Transformation from Organo-Cr (III) to Trivalent Chromium Mineral (Guyanaite/Grimaldiite) and its Environmental Implication

Remediation of heavy-metal contamination by biomineralization has become an environmentally very important issue in the last two decades. Here we describe the transformation of amorphous organo-Cr(III) to chromium hydroxide oxide (guyanaite/grimaldiite) by hydrothermal treatment (HTT). First, glycine-Cr(III) was synthesized to serve as a simple model for exploring the conditions favoring HTT. Cell-bound Cr(III) was obtained by the reduction of hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] by Bacillus cereus. Then the reduced Cr(III) was chelated by ligands at the cell surface, forming cell-bound Cr(III). Subsequently, HTT was applied to treat cell-bound Cr(III) at different temperatures and for different lengths of time. The results showed that, by this treatment at 200°C for 7 days or at 250°C for 1 day, glycine-Cr(III) was converted to trivalent chromium mineral (guyanaite/grimaldiite), having the form of nanosheets with a length of 10～20 nm and a width of 3～5 nm under the described conditions. Cell-bound Cr(III) could also be converted to guyanaite/grimaldiite at 250°C for 9 days if it was bound by an organic compound more complex than glycine. Our finding showed that organo-Cr(III) could be transformed into minerals by an appropriate hydrothermal process, which is applicable to bioremediation of heavy-metal pollution. Our findings also suggest that organo-Cr(III) may play an important role in the biogeochemistry of chromium.     

A Study of the Interaction of Cr(III) Complexes and Their Selective Binding with B-DNA: A Molecular Modeling Approach

Abstract

Molecular modeling and energy minimisation calculations have been used to investigate the interaction of chromium(III) complexes in different ligand environments with various sequences of B-DNA. The complexes are [Cr(salen)(H₂O)₂]⁺; salen denotes 1, 2 bis-salicylideneaminoethane, [Cr(salprn)(H₂O)₂]⁺; salprn denotes 1, 3 bis- salicylideneamino-propane, [Cr(phen)₃]³⁺; phen denotes 1, 10 phenanthroline and [Cr(en)₃]³⁺; en denotes eth- ylenediamine. All the chromium(III) complexes are interacted with the minor groove and major groove of d(AT)₁₂, d(CGCGAATTCGCG)₂ and d(GC)₁₂ sequences of DNA. The binding energy and hydrogen bond parameters of DNA-Cr complex adduct in both the groove have been determined using molecular mechanics approach. The binding energy and formation of hydrogen bonds between chromium(III) complex and DNA has shown that all complexes of chromium(III) prefer minor groove interaction as the favourable binding mode.     

A bacterial flavin reductase system reduces chromate to a soluble chromium(III)-NAD(+) complex

Biological reduction of carcinogenic chromate has been extensively studied in eukaryotic cells partly because the reduction produces stable chromium(III)-DNA adducts, which are mutagenic. Microbial reduction of chromate has been studied for bioremediation purposes, but little is known about the reduction mechanism. In eukaryotic cells chromate is mainly reduced non-enzymatically by ascorbate, which is usually absent in bacterial cells. We have characterized the reduction of chromate by a flavin reductase (Fre) from Escherichia coli with flavins. The Fre-flavin system rapidly reduced chromate, whereas chemical reduction by NADH and glutathione was very slow. Thus, enzymatic chromate reduction is likely the dominant mechanism in bacterial cells. Furthermore, the end-product was a soluble and stable Cr(III)-NAD(+) complex, instead of Cr(III) precipitate. Since intracellularly generated Cr(III) forms adducts with DNA, protein, glutathione, and ascorbate in eukaryotic cells, we suggest that the produced Cr(III) is primarily complexed to NAD(+), DNA, and other cellular components inside bacteria.     

