Intermediary sulfur compounds in pyrite oxidation: implications for bioleaching and biodepyritization of coal

Accumulation of elemental sulfur during pyrite oxidation lowers the efficiency of coal desulfurization and bioleaching. In the case of pyrite bioleaching by Leptospirillum ferrooxidans, an iron(II)-ion-oxidizing organism without sulfur-oxidizing capacity, from the pyritic sulfur moiety about 10% elemental sulfur, 2% pentathionate, and 1% tetrathionate accumulated by a recently described cyclic pyrite oxidation mechanism. In the case of pure cultures of Thiobacillus ferrooxidans and mixed cultures of L. ferrooxidans and T. thiooxidans, pyrite was nearly completely oxidized to sulfate because of the capacity of these cultures to oxidize both iron(II) ions and sulfur compounds. Pyrite oxidation in acidic solutions, mediated chemically by iron(III) ion, resulted in an accumulation of similar amounts of sulfur compounds as obtained with L. ferrooxidans. Changes of pH to values below 2 or in the iron ion concentration are not decisive for diverting the flux of sulfur compounds. The literature on pyrite bioleaching is in agreement with the findings indicating that the chemistry of direct and indirect pyrite leaching is identical. Received: 20 April 1998 / Received revision: 27 August 1998 / Accepted: 3 September 1998     

Leaching of Pyrites of Various Reactivities by Thiobacillus ferrooxidans

Wide variations were found in the rate of chemical and microbiological leaching of iron from pyritic materials from various sources. Thiobacillus ferrooxidans accelerated leaching of iron from all of the pyritic materials tested in shake flask suspensions at loadings of 0.4% (wt/vol) pulp density. The most chemically reactive pyrites exhibited the fastest bioleaching rates. However, at 2.0% pulp density, a delay in onset of bioleaching occurred with two of the pyrites derived from coal sources. T. ferrooxidans was unable to oxidize the most chemically reactive pyrite at 2.0% pulp density. No inhibition of pyrite oxidation by T. ferrooxidans occurred with mineral pyrite at 2.0% pulp density. Experiments with the most chemically reactive pyrite indicated that the leachates from the material were not inhibitory to iron oxidation by T. ferrooxidans.     

Microbiological Redox Potential Control to Improve the Efficiency of Chalcopyrite Bioleaching

The effect of controlling the redox potential (E_h) on chalcopyrite bioleaching kinetics was studied as a new aspect of redox control during chalcopyrite bioleaching, and its mechanism was investigated by employing the “normalized” solution redox potential (E_normal) and the reaction kinetics model. Different E_h ranges were established by use of different acidophiles (Sulfobacillus acidophilus YTF1; Sulfobacillus sibiricus N1; Acidimicrobium ferrooxidans ICP; Acidiplasma sp. Fv-AP). Cu dissolution was very susceptible to real-time change in E_h during the reaction. It was found that efficiency of bioleaching of chalcopyrite can be effectively evaluated on the basis of E_normal, since it is normalized for real-time fluctuations of concentrations of major metal solutes during bioleaching. For steady Cu solubilization during bioleaching at a maximum rate, it was important to maintain a redox potential range of 0 ≤ E_normal ≤ 1 (?0.35 mV optimal) at the mineral surface by employing a “weak” ion-oxidizer. This led to a copper recovery of > 75%. At higher E_normal levels (E_normal > 1 by “strong” microbial Fe²⁺ oxidation), Cu solubilization was slowed by diffusion through the product film at the mineral surface (< 50% Cu recovery) caused by low reactivity of the chalcopyrite and by secondary passivation of the chalcopyrite surface, mainly by jarosite.     

Optimization of polylactic-co-glycolic acid nanoparticles containing itraconazole using 2<Superscript>3</Superscript> factorial design

