Reduction of Hexavalent Chromium by Immobilized Viable Cells of Arthrobacter sp. SUK 1201

Arthrobacter sp. SUK 1201, a potent isolate reported from chromite mine overburden of Orissa, India, has been evaluated for Cr(VI) reduction with immobilized whole cells. For whole-cell immobilization, Ba-alginate was found to be most effective, and the Cr(VI) reduction potential was maximum in minimal salts (MS) medium with cells immobilized in 2% alginate. Fourier transform infrared spectra of depolymerized cells has failed to detect any sign of complexation of Cr(VI) or its reduced products with the cell mass. Reduction efficiency of the beads increased with increase in cell load, but decreased with increase in Cr(VI) concentration in the medium. Glycerol was the most potent electron donor for chromate reduction, followed by glucose and peptone. Optimum pH for Cr(VI) reduction was 7.0, and the process was inhibited by metal ions such as Ni(II), Co(II), Cd(II), Zn(II), and Mn(II) but not by Cu(II) and Fe(III). Similarly, CCCP (carbonyl cyanide-m-chlorophenylhydrazone), DCC (N,N,-dicyclohexylcarbodiimide), sodium azide, and sodium fluoride were inhibitory in nature, whereas chromate reduction was unaffected in the presence of DNP (2,4-dinitrophenol). Moreover, immobilized cells of SUK 1201 remained biologically active for four consecutive cycles, accompanied with an initial increase in cell number in the beads, although a decline in chromate reduction was recorded from the second cycle onward. Immobilized cells of Arthrobacter sp. SUK 1201, therefore, could be a potential tool for long-term uses in chromium detoxification.     

Microbial Reduction of Cr (VI)-loaded Schwertmannite by Shewanella oneidensis MR-1

Microbial transformation of sulfate minerals plays an important role in controlling the behavior of heavy metals in mining areas. Here, the anaerobic reduction of Cr (VI)-loaded schwertmannite by Shewanella oneidensis MR-1 (S. oneidensis MR-1) was investigated. The release of ferrous iron (Fe(II)) to the solution demonstrated the microbial reduction of structural Fe(III) from the schwertmannite to Fe(II). The concentration of Cr in solution decreased in all treatments, indicating that no Cr was released to the solution during this bio-reduction process of schwertmannite. The incorporation of chromate into the mineral structure of schwertmannite increased the microbial stability of the mineral, retarding the formation of secondary phases during bio-reduction process. Analysis of the XRD, SEM and fourier transform infrared spectroscopy (FT-IR) results further showed that goethite formed after 3 or 7 days with a lower content (0.22% or 0.37%) of Cr in schwertmannite, while no secondary mineral was observed with a higher concentration of Cr (0.6 wt%) incorporated in schwertmannite until 22 days. These results imply that microbial reduction of Cr(VI)-loaded schwertmannite does not lead to the release of Cr to the solution, and the microbial stability of schwertmannite will be increased by the incorporation of chromate.     

Hexavalent-chromium reduction by a chromate-resistantBacillus sp. strain

Bacillus strain QC1-2, isolated from a chromium-polluted zone, was selected by its high ability to both tolerate and reduce hexavalent chromium [Cr(VI)] to less-toxic trivalent chromium [Cr(III)]. Cell suspensions of strain QC1-2 rapidly reduced Cr(VI), in both aerobic and anaerobic conditions, to Cr(III) which remained in the supernatant. Cr(VI) reduction was dependent on the addition of glucose but sulfate, an inhibitor of chromate transport, had no effect. Studies with permeabilized cells and cell extracts showed that the Cr(VI) reductase of strain QC1-2 is a soluble NADH-dependent enzyme.     

