Ferrous iron-dependent volatilization of mercury by the plasma membrane of Thiobacillus ferrooxidans

Moving towards the enhancement of extracellular electron transfer in electrogens

Electrogens are very common in nature and becoming a contemporary theme for research as they can be exploited for extracellular electron transfer. Extracellular electron transfer is the key mechanism behind bioelectricity generation and bioremediation of pollutants via microbes. Extracellular electron transfer mechanisms for electrogens other than Shewanella and Geobacter are less explored. An efficient extracellular electron transfer system is crucial for the sustainable future of bioelectrochemical systems. At present, the poor extracellular electron transfer efficiency remains a decisive factor in limiting the development of efficient bioelectrochemical systems. In this review article, the EET mechanisms in different electrogens (bacteria and yeast) have been focused. Apart from the well-known electron transfer mechanisms of Shewanella oneidensis and Geobacter metallireducens, a brief introduction of the EET pathway in Rhodopseudomonas palustris TIE-1, Sideroxydans lithotrophicus ES-1, Thermincola potens JR, Lysinibacillus varians GY32, Carboxydothermus ferrireducens, Enterococcus faecalis and Saccharomyces cerevisiae have been included. In addition to this, the article discusses the several approaches to anode modification and genetic engineering that may be used in order to increase the rate of extracellular electron transfer. In the side lines, this review includes the engagement of the electrogens for different applications followed by the future perspective of efficient extracellular electron transfer.

Microbial mercury transformations: Molecules, functions and organisms

Mercury (Hg) methylation, methylmercury (MeHg) demethylation, and inorganic redox transformations of Hg are microbe-mediating processes that determine the fate and cycling of Hg and MeHg in many environments, and by doing so influence the health of humans and wild life. The discovery of the Hg methylation genes, hgcAB, in the last decade together with advances in high throughput and genome sequencing methods, have resulted in an expanded appreciation of the diversity of Hg methylating microbes. This review aims to describe experimentally confirmed and recently discovered hgcAB gene-carrying Hg methylating microbes; phylogenetic and taxonomic analyses are presented. In addition, the current knowledge on transformation mechanisms, the organisms that carry them out, and the impact of environmental parameters on Hg methylation, MeHg demethylation, and inorganic Hg reduction and oxidation is summarized. This knowledge provides a foundation for future action toward mitigating the impact of environmental Hg pollution.

Bacterial survival strategies and responses under heavy metal stress: a comprehensive overview

Heavy metals bring long-term hazardous consequences and pose a serious threat to all life forms. Being non-biodegradable, they can remain in the food webs for a long period of time. Metal ions are essential for life and indispensable for almost all aspects of metabolism but can be toxic beyond threshold level to all living beings including microbes. Heavy metals are generally present in the environment, but many geogenic and anthropogenic activities has led to excess metal ion accumulation in the environment. To survive in harsh metal contaminated environments, bacteria have certain resistance mechanisms to metabolize and transform heavy metals into less hazardous forms. This also gives rise to different species of heavy metal resistant bacteria. Herein, we have tried to incorporate the different aspects of heavy metal toxicity in bacteria and provide an up-to-date and across-the-board review. The various aspects of heavy metal biology of bacteria encompassed in this review includes the biological notion of heavy metals, toxic effect of heavy metals on bacteria, the factors regulating bacterial heavy metal resistance, the diverse mechanisms governing bacterial heavy metal resistance, bacterial responses to heavy metal stress, and a brief overview of gene regulation under heavy metal stress.