This study investigated the utility of a 2³ factorial design and optimization process for polylactic-co-glycolic acid (PLGA) nanoparticles containing itraconazole with 5 replicates at the center of the design. Nanoparticles were prepared by solvent displacement technique with PLGAX ₁ (10, 100 mg/mL), benzyl benzoateX ₂ (5, 20 μg/mL), and itraconazoleX ₃ (200, 1800 μg/mL). Particle size (Y ₁), the amount of itraconazole entrapped in the nanoparticles (Y ₂), and encapsulation efficiency (Y ₃) were used as responses. A validated statistical model having significant coefficient figures (P<.001) for the particle size (Y ₁), the amount of itraconazole entrapped in the nanoparticles (Y ₂), and encapsulation efficiency (Y ₃) as function of the PLGA (X ₁), benzyl benzoate (X ₂), and itraconazole (X ₃) were developed: Y₁=373.75+66.54X₁+52.09X₂+105.06X₃−4.73X₁X₂+46.30X₁X₃; Y₂=472.93+73.45X₁+ 169.06X₂+333.03X₃+62.40X₁X₃+141.49X₂X₃; Y₃= 57.36+6.53X₁+15.52X₂−12.59X₃+1.01X₁X₃+ 1.73X₂X₃.X ₁,X ₂, andX ₃ had a significant effect (P<.001) onY ₁,Y ₂, andY ₃. The particle size, the amount of itraconazole entrapped in the nanoparticles, and the encapsulation efficiency of the 4 formulas were in agreement with the predictions obtained from the models (P<.05). An overlay plot for the 3 responses shows the boundary in whichY ₁ shows the boundary in which a number of combinations of concentration of PLGA, benzyl benzoate, and itraconazole will result in a satisfactory process. Using the desirability approach with the same constraints, the solution composition having the highest overall desirability (D=0.769) was 10 mg/mL of PLGA, 16.94 μg/mL of benzyl benzoate, and 1001.01 μg/mL of itraconazole. This approach allowed the selection of the optimum formulation ingredients for PLGA nanoparticles containing itraconazole of 500 μg/mL.     

Response surface methodology for optimization and characterization of limonene-based coenzyme Q10 self-nanoemulsified capsule dosage form

The aim of this study was to systematically obtain a model of factors that would yield an optimized self-nanoemulsified capsule dosage form (SNCDF) of a highly lipophilic model compound, Coenzyme Q10 (CoQ). Independent variables such as amount of R-(+)-limonene (X ₁), surfactant (X ₂), and cosurfactant (X ₃), were optimized using a 3-factor, 3-level Box-Behnken statistical design. The dependent variables selected were cumulative percentage of drug released after 5 minutes (Y ₁) with constraints on drug release in 15 minutes (Y ₂), turbidity (Y ₃), particle size (Y ₄), and zeta potential (Y ₅). A mathematical relationship obtained,Y ₁=78.503+6.058X ₁ +13.738X ₂+5.986X ₃−25.831X ₁ ² +9.12X ₁X₂−26.03X ₁X₃−38.67X ₂ ² +11.02X ₂X₃−15.55X ₃ ³ (r ²=0.97), explained the main and quadratic effects, and the interaction of factors that affected the drug release. Response surface methodology (RSM) predicted the levels of factorsX ₁,X ₂, andX ₃ (0.0344, 0.216, and 0.240, respectively), for a maximized response ofY ₁ with constraints of >90% release onY ₂. The observed and predicted values ofY ₁ were in close agreement. In conclusion, the Box-Behnken experimental design allowed us to obtain SNCDF with rapid (>90%) drug release within 5 minutes with desirable properties of low turbidity and particle size.     

The extremophile Acidithiobacillus ferrooxidans possesses a c-di-GMP signalling pathway that could play a significant role during bioleaching of minerals

Aims: The primary goal of this study was to characterize the existence of a functional c‐di‐GMP pathway in the bioleaching bacterium Acidithiobacillus ferrooxidans. Methods and Results: A bioinformatic search revealed that the genome sequence of At. ferrooxidans ATCC 23270 codes for several proteins involved in the c‐di‐GMP pathway, including diguanylate cyclases (DGC), phosphodiesterases and PilZ effector proteins. Overexpression in Escherichia coli demonstrated that four At. ferrooxidans genes code for proteins containing GGDEF/EAL domains with functional DGC activity. MS/MS analysis allowed the identification of c‐di‐GMP in nucleotide preparations obtained from At. ferrooxidans cells. In addition, c‐di‐GMP levels in cells grown on the surface of solid energetic substrates such as sulfur prills or pyrite were higher than those measured in ferrous iron planktonic cells. Conclusions: At. ferrooxidans possesses a functional c‐di‐GMP pathway that could play a key role in At. ferrooxidans biofilm formation during bioleaching processes. Significance and Impact of the Study: This is the first global study about the c‐di‐GMP pathway in an acidophilic bacterium of great interest for the biomining industry. It opens a new way to explore the regulation of biofilm formation by biomining micro‐organisms during the bioleaching process.     