Anaerobic co-reduction of chromate and nitrate by bacterial cultures of Staphylococcus epidermidis L-02

Industrial wastewater is often polluted by Cr(VI) compounds, presenting a serious environmental problem. This study addresses the removal of toxic, mutagenic Cr(VI) by means of microbial reduction to Cr(III), which can then be precipitated as oxides or hydroxides and extracted from the aquatic system. A strain of Staphylococcus epidermidis L-02 was isolated from a bacterial consortium used for the remediation of a chromate-contaminated constructed wetland system. This strain reduced Cr(VI) by using pyruvate as an electron donor under anaerobic conditions. The aims of the present study were to investigate the specific rate of Cr(VI) reduction by the strain L-02, the effects of chromate and nitrate (available as electron acceptors) on the strain, and the interference of chromate and nitrate reduction processes. The presence of Cr(VI) decreased the growth rate of the bacterium. Chromate and nitrate reduction did not occur under sterile conditions but was observed during tests with the strain L-02. The presence of nitrate increased both the specific Cr(VI) reduction rate and the cell number. Under denitrifying conditions, Cr(VI) reduction was not inhibited by nitrite, which was produced during nitrate reduction. The average specific rate of chromate reduction reached 4.4 μmol Cr 10¹⁰ cells⁻¹ h⁻¹, but was only 2.0 μmol Cr 10¹⁰ cells⁻¹ h⁻¹ at 20 °C. The maximum specific rate was as high as 8.8–9.8 μmol Cr 10¹⁰ cells⁻¹ h⁻¹. The role of nitrate in chromate reduction is discussed.     

Reduction of chromate,selenite, tellurite,and iron (III) by the moderately thermophilic bacterium <Emphasis Type="Italic">Bacillus thermoamylovorans</Emphasis> SKC1

A moderately thermophilic, facultatively anaerobic bacterium capable of reducing Cr(VI) (strain SKC1) was isolated from municipal sewage. Based on the analysis of the 16S rRNA gene nucleotide sequence and DNA-DNA hybridization data, strain SKC1 was identified as a representative of the species Bacillus thermoamylovorans. B. thermoamylovorans SKC1 is capable of reducing chromate with L-arabinose as an electron donor with an optimum at 50°C and neutral pH. The culture is able to reduce Cr(VI) at its initial concentration in the medium of up to 150 mg/l. In addition to chromate, strain SKC1 is capable of reducing selenite and tellurite, as well as soluble forms of Fe(III). It was shown that Cr(VI), Te(IV), and Se(IV) exert a bacteriostatic effect on strain SKC1, and the reduction of these anions performs the detoxification function. This is the first communication on the reduction of chromate, selenite, tellurite, and soluble Fe(III) species by a culture of thermophilic bacilli.     

Chromium(VI)-reducing <Emphasis Type="Italic">Chlorella</Emphasis> spp. isolated from disposal sites of paper-pulp and electroplating industry

We isolated four cultures of chromate resistant, unicellular, non-motile green algae from disposal sites of the paper-pulp and electroplating industries. These algae were maintained in Tris-acetate-glycerophosphate medium containing 30 μM K₂Cr₂O₇. The morphological features as well as analysis of the 500-bp fragment of 18S rDNA (NS 12 region) showed that these isolates belong to Chlorella spp. These isolates showed EC₅₀ values for chromate ranging from 60 to 125 μM. Uptake studies with radioactive ⁵¹Cr(VI) showed that 10–19% of total radioactivity was intracellular, and 1–2% was bound to the cell wall. The rest of the activity remained in the medium, suggesting that resistance was not related to accumulation of Cr(VI) in the cells. Interestingly, when these isolates were grown in the presence of 30 μM of K₂Cr₂O₇, a decrease in the Cr(VI) concentration in the medium was observed. Only live cells could deplete Cr(VI) from the supernatant, suggesting the presence of chromium reduction activity in these Chlorella isolates. Cr(VI) reduction activity of the cells of Chlorella was stimulated by light as well as by acetate and glycerophosphate. Treatment of Chlorella cells with 3-(3,4 dichlorophenyl),1,1dimethyl urea (DCMU) did not affect the Cr(VI) reduction. However, if the cells were treated with sodium azide, Cr(VI) reduction was severely affected. Though chromate resistance has been well documented in algae, the information on chromate reduction by algae is scant. This paper discusses the Cr(VI) reduction by Cr(VI) resistant Chlorella, which may find a use in the effective bioremediation of Cr(VI).     