Chromate metabolism in liver microsomes

The carcinogenicity and mutagenicity of various chromium compounds have been found to be markedly dependent on the oxidation state of the metal. The carcinogen chromate was reduced to chromium(III) by rat liver microsomes in vitro. Metabolism of chromate by microsomal enzymes occurred only in the presence of either NADPH or NADH as cofactor. The chromium(III) generated upon metabolism formed a complex with the NADP⁺ cofactor. Significant binding of chromium to DNA occurred only when chromate was incubated in the presence of microsomes and NADPH. Specific inhibitors of the mixed function oxidase enzymes, 2′-AMP, metyrapone, and carbon monoxide, inhibited the rate of reduction of chromate by microsomes and NADPH. The possible relationship of metabolism of chromate and its interaction with nucleic acids to its carcinogenicity and mutagenicity is discussed.     

Exogenously applied sulphate as a tool to investigate transport and reduction of chromate in the duckweed Spirodela polyrhiza

The accumulation of chromium in Spirodela polyrhiza was investigated in the presence and absence of exogenously applied sulphate. Precultivation (10 d) at minimum sulphate concentration (0.013 m m versus 1 m m in controls) enhanced the rate of chromium accumulation. This effect was caused by the increased number of sulphate transporters which transport chromate into cells. Chromate and sulphate compete for the available sulphate transporters. The kinetics of reduction Cr(VI)→Cr(V) was investigated by l -band electron paramagnetic resonance (EPR) spectroscopy. The kinetic model developed previously (Appenroth et al., Journal of Inorganic Biochemistry 78, 235–242, 2000) was refined and extended to include chromate transport and reduction in the presence of competing ions. The following conclusions were drawn from the fitting procedure: without simultaneously applied sulphate, the rate constant of Cr(VI) transport from apoplast into plant cells and the rate constant of Cr(VI) to Cr(V) reduction within the apoplast are comparable (7.0 versus 5.7 h⁻¹) demonstrating that these two processes are competing. Moreover, the rate constant of reduction Cr(V)→Cr(III) is much lower within cells than in apoplast (0.39 versus 7.0 h⁻¹) showing that Cr(V) is stabilized in the symplast. The rate of transport of Cr(VI) into plant cells is at least one order of magnitude higher than that of Cr(V) or Cr(III). The treatment with sulphate (10 m m ) decreases the rate constant of the transport of Cr(VI) into cells (2.0 h⁻¹) confirming the competition of chromate and sulphate for the same transporters. Simultaneously, the rate constant of Cr(V)→Cr(III) reduction is increased in the apoplast (by the factor of 3) and decreased in the symplast (by the factor of 5). Treatment with higher sulphate concentrations (100 m m ) increases the accumulation of chromium by enhancing the rate constant of Cr(VI) transport into cells leaving other processes essentially unchanged. We suggest that 100 m m sulphate opens a new pathway for chromate transport into cells.     

Bioreduction of Hexavalent Chromium from Soil Column Leachate by Pseudomonas stutzeri

Bioreduction of Cr(VI) to less toxic Cr(III) by chromate-reducing bacteria has offered an ecological and economical option for chromate detoxification. The present study reports isolation of chromate-resistant bacterial strain Cr8 from chromium slag, identified as Pseudomonas stutzeri, based on 16S rRNA gene sequencing and their potential use in Cr(VI) reduction. The reduced product associated with bacterial cell was characterized by scanning electron microscopy–energy-dispersive x-ray spectroscopy (SEM-EDS) and x-ray diffraction (XRD) analyses. At initial concentrations of 100 and 200 mg L^?1 Cr(VI), P. stutzeri Cr8 reduced Cr(VI) completely within 24 h, whereas it reduced almost 1000 mg L^?1 Cr(VI) at the end of 120 h. Further, soil column leaching experiments were performed and found that bacterial cells reduced Cr(VI) leachate at faster rate that almost disappeared at the end of 168 h. The leachate precipitates also revealed efficient chromate bioreduction. The remediation process utilizing P. stutzeri could be considered as a viable alternative to reduce Cr(VI) contamination, especially emanating from the overburden dumps of chromite ores and mine drainage.     

