Cross-comparison of leaching strains isolated from two different regions: Chambishi and Dexing copper mines.

A cross-comparison of six strains isolated from two different regions, Chambishi copper mine (Zambia, Africa) and Dexing copper mine (China, Asia), was conducted to study the leaching efficiency of low grade copper ores. The strains belong to the three major species often encountered in bioleaching of copper sulfide ores under mesophilic conditions: Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Leptospirillum ferriphilum. Prior to their study in bioleaching, the different strains were characterized and compared at physiological level. The results revealed that, except for copper tolerance, strains within species presented almost similar physiological traits with slight advantages of Chambishi strains. However, in terms of leaching efficiency, native strains always achieved higher cell density and greater iron and copper extraction rates than the foreign microorganisms. In addition, microbial community analysis revealed that the different mixed cultures shared almost the same profile, and At. ferrooxidans strains always outcompeted the other strains.

Adhesion forces between cells of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans or Leptospirillum ferrooxidans and chalcopyrite.

The efficiency of copper leaching is improved by bacteria attached to chalcopyrite. Therefore, the study of the attachment mechanism to control leaching is important. The adhesion of three species of leaching microorganisms including Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans to chalcopyrite was investigated by using atomic force microscopy (AFM). The forces were measured with tip-immobilized cells approached to and retracted from the mineral. The results show that both the surface charge and the hydrophobicity of bacteria cells influence the adhesion force. Furthermore, the adhesion force decreased in case the extracellular polymeric substances (EPS) had been removed. In addition, the data indicate that the amount of attached cells increased with increasing adhesion force.

Metal recovery from spent refinery catalysts by means of biotechnological strategies.

A bioleaching study aimed at recovering metals from hazardous spent hydroprocessing catalysts was carried out. The exhaust catalyst was rich in nickel (4.5 mg/g), vanadium (9.4 mg/g) and molybdenum (4.4 mg/g). Involved microorganisms were iron/sulphur oxidizing bacteria. Investigated factors were elemental sulphur addition, ferrous iron addition and actions contrasting a possible metal toxicity (either adding powdered activated charcoal or simulating a cross current process by means of periodical filtration). Ferrous iron resulted to be essential for metal extraction: nickel and vanadium extraction yields were 83% and 90%, respectively, while about 50% with no iron. The observed values for molybdenum extraction yields were not as high as Ni and V ones (the highest values were around 30-40%). The investigated actions aimed at contrasting a possible metal toxicity resulted not to be effective; in contrast, sequential filtration of the liquor leach had a significant negative effect on metals extraction. Nickel and vanadium dissolution kinetics resulted to be significantly faster than molybdenum dissolution ones. Furthermore, a simple first order kinetic model was successfully fitted to experimental data. All the observed results supported the important role of the indirect mechanism in bioleaching of LC-Finer catalysts.

Effects of dissolved low molecular weight organic acids on oxidation of ferrous iron by Acidithiobacillus ferrooxidans.

A few researchers have reported on work concerning bioleaching of heavy-metal-contaminated soil using Acidithiobacillus ferrooxidans, since this acidophile is sensitive to dissolved low molecular weight (LMW) organic acids. Iron oxidation by A. ferrooxidans R2 as well as growth on ferrous iron was inhibited by a variety of dissolved LMW organic acids. Growth experiments with ferrous iron as an oxidant showed that the inhibition capability sequence was formic acid>acetic acid>propionic acid>oxalic acid>malic acid>citric acid. The concentrations that R2 might tolerate were formic acid 0.1mmolL(-1) (2mmolkg(-1)soil), acetic and propionic acids 0.4mmolL(-1) (8mmolkg(-1)soil), oxalic acid 2.0mmolL(-1) (40mmolkg(-1)soil), malic acid 20mmolL(-1) (400mmolkg(-1)soil), citric acid 40mmolL(-1) (800mmolkg(-1)soil), respectively. Although R2 was sensitive to organic acids, the concentrations of LMW organic acids in the contaminated soils were rather lower than the tolerable levels. Hence, it is feasible that R2 might be used for bioleaching of soils contaminated with metals or metals coupled with organic compounds because of the higher concentrations of LMW organic acids to which R2 is tolerant.