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The bioreduction capacity of Cr(VI) by Shewanella is mainly governed by its bidirectional extracellular electron transfer (EET). However, the low bidirectional EET efficiency restricts its wider applications in remediation of the environments contaminated by Cr(VI). Cyclic adenosine 3′,5′-monophosphate (cAMP) commonly exists in Shewanella strains and cAMP–cyclic adenosine 3′,5′-monophosphate receptor protein (CRP) system regulates multiple bidirectional EET-related pathways. This inspires us to strengthen the bidirectional EET through elevating the intracellular cAMP level in Shewanella strains. In this study, an exogenous gene encoding adenylate cyclase from the soil bacterium Beggiatoa sp. PS is functionally expressed in Shewanella oneidensis MR-1 (the strain MR-1/pbPAC) and a MR-1 mutant lacking all endogenous adenylate cyclase encoding genes (the strain Δca/pbPAC). The engineered strains exhibit the enhanced bidirectional EET capacities in microbial electrochemical systems compared with their counterparts. Meanwhile, a three times more rapid reduction rate of Cr(VI) is achieved by the strain MR-1/pbPAC than the control in batch experiments. Furthermore, a higher Cr(VI) reduction efficiency is also achieved by the strain MR-1/pbPAC in the Cr(VI)-reducing biocathode experiments. Such a bidirectional enhancement is attributed to the improved production of cAMP–CRP complex, which upregulates the expression levels of the genes encoding the c-type cytochromes and flavins synthetic pathways. Specially, this strategy could be used as a broad-spectrum approach for the other Shewanella strains. Our results demonstrate that elevating the intracellular cAMP levels could be an efficient strategy to enhance the bidirectional EET of Shewanella strains and improve their pollutant transformation capacity.     

Evaluation of biosorption potency of <Emphasis Type="Italic">Acinetobacter</Emphasis> sp. for removal of hexavalent chromium from tannery effluent

Two bacterial consortia were developed by continuous enrichment of microbial population of tannery and pulp and paper mill effluent contained Serratia mercascens, Pseudomonas fluorescence, Escherichia coli, Pseudomonas aeruginosa and Acinetobacter sp. identified by 16S rDNA method. The consortia evaluated for removal of chromate [(Cr(VI)] in shake flask culture indicated pulp and paper mill consortium had more potential for removal of chromate. Acinetobacter sp. isolated from pulp and paper mill consortium removed higher amount of chromate [Cr(VI)] under aerobic conditions. Parameters optimized in different carbon, nitrogen sources, and pH, indicated maximum removal of chromate in sodium acetate (0.2%), sodium nitrate (0.1%) and pH 7 by Acinetobacter sp. Bacteria was applied in 2-l bioreactor significantly removed chromate after 3 days. The results of the study indicated removal of more than 75% chromium by Acinetobacter sp. determined by diphenylcarbazide colorimetric assay and atomic absorption spectrophotometer after 7 days. Study of microbial [Cr(VI)] removal and identification of reduction intermediates has been hindered by the lack of analytical techniques. Therefore, removal of chromium was further substantiated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) which indicated bioaccumulation of chromium in the bacterial cells.     