Conserved water-mediated recognition and dynamics of NAD+ (carboxamide group) to hIMPDH enzyme: water mimic approach toward the design of isoform-selective inhibitor

Inosine monophosphate dehydrogenase (IMPDH) enzyme involves in GMP biosynthesis pathway. Type I hIMPDH is expressed at lower levels in all cells, whereas type II is especially observed in acute myelogenous leukemia, chronic myelogenous leukemia cancer cells, and 10?ns simulation of the IMP–NAD⁺ complex structures (PDB ID. 1B3O and 1JCN) have revealed the presence of a few conserved hydrophilic centers near carboxamide group of NAD⁺. Three conserved water molecules (W1, W, and W1′) in di-nucleotide binding pocket of enzyme have played a significant role in the recognition of carboxamide group (of NAD⁺) to D274 and H93 residues. Based on H-bonding interaction of conserved hydrophilic (water molecular) centers within IMP–NAD⁺-enzyme complexes and their recognition to NAD⁺, some covalent modification at carboxamide group of di-nucleotide (NAD⁺) has been made by substituting the –CONH₂group by –CONHNH₂ (carboxyl hydrazide group) using water mimic inhibitor design protocol. The modeled structure of modified ligand may, though, be useful for the development of antileukemic agent or it could be act as better inhibitor for hIMPDH-II.     

Extra-cellular chromate-reducing activity of the yeast cultures

This paper reports on the experimental data supporting an essential role of extra-cellular reduction in chromate detoxification by baker’s and non-conventional yeasts. A decrease of chromate content in the yeast culture coincides with an increase of Cr(III) content in extra-cellular liquid. At these conditions, cell-bound chromium level was insignificant and a dominant part of extra-cellular Cr(III) species was detected in the reaction with chromazurol S only after mineralization of the cell-free samples. This phenomenon of chromium “disappearance” can be explained by the formation of Cr(III) stable complexes with extra-cellular yeast-secreted components which are “inaccessible” in the reaction with chromazurol S without mineralization. It was shown that increasing sucrose concentration in a growth medium resulted in an increase of chromate reduction. A strong inhibition of chromate reduction by 0.25 mM sodium azide, a respiration inhibitor and a protonophore, testifies that extra-cellular chromate detoxification depends on energetic status of the yeast cells. It was shown that Cr(III)-biochelates produced in extra-cellular medium are of a different chemical nature and can be separated into at least two components by ion-exchange chromatography on anionit Dowex 1x10. A total yield of the isolated Cr(III)-biocomplexes is approximately 65 % (from initial level of chromate) with a relative molar ratio 8:5.     

Cr(VI) detoxification by Desulfovibrio vulgaris strain Hildenborough: microbe–metal interactions studies

Toxic heavy metals constitute a worldwide environmental pollution problem. Bioremediation technologies represent efficient alternatives to the classic cleaning-up of contaminated soil and ground water. Most toxic heavy metals such as chromium are less soluble and toxic when reduced than when oxidized. Sulfate-reducing bacteria (SRB) are able to reduce heavy metals by a chemical reduction via the production of H₂S and by a direct enzymatic process involving hydrogenases and c₃ cytochromes. We have previously reported the effects of chromate [Cr(VI)] on SRB bioenergetic metabolism and the molecular mechanism of the metal reduction by polyhemic cytochromes. In the current work, we pinpoint the bacteria–metal interactions using Desulfovibrio vulgaris strain Hildenborough as a model. The bacteria were grown in the presence of high Cr(VI) concentration, where they accumulated precipitates of a reduced form of chromium, trivalent chromium [Cr(III)], on their cell surfaces. Moreover, the inner and outer membranes exhibited precipitates that shared the spectroscopic signature of trivalent chromium. This subcellular localization is consistent with enzymatic metal reduction by cytochromes and hydrogenases. Regarding environmental significance, our findings point out the Cr(VI) immobilization mechanisms of SRB; suggesting that SRB are highly important in metal biogeochemistry.     