Optimization of microbial detoxification for an aquatic mercury-contaminated environment

Mercury (Hg) reduction performed by microorganisms is well recognized as a biological means for remediation of contaminated environment. Recently, studies demonstrated that Hg-resistant microorganisms of Tagus Estuary are involved in metal reduction processes. In the present study, aerobic microbial community isolated from a highly Hg-contaminated area of Tagus Estuary was used to determine the optimization of the reduction process in conditions such as the contaminated ecosystem. Factorial design methodology was employed to examine the influence of glucose, sulfate, iron, and chloride on Hg reduction. In the presence of several concentrations of these elements, microbial community reduced Hg in a range of 37-61% of the initial 0.1 mg/ml Hg²⁺ levels. The response prediction through central composite design showed that the increase of sulfate concentration led to an optimal response in Hg reduction by microbial community, while the rise in chloride levels markedly decreased metal reduction. Iron may exert antagonistic effects depending upon the media composition. These results are useful in understanding the persistence of Hg contamination in Tagus Estuary after inactivation of critical industrial units, as well as data might also be beneficial for development of new bioremediation strategies either in Tagus Estuary and/or in other Hg-contaminated aquatic environments.

Quantifying the effects of soil temperature, moisture and sterilization on elemental mercury formation in boreal soils

Soils are a source of elemental mercury (Hg(0)) to the atmosphere, however the effects of soil temperature and moisture on Hg(0) formation is not well defined. This research quantifies the effect of varying soil temperature (278-303 K), moisture (15-80% water filled pore space (WFPS)) and sterilization on the kinetics of Hg(0) formation in forested soils of Nova Scotia, Canada. Both, the logarithm of cumulative mass of Hg(0) formed in soils and the reduction rate constants (k values) increased with temperature and moisture respectively. Sterilizing soils significantly (p < 0.05, n = 10) decreased the percent of total Hg reduced to Hg(0). We describe the fundamentals of Hg(0) formation in soils and our results highlight two key processes: (i) a fast abiotic process that peaks at 45% WFPS and depletes a small pool of Hg(0) and; (ii) a slower, rate limiting biotic process that generates a large pool of reducible Hg(II).

Construction of recombinant mercury resistant Acidithiobacillus caldus

A mercury-resistant plasmid of pTMJ212 which was able to shuttle between Acidithiobacillus caldus and Escherichia coli was constructed by inserting the mercury resistant determinants, the mer operon of Acidithiobacillus ferrooxidans, into the IncQ plasmid of pJRD215. pTMJ212 was transferred from Escherichia coli into Acidithiobacillus caldus through conjugation. Furthermore, pTMJ212 was transferred back from Acidithiobacillus caldus into Escherichia coli, thereby confirming the initial transfer of pTMJ212 from Escherichia coli to Acidithiobacillus caldus. Compared to the control, the cell growth of the recombinant Acidithiobacillus caldus increased markedly under mercury (Hg(2+)) stress especially at Hg(2+) concentrations ranging from 2.0 to 4.5 μg/ml.

ribB and ribBA genes from Acidithiobacillus ferrooxidans: expression levels under different growth conditions and phylogenetic analysis

Acidithiobacillus ferrooxidans is a Gram-negative, chemolithoautotrophic bacterium involved in metal bioleaching. Using the RNA arbitrarily primed polymerase chain reaction (RAP-PCR), we have identified several cDNAs that were differentially expressed when A. ferrooxidans LR was submitted to potassium- and phosphate-limiting conditions. One of these cDNAs showed similarity with ribB. An analysis of the A. ferrooxidans ATCC 23270 genome, made available by The Institute for Genomic Research, showed that the ribB gene was not located in the rib operon, but a ribBA gene was present in this operon instead. The ribBA gene was isolated from A. ferrooxidans LR and expression of both ribB and ribBA was investigated. Transcript levels of both genes were enhanced in cells grown in the absence of K2HPO4, in the presence of zinc and copper sulfate and in different pHs. Transcript levels decreased upon exposure to a temperature higher than the ideal 30 degrees C and at pH 1.2. A comparative genomic analysis using the A. ferrooxidans ATCC 23270 genome revealed similar putative regulatory elements for both genes. Moreover, an RFN element was identified upstream from the ribB gene. Phylogenetic analysis of the distribution of RibB and RibBA in bacteria showed six different combinations. We suggest that the presence of duplicated riboflavin synthesis genes in bacteria must provide their host with some benefit in certain stressful situations.