Pyridine-2,6-bis(thiocarboxylic acid) Produced by <Emphasis Type="BoldItalic">Pseudomonas stutzeri</Emphasis> KC Reduces Chromium(VI) and Precipitates Mercury,Cadmium, Lead and Arsenic

Interactions of the Pseudomonas stutzeri KC siderophore pyridine-2,6-bis(thiocarboxylic acid) (pdtc) with chromium(VI), mercury(II), cadmium(II), lead(II), and arsenic(III) are described. Pdtc was found to reduce Cr(VI) to Cr(III) in both bacterial cultures and in abiotic reactions with chemically synthesized pdtc. Cr(III) subsequently formed complexes with pdtc and pdtc hydrolysis products, and their presence was confirmed using electrospray ionization-mass spectrometry (ESI-MS). Cr(III):pdtc complexes were found to slowly release Cr(III) as chromium sulfide and possibly Cr(III) oxides. Pdtc also formed poorly soluble complexes with Hg, Cd, Pb, and As(III). Hydrolysis of those complexes led to the formation of their respective metal sulfides as confirmed by energy dispersive X-ray spectroscopy (EDS) elemental analysis. The pdtc-producing strain P. stutzeri KC showed higher tolerance to most of these metals as compared to a pdtc-negative mutant. A novel role of pdtc is postulated as its involvement in providing an extracellular pool of thiols that are used for redox processes in detoxification of the bacterial extracellular environment. These redox processes can be mediated by transition metal:pdtc complexes.     

Different physiological responses to chromate and dichromate in the chromium resistant and reducing strain <Emphasis Type="Italic">Ochrobactrum tritici</Emphasis> 5bvl1

Studies of Cr(VI) toxicity are generally performed using chromate salts in solution, both when studying the effects on prokaryotes and eukaryotes. Some studies on human carcinogenesis and toxicology on bacteria were done using dichromate, but comparison with chromate was never reported before, and dichromate existence was never taken into consideration and usually overlooked. This paper studied comparatively the effect of dichromate and chromate on the physiology of Ochrobactrum tritici strain 5bvl1, a highly Cr(VI)-resistant and reducing microorganism. This study demonstrated that the addition of chromate or dichromate sodium salts to growth medium at neutral pH ended-up in two different solutions with a different balance of chemical species. Cr(VI) was toxic to O. tritici strain 5bvl1, as clearly shown on growth, reduction, respiration, glucose accumulation assays and by comparing cell morphology. Moreover, the addition of sodium dichromate was always more toxic to cells when compared to chromate and achieved a higher inhibition of every parameter studied. The toxicity differences between the two Cr(VI) oxyanions indicate the possibility of a different impact of Cr(VI) contamination on the environment. This may be of major importance, considering the slight acidity of most of the arable lands which favours the presence of dichromate, the more toxic species.     

Chromium resistance strategies and toxicity: what makes <Emphasis Type="Italic">Ochrobactrum tritici</Emphasis> 5bvl1 a strain highly resistant

Large-scale industrial use of chromium (Cr) resulted in widespread environmental contamination with hexavalent chromium (Cr(VI)). The ability of microorganisms to survive in these environments and detoxify chromate requires the presence of specific resistance systems. Several Cr(VI) resistant species, belonging to a variety of genera, have been isolated in recent years. Ochrobactrum tritici strain 5bvl1 is a model for a highly Cr(VI)-resistant and reducing microorganism, with different strategies to cope with chromium. The strain contains the transposon-located (TnOtChr) chromate resistance genes chrB, chrA, chrC, chrF. The chrB and chrA genes were found to be essential for the establishment of high resistance but not chrC or chrF genes. Other mechanisms involved in chromium resistance in this strain were related to strategies such as specific or unspecific Cr(VI) reduction, free-radical detoxifying activities, and repairing DNA damage. Expression of the chrB, chrC or chrF genes was related to increased resistance to superoxide-generating agents. Genetic analyses also showed that, the ruvB gene is related to chromium resistance in O. tritici 5bvl1. The RuvABC complex probably does not form when ruvB gene is interrupted, and the repair of DNA damage induced by chromium is prevented. Aerobic or anaerobic chromate reductase activity and other unspecific mechanisms for chromium reduction have been identified in different bacteria. In the strain O. tritici 5bvl1, several unspecific mechanisms were found. Dichromate and chromate have different effects on the physiology of the chromium resistant strains and dichromate seems to be more toxic. Toxicity of Cr(VI) was evaluated by following growth, reduction, respiration, glucose uptake assays and by comparing cell morphology.     