Isolation of a strain of <Emphasis Type="Italic">Acidithiobacillus caldus</Emphasis> and its role in bioleaching of chalcopyrite

A moderately thermophilic and acidophilic sulfur-oxidizing bacterium named S₂, was isolated from coal heap drainage. The bacterium was motile, Gram-negative, rod-shaped, measured 0.4 to 0.6 by 1 to 2 μm, and grew optimally at 42–45°C and an initial pH of 2.5. The strain S₂ grew autotrophically by using elemental sulfur, sodium thiosulfate and potassium tetrathionate as energy sources. The strain did not use organic matter and inorganic minerals including ferrous sulfate, pyrite and chalcopyrite as energy sources. The morphological, biochemical, physiological characterization and analysis based on 16S rRNA gene sequence indicated that the strain S₂ is most closely related to Acidithiobacillus caldus (>99% similarity in gene sequence). The combination of the strain S₂ with Leptospirillum ferriphilum or Acidithiobacillus ferrooxidans in chalcopyrite bioleaching improved the copper-leaching efficiency. Scanning electron microscope (SEM) analysis revealed that the chalcopyrite surface in a mixed culture of Leptospirillum ferriphilum and Acidithiobacillus caldus was heavily etched. The energy dispersive X-ray (EDX) analysis indicated that Acidithiobacillus caldus has the potential role to enhance the recovery of copper from chalcopyrite by oxidizing the sulfur formed during the bioleaching progress.     

Direct and Indirect Bioleaching of Cobalt from Low Grade Laterite and Pyritic Ores by Aspergillus niger

Abstract

The bioleaching efficiency and mechanism of recovery of cobalt (Co) and nickel from laterites and pyritic ores by Aspergillus niger were investigated. Recoveries of Co from laterites and pyritic ores by direct bioleaching were 65.9?±?1.8% and 4.9?±?2.7%, respectively, while 30.9?±?0.6% and 10.9?±?6.2% recovery of Ni were obtained from laterites and pyritic ores, respectively. Recovery of Co via indirect bioleaching in the absence of the fungal biomass from laterite was significantly lower when compared with Co released by direct bioleaching. In the latter, hyphal penetration and colonization of the laterites were clearly observed by scanning electron microscopy (SEM). X-ray powder diffraction (XRPD) analysis of mineral phases before and after bioleaching indicated that cobalt-bearing goethite was the main phase bioleached in the laterites. No significant difference was found between Co recoveries from synthesized cobalt-bearing goethite by both direct and indirect bioleaching. Therefore, we propose that two processes are involved in bioleaching from laterites: (1) cobalt-bearing goethite was exposed via direct interactions between the fungus and the minerals and (2) cobalt-bearing goethite was dissolved by released metabolites of A. niger, such as organic acids. An incongruent pattern of Co and Fe bioleaching from the laterites was also a feature of the metal recovery process.     

Formulation of Dacarbazine-loaded Cubosomes—Part II: Influence of Process Parameters

The purpose of this study was to investigate the combined influence of three-level, three-factor variables on the formulation of dacarbazine (a water-soluble drug) loaded cubosomes. Box–Behnken design was used to obtain a second-order polynomial equation with interaction terms to predict response values. In this study, the selected and coded variables X ₁, X ₂, and X ₃ representing the amount of monoolein, polymer, and drug as the independent variables, respectively. Fifteen runs of experiments were conducted, and the particle size (Y ₁) and encapsulation efficiency (Y ₂) were evaluated as dependent variables. We performed multiple regression to establish a full-model second-order polynomial equation relating independent and dependent variables. A second-order polynomial regression model was constructed for Y ₁ and confirmed by performing checkpoint analysis. The optimization process and Pareto charts were obtained automatically, and they predicted the levels of independent coded variables X ₁, X ₂, and X ₃ (−1, 0.53485, and −1, respectively) and minimized Y ₁ while maximizing Y ₂. These corresponded to a cubosome formulation made from 100 mg of monoolein, 107 mg of polymer, and 2 mg with average diameter of 104.7 nm and an encapsulation efficiency of 6.9%. The Box–Behnken design proved to be a useful tool to optimize the particle size of these drug-loaded cubosomes. For encapsulation efficiency (Y ₂), further studies are needed to identify appropriate regression model.     