Isolation and characterization of a chromium-reducing bacterium from a chromated copper arsenate-contaminated site

A Gram-negative bacterium (CRB5) was isolated from a chromium-contaminated site that was capable of reducing hexavalent chromium to an insoluble precipitate, thereby removing this toxic chromium species from solution. Analysis of the 16S rRNA from the isolate revealed that it was a pseudomonad with high similarity to Pseudomaonas synxantha . CRB5 was tolerant to high concentrations of chromate (500 mg l⁻¹) and can reduce Cr(VI) under aerobic and anaerobic conditions. It also exhibited a broad range of reduction efficiencies under minimal nutrient conditions at temperatures between 4°C and 37°C and at pH levels from 4 to 9. As reduction increased, so did total cellular protein, indicating that cell growth was a requirement for reduction. Under low nutrient conditions with CRB5 or when using non-sterile contaminated groundwater from the site, reduction of Cr(VI) was followed by a increase in solution turbidity as a result of the formation of fine-grained Cr(III) precipitates, most probably chromium hydroxide mineral phases such as Cr(OH)₃. Chromium adsorption and precipitation, as observed by transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (TEM/EDS), revealed that the surfaces of the cells were uniformly stained with bound Cr(III) and amorphous precipitates (as determined by selected area electron diffraction; SAED). A mass balance of chromium in a batch bioreactor revealed that up to 30% of the total Cr was as settable precipitates or bound to cells.     

Origin of the regioselectivity in an intramolecular nucleophilic addition to arene chromium tricarbonyl complexes

The source of the regioselectivity in the intramolecular nucleophilic addition of nitrile-stabilized carbanions to arene chromium tricarbonyl complexes was investigated for seven different substitution patterns on the arenes. All of the arenes are 1,4-dioxygenated and the substitution varies in the oxygen substituent and in the substituents of the arene carbons (hydrogen and alkyl). The regioselectivity is correlated with the preferred conformations of the chromium tricarbonyl group which in turn was determined by solution and solid-state ¹³C NMR spectroscopy, ¹H NMR spectroscopy in solution as well as X-ray diffraction. In the four complexes analyzed by X-ray diffraction and the three complexes analyzed by solid-state ¹³C NMR spectroscopy, there was only one complex where it was found that the preferred conformation of the -Cr(CO)₃ is different in solution than it is in the solid-state.     

Chromate Reduction by a Pseudomonad Isolated from a Site Contaminated with Chromated Copper Arsenate

A pseudomonad (CRB5) isolated from a decommissioned wood preservation site reduced toxic chromate [Cr(VI)] to an insoluble Cr(III) precipitate under aerobic and anaerobic conditions. CRB5 tolerated up to 520 mg of Cr(VI) liter⁻¹ and reduced chromate in the presence of copper and arsenate. Under anaerobic conditions it also reduced Co(III) and U(VI), partially internalizing each metal. Metal precipitates were also found on the surface of the outer membrane and (sometimes) on a capsule. The results showed that chromate reduction by CRB5 was mediated by a soluble enzyme that was largely contained in the cytoplasm but also found outside of the cells. The crude reductase activity in the soluble fraction showed a K_m of 23 mg liter⁻¹ (437 μM) and a V_max of 0.98 mg of Cr h⁻¹ mg of protein⁻¹ (317 nmol min⁻¹ mg of protein⁻¹). Minor membrane-associated Cr(VI) reduction under anaerobiosis may account for anaerobic reduction of chromate under nongrowth conditions with an organic electron donor present. Chromate reduction under both aerobic and anaerobic conditions may be a detoxification strategy for the bacterium which could be exploited to bioremediate chromate-contaminated or other toxic heavy metal-contaminated environments.     

Chromate reduction by a chromate-resistant bacterium, <Emphasis Type="Italic">Microbacterium</Emphasis> sp.

Heavy-metal chromium [Cr(VI)] is a ubiquitous environmental pollutant. Comparing with chemical reduction, microbiological reduction is considered to be a friendly and cheaper way to decrease the damage caused by chromate. A bacterial strain, CR-07, which is resistant to and capable of reducing chromate was isolated from a mud sample of iron ore and identified as a Microbacterium sp. The bacterium had a high degree of tolerance to chromate, and could grow in LB medium containing 4.08 mM of K₂Cr₂O₇. It also had a degree of resistance to other heavy metals, e.g. Cd²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Co²⁺, Hg²⁺ and Ag⁺. The bacterium could remove 1.02 mM of Cr(VI) from LB medium within 36 h of incubation. Chromate removal was achieved in the supernatant from the bacterial cultures, and corresponded to chromate reduction. The activity of chromate reduction by the bacterium was not related to enzymes or reducing sugars, while fluorometric assay suggested that glutathione, a chromate-reducing substance which was produced by the bacterium, was one of the factors that contributed to the reduction of Cr(VI).     