Characterization and genomic analysis of chromate resistant and reducing <Emphasis Type="Italic">Bacillus cereus</Emphasis> strain SJ1

Background  

Chromium is a toxic heavy metal, which primarily exists in two inorganic forms, Cr(VI) and Cr(III). Chromate [Cr(VI)] is carcinogenic, mutational, and teratogenic due to its strong oxidizing nature. Biotransformation of Cr(VI) to less-toxic Cr(III) by chromate-resistant and reducing bacteria has offered an ecological and economical option for chromate detoxification and bioremediation. However, knowledge of the genetic determinants for chromate resistance and reduction has been limited so far. Our main aim was to investigate chromate resistance and reduction by Bacillus cereus SJ1, and to further study the underlying mechanisms at the molecular level using the obtained genome sequence.     

Role of glutathione in detoxification of metal(loid)s by Saccharomyces cerevisiae

Cellular glutathione (GSH) was implicated in tolerance to potentially toxic metal(loid)s using two strains of Saccharomyces cerevisiae, a wild-type (sigma 1278b) and a GSH-deficient mutant strain (gshA-2). Both yeast strains exhibited no significant difference in tolerance to tellurite, zinc, cobalt, copper, manganese, nickel and chromate. There was no marked influence of glutathione on the accumulation of Te, Co, Cu, and Mn, although the absence of cellular glutathione significantly increased the cellular content of Zn and Ni, but greatly decreased Cr content without significant alteration of tolerance. These results indicated the independence of cellular glutathione activity from tolerance to Te, Zn, Co, Cu, Mn, Ni, and Cr. However, involvement of glutathione in Zn, Ni and Cr uptake is possible. The glutathione-deficient strain displayed a high sensitivity to selenite and cadmium in comparison to the wild-type strain of S. cerevisiae. The minimum inhibitory concentrations of Se and Cd for the glutathione-deficient strain were 980 +/- 13 and 32 +/- 4 microM, respectively, whereas the wild strain tolerated up to 4080 +/- 198 microM Se and 148 +/- 5 microM Cd. A relationship between tolerance and reduced cellular content of both Se and Cd was also shown: the mutant strain accumulated approximately three-fold more Se and two-fold more Cd than that accumulated by the wild-type strain. This suggests an influence of GSH on cellular uptake of Se and Cd, and also directly confirms the protective action of such a cellular thiol compound against Se and Cd toxicity.     

Dosage-dependent proteome response of Shewanella oneidensis MR-1 to acute chromate challenge

Proteome alterations in the metal-reducing bacterium Shewanella oneidensis MR-1 in response to different acute dose challenges (0.3, 0.5, or 1 mM) of the toxic metal chromate [Cr(VI)] were characterized with multidimensional HPLC-MS/MS. Proteome measurements were performed and compared on both quadrupole ion traps as well as linear trapping quadrupole mass spectrometers. We have found that the implementation of multidimensional liquid chromatography on-line with the rapid scanning, high throughput linear trapping quadrupole platform resulted in a dramatic increase in the number of measured peptides and, thus, the number of identified proteins. A total of 2406 functionally diverse, nonredundant proteins were identified in this study, representing a relatively deep proteome coverage for this organism. The core molecular response to chromate challenge under all three concentrations consisted predominantly of proteins with annotated functions in transport and binding (e.g., components of the TonB1 iron transport system, TonB-dependent receptors, and sulfate transporters) as well as a functionally undefined DNA-binding response regulator (SO2426) that might play a role in mediating metal stress responses. In addition, proteins annotated as a cytochrome c, a putative azoreductase, and various proteins involved in general stress protection were up-regulated at the higher Cr(VI) doses (0.5 and 1 mM) only. Proteins down-regulated in response to metal treatment were distributed across diverse functional categories, with energy metabolism proteins dominating. The results presented in this work demonstrate the dynamic dosage response of S. oneidensis to sub-toxic levels of chromate.     