The Mean of the Inverse of a Punctured Normal Distribution and Its Application

The fundamental properties of a punctured normal distribution are studied. The results are applied to three issues concerning X/Y where X and Y are independent normal random variables with means μ_X and μ_Y respectively. First, estimation of μ_X/μ_Y as a surrogate for E(X/Y) is justified, then the reason for preference of a weighted average, over an arithmetic average, as an estimator of μ_X/μ_Y is given. Finally, an approximate confidence interval for μ_X/μ_Y is provided. A grain yield data set is used to illustrate the results. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of energy gene expressions and community structures of free and attached acidophilic bacteria in chalcopyrite bioleaching

In order to better understand the bioleaching mechanism, expression of genes involved in energy conservation and community structure of free and attached acidophilic bacteria in chalcopyrite bioleaching were investigated. Using quantitative real-time PCR, we studied the expression of genes involved in energy conservation in free and attached Acidithiobacillus ferrooxidans during bioleaching of chalcopyrite. Sulfur oxidation genes of attached A. ferrooxidans were up-regulated while ferrous iron oxidation genes were down-regulated compared with free A. ferrooxidans in the solution. The up-regulation may be induced by elemental sulfur on the mineral surface. This conclusion was supported by the results of HPLC analysis. Sulfur-oxidizing Acidithiobacillus thiooxidans and ferrous-oxidizing Leptospirillum ferrooxidans were the members of the mixed culture in chalcopyrite bioleaching. Study of the community structure of free and attached bacteria showed that A. thiooxidans dominated the attached bacteria while L. ferrooxidans dominated the free bacteria. With respect to available energy sources during bioleaching of chalcopyrite, sulfur-oxidizers tend to be on the mineral surfaces whereas ferrous iron-oxidizers tend to be suspended in the aqueous phase. Taken together, these results indicate that the main role of attached acidophilic bacteria was to oxidize elemental sulfur and dissolution of chalcopyrite involved chiefly an indirect bioleaching mechanism.     

Sulfite Oxidation Catalyzed by aa 3-Type Cytochrome c Oxidase in Acidithiobacillus ferrooxidans

Sulfite is produced as a toxic intermediate during Acidithiobacillus ferrooxidans sulfur oxidation. A. ferrooxidans D3-2, which posseses the highest copper bioleaching activity, is more resistant to sulfite than other A. ferrooxidans strains, including ATCC 23270. When sulfite oxidase was purified homogeneously from strain D3-2, the oxidized and reduced forms of the purified sulfite oxidase absorption spectra corresponded to those of A. ferrooxidans aa ₃-type cytochrome c oxidase. The confirmed molecular weights of the α-subunit (52.5 kDa), the β-subunit (25 kDa), and the γ-subunit (20 kDa) of the purified sulfite oxidase and the N-terminal amino acid sequences of the γ-subunit of sulfite oxidase (AAKKG) corresponded to those of A. ferrooxidans ATCC 23270 cytochrome c oxidase. The sulfite oxidase activities of the iron- and sulfur-grown A. ferrooxidans D3-2 were much higher than those cytochrome c oxidases purified from A. ferrooxidans strains ATCC 23270, MON-1 and AP19-3. The activities of sulfite oxidase purified from iron- and sulfur-grown strain D3-2 were completely inhibited by an antibody raised against a purified A. ferrooxidans MON-1 aa ₃-type cytochrome c oxidase. This is the first report to indicate that aa ₃-type cytochrome c oxidase catalyzed sulfite oxidation in A. ferrooxidans.     

Fed-Batch Cultivation of Arthrospira platensis Using Carbon Dioxide from Alcoholic Fermentation and Urea as Carbon and Nitrogen Sources

To reduce CO₂ emissions from alcoholic fermentation, Arthrospira platensis was cultivated in tubular photobioreactor using either urea or nitrate as nitrogen sources at different light intensities (60 μmol m^?2 s^?1?≤?I?≤?240 μmol m^?2 s^?1). The type of carbon source (pure CO₂ or CO₂ from fermentation) did not show any appreciable influence on the main cultivation parameters, whereas substitution of nitrate for urea increased the nitrogen-to-cell conversion factor (Y _X/N ), and the maximum cell concentration (X _m ) and productivity (P _X ) increased with I. As a result, the best performance using gaseous emissions from alcoholic fermentation (X _m ?=?2,960?±?35 g m^?3, P _X ?=?425?±?5.9 g m^?3 day^?1 and Y _X/N ?=?15?±?0.2 g g^?1) was obtained at I?=?120 μmol m^?2 s^?1 using urea as nitrogen source. The results obtained in this work demonstrate that the combined use of effluents rich in urea and carbon dioxide could be exploited in large-scale cyanobacteria cultivations to reduce not only the production costs of these photosynthetic microorganisms but also the environmental impact associated to the release of greenhouse emissions.     