Bioremediation of toxic heavy metals using acidothermophilic autotrophes

Investigations were carried out to isolate microbial strains from soil, mud and water samples from metallurgically polluted environment for bioremediation of toxic heavy metals. As a result of primary and secondary screening various 72 acidothermophilic autotrophic microbes were isolated and adapted for metal tolerance and biosorption potentiality. The multi-metal tolerance was developed with higher gradient of concentrations of Ag, As, Bi, Cd, Cr, Co, Cu, Hg, Li, Mo, Pb, Sn and Zn. The isolates were checked for their biosolubilization ability with copper containing metal sulfide ores. In case of chalcopyrite 85.82% and in covellite as high as 97.5% copper solubilization occurred in presence of 10(-3) M multi-heavy metals on fifth day at 55 degrees C and pH 2.5. Chemical analyses were carried out by inductively coupled plasma spectroscopy (ICP) for metal absorption. The selected highly potential isolate (ATh-14) showed maximum adsorption of Ag 73%, followed by Pb 35%, Zn 34%, As 19%, Ni 15% and Cr 9% in chalcopyrite.

Mercury analysis of acid- and alkaline-reduced biological samples: identification of meta-cinnabar as the major biotransformed compound in algae

The biotransformation of Hg(II) in pH-controlled and aerated algal cultures was investigated. Previous researchers have observed losses in Hg detection in vitro with the addition of cysteine under acid reduction conditions in the presence of SnCl2. They proposed that this was the effect of Hg-thiol complexing. The present study found that cysteine-Hg, protein and nonprotein thiol chelates, and nucleoside chelates of Hg were all fully detectable under acid reduction conditions without previous digestion. Furthermore, organic (R-Hg) mercury compounds could not be detected under either the acid or alkaline reduction conditions, and only beta-HgS was detected under alkaline and not under acid SnCl2 reduction conditions. The blue-green alga Limnothrix planctonica biotransformed the bulk of Hg(II) applied as HgCl2 into a form with the analytical properties of beta-HgS. Similar results were obtained for the eukaryotic alga Selenastrum minutum. No evidence for the synthesis of organomercurials such as CH3Hg+ was obtained from analysis of either airstream or biomass samples under the aerobic conditions of the study. An analytical procedure that involved both acid and alkaline reduction was developed. It provides the first selective method for the determination of beta-HgS in biological samples. Under aerobic conditions, Hg(II) is biotransformed mainly into beta-HgS (meta-cinnabar), and this occurs in both prokaryotic and eukaryotic algae. This has important implications with respect to identification of mercury species and cycling in aquatic habitats.

Cytochrome c oxidase purified from a mercury-resistant strain of Acidithiobacillus ferrooxidans volatilizes mercury

We suggested in our previous study that the plasma membrane cytochrome c oxidase of the mercury-resistant iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans, SUG 202, is involved in Fe2+-dependent mercury volatilization. To study the involvement of A. ferrooxidans cytochrome c oxidase in mercury reduction, the cytochrome c oxidase was extracted from mercury-resistant and mercury-sensitive strains and purified. The Fe2+-dependent mercury volatilization activities of the oxidases from these strains were compared. The cytochrome c oxidase from strain SUG 2-2 volatilized 39% of the total Hg2+ (7 nmol) that had been added to a 10-ml reaction mixture (pH 3.8) in the presence of 10 micromol of Fe2+ after a 7-d incubation period at 30 degrees C. In contrast, the enzyme purified from the mercury-sensitive strain AP19-3 volatilized 3.5% of the total mercury under the same conditions. The boiled SUG 2-2 oxidase did not exhibit activity to volatilize mercury. Fe2+ reduced the oxidase from SUG 2-2 and Hg2+ oxidized the reduced enzyme. The purified SUG 2-2 oxidase is composed of three protein subunits with apparent molecular weights of 56,000 Da (alpha), 24,000 Da (beta), and 19,000 Da (gamma). The amount of mercury bound to the purified SUG 2-2 oxidase was 6.2 microg/mg protein and those bound to alpha-, beta- and gamma-subunits of the cytochrome c oxidase were 3.5, 2.6 and 0.7 microg/mg protein, respectively.