Microbial reduction of Cr(VI) in the presence of chromate conversion coating constituents

The reduction of Cr(VI) by the metal-reducing bacterium Shewanella oneidensis MR-1 was evaluated, to determine the potential for exploiting Cr(VI) bioreduction as a means of treating chromate conversion coating (CCC) waste streams. Inclusion of Cr(VI) at concentrations ≥1 mM inhibited aerobic growth of S. oneidensis, but that organism was able to reduce Cr(VI) at a concentration of up to 1 mM under anaerobic, nongrowth conditions. S. oneidensis reduced Cr(VI) in the presence of common CCC constituents, with the exception of ferricyanide, when these CCC constituents were included at concentrations typical of CCC waste streams. Ferricyanide inhibited neither aerobic growth nor metabolism under aerobic, nitrate- or iron-reducing conditions, suggesting that the ferricyanide-depended inhibition of Cr(VI) reduction is not due to broad metabolic inhibition, but is specific to Cr(VI) reduction. Results indicate that under some conditions, the activities of metal-reducing bacteria, such as S. oneidensis, could be exploited for the removal of Cr(VI) from CCC waste streams under appropriate conditions.     

Hydrogenases in sulfate-reducing bacteria function as chromium reductase

The ability of sulfate-reducing bacteria (SRB) to reduce chromate VI has been studied for possible application to the decontamination of polluted environments. Metal reduction can be achieved both chemically, by H₂S produced by the bacteria, and enzymatically, by polyhemic cytochromes c₃. We demonstrate that, in addition to low potential polyheme c-type cytochromes, the ability to reduce chromate is widespread among [Fe], [NiFe], and [NiFeSe] hydrogenases isolated from SRB of the genera Desulfovibrio and Desulfomicrobium. Among them, the [Fe] hydrogenase from Desulfovibrio vulgaris strain Hildenborough reduces Cr(VI) with the highest rate. Both [Fe] and [NiFeSe] enzymes exhibit the same K_m towards Cr(VI), suggesting that Cr(VI) reduction rates are directly correlated with hydrogen consumption rates. Electron paramagnetic resonance spectroscopy enabled us to probe the oxidation by Cr(VI) of the various metal centers in both [NiFe] and [Fe] hydrogenases. These experiments showed that Cr(VI) is reduced to paramagnetic Cr(III), and revealed inhibition of the enzyme at high Cr(VI) concentrations. The significant decrease of both hydrogenase and Cr(VI)-reductase activities in a mutant lacking [Fe] hydrogenase demonstrated the involvement of this enzyme in Cr(VI) reduction in vivo. Experiments with [3Fe-4S] ferredoxin from Desulfovibrio gigas demonstrated that the low redox [Fe-S] (non-heme iron) clusters are involved in the mechanism of metal reduction by hydrogenases.     

Evaluation of the effects of various culture conditions on Cr(VI) reduction by Shewanella oneidensis MR-1 in a novel high-throughput mini-bioreactor

