO-B)+ and uncomplexed oxalate, the latter of which, in turn, could adsorb to the goethite surface again. For Al-goethites with mol % Al > 4, synergism was not apparent, which may signal the effect of a decreased surface density of Fe-OH sites associated with Al for Fe substitution. The oxalate-promoted release rates of Al did not scale with those of Fe, indicating incongruent dissolution. However, Al release rates in the presence of DFO-B did scale approximately with those of Fe but were not affected by the concentration of siderophore. These results are consistent with the presence of Al(OH)3 inclusions in Al-goethite.

Reduction of the Fe(III)-desferrioxamine-B complexes by caffeic acid: A reduction mechanism of biochemical significance

The reduction of Fe(III) by caffeic acid (CAF) in the presence of desferrioxamine-B (DFOB) was studied as a function of pH and time. The HPLC analyses, UV/vis spectra and kinetic data provide evidence that the reduction of Fe(III) by caffeic acid is strongly inhibited by DFOB and that it can be induced by the siderophore at low pH values. The stoichiometry and the possible mechanism of the reduction process are reported.

Interaction of polymorphonuclear leukocytes with Yersinia enterocolitica: role of the Yersinia virulence plasmid and modulation by the iron-chelator desferrioxamine B.

The impact of desferrioxamine B (DFOB) on intracellular calcium mobilization, chemoluminescence (CL) signaling, phagocytosis, and bacterial killing of Yersinia enterocolitica by polymorphonuclear leukocytes (PMNL) was investigated. DFOB did not alter calcium signaling in PMNL but transiently increased phagocytosis. Likewise, higher counts of viable bacteria in PMNL and increased CL signals after stimulation with Y. enterocolitica were observed in the presence of DFOB. However, DFOB reduced the CL signal elicited individually by zymosan, PMA, and FMLP. Comparison of plasmid-cured with plasmid-harboring Y. enterocolitica revealed that plasmid-encoded determinants modulate phagocytosis, CL signal, and bactericidal properties of PMNL. These data demonstrate that DFOB alters microbicidal properties of PMNL and modulates their interaction with Y. enterocolitica and provide further evidence for a putative dual role of DFOB in the pathogenesis of Yersinia infection, namely growth support of the pathogen and modulation of the antimicrobial host response.