Metal Removal from Contaminated Soils Through Bioleaching with Oxidizing Bacteria and Rhamnolipid Biosurfactants

The use of surfactants as a method for solubilization and removal of heavy metal contamination from soil has been reported before. Biosurfactants produced by some microorganisms are able to modify the surface of various metals and aggregate on interphases favoring the metal separation process from contaminated environments. We evaluated the feasibility of enhancing the removal of metal ions from mineral waste/contaminated soils using alternate cycles of treatment with rhamnolipid biosurfactants and bioleaching with a mixed bacterial culture of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans. Bioleaching alone removed 50% Zn and 19% Fe. When rhamnolipids were used at low concentration (0.4 mg/mL), 11% Fe and 25% Zn were removed, while at 1 mg/mL 19% Fe and 52% Zn removal were achieved. When using a cyclic treatment combining bioleaching and biosurfactants, metal removal reached up to 36% for Fe and 63% to 70% for Zn.     

Analysis of molybdenite bioleaching by Thiobacillus ferrooxidans in the absence of iron (II)

Summary Thiobacillus ferrooxidans attachment on MoS² and Mo dissolution are increased by the addition of the tensioactive agent Tween 80 in absence of iron(II), which suggests that the poor bioleaching of MoS² is caused by its hydrophobic character. Additionally, inhibition ofThiobacillus ferrooxidans growth by the presence of MoO₄ ^2– and the effect of variable amounts of Tween 80 on bacteria growth and on MoS² bioleaching are considered in this paper. Data confirm the need of bacterial attachment to insoluble substrate for bioleaching by the direct mechanism.     

Acidithiobacillus ferrooxidans and its potential application

The widely distributed Acidithiobacillus ferrooxidans (A. ferrooxidans) lives in extremely acidic conditions by fixing CO₂ and nitrogen, and by obtaining energy from Fe²⁺ oxidation with either downhill or uphill electron transfer pathway and from reduced sulfur oxidation. A. ferrooxidans exists as different genomovars and its genome size is 2.89–4.18 Mb. The chemotactic movement of A. ferrooxidans is regulated by quorum sensing. A. ferrooxidans shows weak magnetotaxis due to formation of 15–70 nm magnetite magnetosomes with surface functional groups. The room- and low-temperature magnetic features of A. ferrooxidans are different from other magnetotactic bacteria. A. ferrooxidans has potential for removing sulfur from solids and gases, metals recycling from metal-bearing ores, electric wastes and sludge, biochemical production synthesizing, and metal workpiece machining.

     

Evaluation of the Biotechnological Potential of Pure and Mixture of Indigenous Iron-Oxidizing Bacteria to Dissolve Trace Metals from Cu Bearing Ore

Abstract

This study aimed to investigate the ability of pure and consortia of indigenous iron-oxidizing bacteria to enhance the dissolution of trace metals from Cu and Zn-bearing ore. Three bacterial strains Acidithiobacillus ferrooxidans strain WG101, Leptospirillum ferriphilum strain WG102, Leptospirillum ferrooxidans strain WG103 isolated from Baiyin copper mine, China were used in this study. The biotechnological potential of these indigenous isolates was evaluated both in pure and in consortia to extract cobalt, chromium, and lead from the copper and zinc bearing ore. The sulfur and iron-oxidizing bacterial isolate Acidithiobacillus ferrooxidans strain WG101 exhibited efficient dissolution compared to sole iron-oxidizing Leptospirillum ferriphilum strain WG102, and Leptospirillum ferrooxidans strain WG103. Initial medium pH, pulp density, and temperature were studied as influential parameters in bioleaching carried out by bacterial consortia. The achieved optimum conditions were; initial pH of 1.5, 10% of pulp density, and temperature 30?°C with 68.7?±?3.9% cobalt, 56.6?±?3.9% chromium, and 36?±?3.7% lead recovery. Analytical study of oxidation-reduction potential and pH fluctuation were observed during this whole process that shows the metal dissolution efficiency of bacterial consortia. Alterations in spectral bands of processed residues were reported through FTIR analysis compared with control ore sample. Mössbauer spectroscopy analysis showed the influence of bacterial consortia on iron speciation in bioleached samples. The findings confirm that the indigenous acidophilic iron-oxidizing bacterial strains are highly effective in the dissolution of trace elements present in ore samples. This study not only supports the notion that indigenous bacterial strains are highly effectual in metal dissolution but provides the basic vital conditions to upscale the bioleaching technique for metals dissolution.     