The growth and Cr(VI) reduction by Shewanella oneidensis MR-1 was examined using a mini-bioreactor system that independently monitors and controls pH, dissolved oxygen (DO), and temperature for each of its 24, 10-mL reactors. Independent monitoring and control of each reactor in the cassette allows the exploration of a matrix of environmental conditions known to influence S. oneidensis chromium reduction. S. oneidensis MR-1 grew in minimal medium without amino acid or vitamin supplementation under aerobic conditions but required serine and glycine supplementation under anaerobic conditions. Growth was inhibited by DO concentrations >80%. Lactate transformation to acetate was enhanced by low concentration of DO during the logarithmic growth phase. Between 11 and 35 degrees C, the growth rate obeyed the Arrhenius reaction rate-temperature relationship, with a maximum growth rate occurring at 35 degrees C. S. oneidensis MR-1 was able to grow over a wide range of pH (6-9). At neutral pH and temperatures ranging from 30 to 35 degrees C, S. oneidensis MR-1 reduced 100 microM Cr(VI) to Cr(III) within 20 min in the exponential growth phase, and the growth rate was not affected by the addition of chromate; it reduced chromate even faster at temperatures between 35 and 39 degrees C. At low temperatures (<25 degrees C), acidic (pH < 6.5), or alkaline (pH > 8.5) conditions, 100 microM Cr(VI) strongly inhibited growth and chromate reduction. The mini-bioreactor system enabled the rapid determination of these parameters reproducibly and easily by performing very few experiments. Besides its use for examining parameters of interest to environmental remediation, the device will also allow one to quickly assess parameters for optimal production of recombinant proteins or secondary metabolites.     

METAL‐INDUCED REACTIVE OXYGEN SPECIES PRODUCTION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)1

Toxic effects of metals appear to be partly related to the production of reactive oxygen species (ROS), which can cause oxidative damage to cells. The ability of several redox active metals [Fe(III), Cu(II), Ag(I), Cr(III), Cr(VI)], nonredox active metals [Pb(II), Cd(II), Zn(II)], and the metalloid As(III) and As(V) to produce ROS at environmentally relevant metal concentrations was assessed. Cells of the freshwater alga Chlamydomonas reinhardtii P. A. Dang. were exposed to various metal concentrations for 2.5 h. Intracellular ROS accumulation was detected using an oxidation‐sensitive reporter dye, 5‐(and‐6)‐carboxy‐2′,7′‐dihydrodifluorofluorescein diacetate (H₂DFFDA), and changes in the fluorescence signal were quantified by flow cytometry (FCM). In almost all cases, low concentrations of both redox and nonredox active metals enhanced intracellular ROS levels. The hierarchy of maximal ROS induction indicated by the increased number of stained cells compared to the control sample was as follows: Pb(II) > Fe(III) > Cd(II) > Ag(I) > Cu(II) > As(V) > Cr(VI) > Zn(II). As(III) and Cr(III) had no detectable effect. The effective free metal ion concentrations ranged from 10^?6 to 10^?9 M, except in the case of Fe(III), which was effective at 10^?18 M. These metal concentrations did not affect algal photosynthesis. Therefore, a slightly enhanced ROS production is a general and early response to elevated, environmentally relevant metal concentrations.     

Chromate removal by yeasts isolated from sediments of a tanning factory and a mine site in Argentina

Twenty-one yeast-like microorganisms were isolated from tannery effluents and from a nickel–copper mine in Argentina. They were tested for their Cu(II), Ni(II), Cd(II) and Cr(VI) tolerance in qualitative assays on solid medium. Three isolates were selected for their multiple tolerance to the different heavy metals and highest tolerance to Cr(VI). According to morphological and physiological analysis and 26S rDNA D1/D2 domain sequences the isolates were characterized as: Lecythophora sp. NGV-1, Candida sp. NGV-9 and Aureobasidium pullulans VR-8. Resistance of the three strains to high Cr(VI) concentrations and their ability to remove Cr(VI) were assessed using YNB-glucose medium supplemented with 0.5 and 1 mM Cr(VI). Chromate removal activity was estimated by measuring remaining Cr(VI) concentration in the supernatant using the colorimetric 1,5-diphenylcarbazide method and total chromium was determined by flame atomic absorption spectroscopy. The results indicate that the initial Cr(VI) concentration negatively influenced growth and the specific growth rate but stimulated the metabolic activity of the three strains; resistance to Cr(VI) by these strains was mainly due to reduction of Cr(VI) rather than chromium bioaccumulation. This study showed the potential ability of these strains as tools for bioremediation of Cr(VI) from contaminated sites.