Existence of an iron-oxidizing bacterium Acidithiobacillus ferrooxidans resistant to organomercurial compounds

Acidithiobacillus ferrooxidans MON-1 which is highly resistant to Hg2+ could grow in a ferrous sulfate medium (pH 2.5) with 0.1 microM p-chloromercuribenzoic acid (PCMB) with a lag time of 2 d. In contrast, A. ferrooxidans AP19-3 which is sensitive to Hg2+ did not grow in the medium. Nine strains of A. ferrooxidans, including seven strains of the American Type Culture Collection grew in the medium with a lag time ranging from 5 to 12 d. The resting cells of MON-1, which has NADPH-dependent mercuric reductase activity, could volatilize Hg0 when incubated in acidic water (pH 3.0) containing 0.1 microM PCMB. However, the resting cells of AP19-3, which has a similar level of NADPH-dependent mercuric reductase activity compared with MON-1, did not volatilize Hg0 from the reaction mixture with 0.1 microM PCMB. The activity level of the 11 strains of A. ferrooxidans to volatilize Hg0 from PCMB corresponded well with the level of growth inhibition by PCMB observed in the growth experiments. The resting cells of MON-1 volatilized Hg0 from phenylmercury acetate (PMA) and methylmercury chloride (MMC) as well as PCMB. The cytosol prepared from MON-1 could volatilize Hg0 from PCMB (0.015 nmol mg(-1) h(-1)), PMA (0.33 nmol mg(-1) h(-1)) and MMC (0.005 nmol mg(-1) h(-1)) in the presence of NADPH and beta-mercaptoethanol.

Characteristics and adaptability of iron- and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates

Microorganisms are used in large-scale heap or tank aeration processes for the commercial extraction of a variety of metals from their ores or concentrates. These include copper, cobalt, gold and, in the past, uranium. The metal solubilization processes are considered to be largely chemical with the microorganisms providing the chemicals and the space (exopolysaccharide layer) where the mineral dissolution reactions occur. Temperatures at which these processes are carried out can vary from ambient to 80 degrees C and the types of organisms present depends to a large extent on the process temperature used. Irrespective of the operation temperature, biomining microbes have several characteristics in common. One shared characteristic is their ability to produce the ferric iron and sulfuric acid required to degrade the mineral and facilitate metal recovery. Other characteristics are their ability to grow autotrophically, their acid-tolerance and their inherent metal resistance or ability to acquire metal resistance. Although the microorganisms that drive the process have the above properties in common, biomining microbes usually occur in consortia in which cross-feeding may occur such that a combination of microbes including some with heterotrophic tendencies may contribute to the efficiency of the process. The remarkable adaptability of these organisms is assisted by several of the processes being continuous-flow systems that enable the continual selection of microorganisms that are more efficient at mineral degradation. Adaptability is also assisted by the processes being open and non-sterile thereby permitting new organisms to enter. This openness allows for the possibility of new genes that improve cell fitness to be selected from the horizontal gene pool. Characteristics that biomining microorganisms have in common and examples of their remarkable adaptability are described.

Existence of a tungsten-binding protein in Acidithiobacillus ferrooxidans AP19-3

A tungsten-binding protein was purified from a plasma membrane preparation of the iron-oxidizing bacterium, Acidithiobacillus ferrooxidans AP19-3 in an electrophoretically homogenous state. The protein was composed of two subunits with apparent molecular masses of 12 and 20.7 kDa. The molecular mass of the native protein was estimated to be 26.4 kDa in the presence of 1.5% 1-o-octyl-D -glucopyranoside (OGL), indicating that the native tungsten-binding protein is a heterodimeric protein. The amounts of tungsten bound to 1 mg of plasma membranes of A. ferrooxidans AP19-3 and the purified tungsten-binding protein at pH 3.0 were 191 and 1506 mug, respectively. In contrast, the amounts of tungsten bound to 1 mg of albumin, aldolase, catalase, chymotrypsinogen A, ferritin, and ferredoxin at pH 3.0 were 13.1, 18.6, 12.8, 16.6, 11.4, and 6.1 mug, respectively. Incubation of the tungsten-binding protein for 1 h with 10 mM Na(2)WO(4) plus 10 mM metal ion, such as NaVO(3), Na(2)MoO(4), CuSO(4), NiSO(4), MnSO(4), CoSO(4), or CdCl(2), did not markedly affect the amount of tungsten bound to the tungsten-binding protein, suggesting that the protein specifically binds tungsten.