Fabrication of Modified Transport Fluconazole Transdermal Spray Containing Ethyl Cellulose and Eudragit® RS100 as Film Formers

The present investigation was undertaken to fabricate modified transport fluconazole transdermal spray using ethyl cellulose and Eudragit® RS100 as film-forming polymers. Eudragit® RS100 (X ₁) and ethyl cellulose (X ₂) were selected as independent variables in 3² full factorial design, whereas drug transport in first hour (Y ₁) and the time required for 50% drug transport (Y ₂) were selected as dependent variables. Eutectic blend of camphor and menthol was used as permeation enhancer cum solvent for film-forming polymers. The pH, viscosity, volume of solution delivered upon each actuation, spray angle, ex–in vivo physical evaluation and in vitro drug transport of the formulated products were evaluated. The optimized batch B16 containing 5.25% w/w ethyl cellulose and 10.6% w/w Eudragit® RS100 was formulated by overlapping the contour plots of Y ₁ and Y ₂. The pH, viscosity, volume of solution sprayed upon each actuation and spray angle of the batch B16 was 6.3, 52.9 cPs, 0.24 ml and 82.6° respectively. The film of optimized batch was flexible and dermal-adhesive. The responses Y ₁ and Y ₂ of batch B16 were 7.91 μg/ml and 347 min respectively. The kinetics of drug transport was best explained by the Korsmeyer and Peppas model. The eutectic mixture consisting of equal parts of camphor and menthol showed improved drug permeation through shed snake skin. Short-term stability study demonstrated insignificant changes in performance characteristics.     

Bacterial leaching of metals from sewage sludge

Summary Thiobacillus thiooxidans is capable of oxidizing sulfur in digested sludge, while decreasing the pH value from about 5.5 to, say, 1.0 to 1.5. Insoluble metal sulfides can be solubilized through this acidification. Thiobacillus ferrooxidans oxidises pyritic ore in the presence of 6% centrifuged sludge if the pH value is adjusted to about 2.5. When mixing T. thiooxidans and T. ferrooxidans with sludge and 1% sulfur, the former acidifies the sludge and the latter oxidizes metal sulfides; together they solubilize more metal than T. thiooxidans alone. The following metals solubilized from their sulfides have been investigated so far: iron, copper, zinc, nickel, and cadmium. The possibility of recycling metals from sewage sludge with this method is discussed.     

Genomic insights into the iron uptake mechanisms of the biomining microorganism Acidithiobacillus ferrooxidans

Commercial bioleaching of copper and the biooxidation of gold is a cost-effective and environmentally friendly process for metal recovery. A partial genome sequence of the acidophilic, bioleaching bacterium Acidithiobacillus ferrooxidans is available from two public sources. This information has been used to build preliminary models that describe how this microorganism confronts unusually high iron loads in the extremely acidic conditions (pH 2) found in natural environments and in bioleaching operations. A. ferrooxidans contains candidate genes for iron uptake, sensing, storage, and regulation of iron homeostasis. Predicted proteins exhibit significant amino acid similarity with known proteins from neutrophilic organisms, including conservation of functional motifs, permitting their identification by bioinformatics tools and allowing the recognition of common themes in iron transport across distantly related species. However, significant differences in amino acid sequence were detected in pertinent domains that suggest ways in which the periplasmic and outer membrane proteins of A. ferrooxidans maintain structural integrity and relevant protein-protein contacts at low pH. Unexpectedly, the microorganism also contains candidate genes, organized in operon-like structures that potentially encode at least 11 siderophore systems for the uptake of Fe(III), although it does not exhibit genes that could encode the biosynthesis of the siderophores themselves. The presence of multiple Fe(III) uptake systems suggests that A. ferrooxidans can inhabit aerobic environments where iron is scarce and where siderophore producers are present. It may also help to explain why it cannot tolerate high Fe(III) concentrations in bioleaching operations where it is out-competed by Leptospirillum species.