Convergent evolution of a new arsenic binding site in the ArsR/SmtB family of metalloregulators.

Acidithiobacillus ferrooxidans has an arsenic resistance operon that is controlled by an As(III)-responsive transcriptional repressor, AfArsR, a member of the ArsR/SmtB family of metalloregulators. AfArsR lacks the As(III) binding site of the ArsRs from plasmid R773 and Escherichia coli, which have a Cys(32)-Val-Cys(34)-Asp-Leu-Cys(37) sequence in the DNA binding site. In contrast, it has three cysteine residues, Cys(95), Cys(96), and Cys(102), that are not present in the R773 and E. coli ArsRs. The results of direct As(III) binding measurements and x-ray absorption spectroscopy show that these three cysteine residues form a 3-coordinate As(III) binding site. DNA binding studies indicate that binding of As(III) to these cysteine residues produces derepression. Homology modeling indicates that As(III) binding sites in AfArsR are located at the ends of antiparallel C-terminal helices in each monomer that form a dimerization domain. These results suggest that the As(III)-S(3) binding sites in AfArsR and R773 ArsR arose independently at spatially distinct locations in their three-dimensional structures.

Uranium(VI) complexes with phospholipid model compounds--a laser spectroscopic study.

We present the complex formation of the uranyl ion (UO(2)(2+)) in the aqueous system with phosphocholine, O-phosphoethanolamine and O-phosphoserine. These phosphonates (R-O-PO(3)(2-)) represent the hydrophilic head groups of phospholipids. The complexation was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH=2-6. An increase of the fluorescence intensity, connected with a strong red-shift of about 8 nm compared to the free uranyl ion, indicates a complex formation between UO(2)(2+) and the phosphonates already at pH=2. Even at pH=6 these complexes prevail over the uranyl hydroxide and carbonate species, which are generated naturally at this pH. At pH=4 and higher a 1:2 complex between uranyl and O-phosphoserine was found. Complexes with a metal-to-ligand ratio of 1:1 were observed for all other ligands. Fluorescence lifetimes, emission maxima and complex stability constants at T=22+/-1 degrees C are reported. The TRLFS spectra of uranyl complexes with two phosphatidic acids (1,2-dimyristoyl-sn-glycero-3-phosphate and 1,2-dipalmitoyl-sn-glycero-3-phosphate), which represent the apolaric site of phospholipids, show in each case two different species.

A Lux-like Quorum Sensing System in the Extreme Acidophile Acidithiobacillus ferrooxidans

The genome of the acidophilic, proteobacterium Acidithiobacillus ferrooxidans, contains linked but divergently oriented genes, termed afeI and afeR, whose predicted protein products are significantly similar to the LuxI and LuxR families of proteins. A possible promoter and Lux box are predicted upstream of afeI. A cloned copy of afeI, expressed in E. coli, encodes an enzyme that catalyzes the production of a diffusible compound identified by gas chromatography and mass spectrometry as an unsubstituted N-acyl homoserine lactone (AHL) of chain length C14. This AHL can be detected by a reporter strain of Sinorhizobium meliloti Rm41 suggesting that it is biologically active. The reporter strain also responds to extracts of the supernatant of A. ferrooxidans grown to early stationary phase in sulfur medium indicating that a diffusible AHL is produced by this microorganism. Semi-quantitative RT-PCR experiments indicate that afeI and afeR are expressed maximally in early stationary phase and are more expressed when A. ferrooxidans is grown in sulfur- rather than iron-containing medium. Given the predicted amino acid sequence and functional properties of AfeI and AfeR it is proposed that A. ferrooxidans has a quorum sensing system similar to the LuxI-LuxR paradigm.