Microbial reduction of metals and radionuclides

The microbial reduction of metals has attracted recent interest as these transformations can play crucial roles in the cycling of both inorganic and organic species in a range of environments and, if harnessed, may offer the basis for a wide range of innovative biotechnological processes. Under certain conditions, however, microbial metal reduction can also mobilise toxic metals with potentially calamitous effects on human health. This review focuses on recent research on the reduction of a wide range of metals including Fe(III), Mn(IV) and other more toxic metals such as Cr(VI), Hg(II), Co(III), Pd(II), Au(III), Ag(I), Mo(VI) and V(V). The reduction of metalloids including As(V) and Se(VI) and radionuclides including U(VI), Np(V) and Tc(VII) is also reviewed. Rapid advances over the last decade have resulted in a detailed understanding of some of these transformations at a molecular level. Where known, the mechanisms of metal reduction are discussed, alongside the environmental impact of such transformations and possible biotechnological applications that could utilise these activities.

Metal and radionuclide bioremediation: issues, considerations and potentials

Recent demonstrations of the removal and immobilization of inorganic contaminants by microbial transformations, sorption and mineralization show the potential of both natural and engineered microbes as bioremedial tools. Demonstrations of microbe-mediated mineral formation in biofilms implicate this mode of microbial life in geological evolution and remediation of inorganic contaminants.

Microbial detoxification of metals and radionuclides

Microorganisms have important roles in the biogeochemical cycling of toxic metals and radionuclides. Recent advances have been made in understanding metal-microbe interactions and new applications of these processes to the detoxification of metal and radionuclide contamination have been developed.

Mechanism of growth inhibition by tungsten in Acidithiobacillus ferrooxidans

Cell growth of three hundred iron-oxidizing bacteria isolated from natural environments was inhibited strongly by 0.05 mM, and completely by 0.2 mM of sodium tungstate (Na2WO4), respectively. Since no great difference in the level of tungsten inhibition was observed among the 300 strains tested, the mechanism of inhibition by Na2WO4 was studied with Acidithiobacillus ferrooxidans strain AP19-3. When resting cells of AP19-3 were incubated in 0.1 M beta-alanine-SO4(2-) buffer (pH 3.0) with 0.1 mM Na2WO4 for 1 h, the amount of tungsten bound to the cells was 12 microg/mg protein. The optimum pH for tungsten binding to the resting cells was 2 to approximately 3. Approximately 2 times more tungsten bound to the cells at pH 3.0 than at pH 6.0. The tungsten binding was specifically inhibited by sodium molybdenum. However, copper, nickel, cadmium, zinc, manganese, cobalt, and vanadate did not disturb tungsten binding to the resting cells. The iron-oxidizing activity of AP19-3 was inhibited 24, 62, and 77% by 1, 5, and 10 mM of Na2WO4, respectively. Among the components of iron oxidation enzyme system, iron:cytochrome c oxidoreductase activity was not inhibited by 10 mM of Na2WO4. In contrast, the activity of cytochrome c oxidase purified highly from the strain was inhibited 50 and 72%, respectively, by 0.05 and 0.1 mM of Na2WO4. The amounts of tungsten bound to plasma membrane, cytosol fraction, and a purified cytochrome c oxidase were 8, 0.5, and 191 microg/mg protein, respectively. From the results, the growth inhibition by Na2WO4 observed in A. ferrooxidans is explained as follows: tungsten binds to cytochrome c oxidase in plasma membranes and inhibits cytochrome c oxidase activity, and as a results, the generation of energy needed for cell growth from the oxidation of Fe2+ is stopped.