Community structure and function in a H2-based membrane biofilm reactor capable of bioreduction of selenate and chromate

Two different H₂-based, denitrifying membrane-biofilm reactors (MBfRs) initially reduced Se(VI) or Cr(VI) stably to Se⁰ or Cr(III). When the oxidized contaminants in the influent were switched, each new oxidized contaminant was reduced immediately, and its reduction soon was approximately the same or greater than it had been in its original MBfR. The precipitation of reduced selenium and chromium in the biofilm was verified by scanning electron microscopy and energy dispersive X-ray analysis. These results on selenate and chromate reduction are consistent with the interpretation that the H₂-based biofilm community had a high level of functional diversity. The communities’ structures were assessed by cloning analysis. Dechloromonas spp., a known perchlorate-reducing bacteria, dominated the clones from both reactors during selenate and chromate reductions, which suggests that it may have functional diversity capable of reducing selenate and chromate as secondary and dissimilatory acceptors.     

Generation of PM2 DNA breaks in the course of reduction of chromium(VI) by glutathione

The carcinogen chromate is efficiently taken up and reduced to chromium(III) compounds by various biological systems. To test the possible DNA damage induced in the course of chromium(VI) reduction, we used a combination of chromate with the reductant glutathione (GSH) as well as a green complex of chromium(V), which is formed in the reaction of chromate with GSH. The combination of chromate and glutathione was found to cause single-strand breaks in supercoiled circular DNA of the bacteriophage PM2. The green chromium(V) complex Na4(GSH)4Cr(V).8H2O, prepared from chromate and glutathione, also cleaved supercoiled PM2 DNA. No DNA-degrading effects were observed with either chromate or the final product of the reaction with GSH, a purple anionic chromium(III) GSH complex. The nature of the buffering agents revealed a strong influence on the extent of DNA strand breaks produced by chromate and GSH. A variation of the GSH concentration in the reaction with chromate and PM2 DNA, performed in sodium phosphate-buffered solutions showed an initial increase in the number of strand breaks at GSH concentrations up to 1 mM followed by a decline at higher GSH concentrations. Since neither chromate, when administered individually, nor the final product of chromium(VI) reduction, the purple chromium(III) GSH complex, produced any detectable DNA cleavage, the critical steps leading to DNA strand breaks occur in the course of the conversion of chromium(VI) to chromium(III) by GSH, the most abundant intracellular low molecular thiol. Moreover, the demonstration that DNA cleavage is induced in the presence of the chromium(V) complex identifies chromium(V) as the oxidation state of the metal, which is involved in the steps leading to DNA-damaging effects of chromate.     

Photocatalyzed Anaerobic Oxidation of Nicotinamide Coenzyme Dimers to NAD+ by Adriamycin

The nicotinamide adenine dinucleotide dimers (NAD)₂ obtained by electrochemical reduction of NAD⁺ are oxidized by adriamycin in anaerobic photocatalyzed reaction yielding NAD⁺ and 7-deoxyadriamyci-none. Under the same conditions NADH is not oxidized.     

Use of Experimental Factorial Design for Optimization of Hexavalent Chromium Removal by a Bacterial Consortium: Soil Microcosm Bioremediation

Hexavalent chromium Cr(VI) is regularly introduced into the environment through diverse anthropogenic activities. It is highly toxic, mutagenic and carcinogenic, and because of its solubility in water, chromate contamination can be difficult to contain. Bacteria can reduce chromate to insoluble and less toxic trivalent chromium Cr(III), and thus increasing attention is paid to chromate bioremediation to reduce its ecotoxicological impacts. In this study, the factorial design 2³ was employed to optimize critical parameters responsible for higher Cr(VI) removal by a bacterial consortium. The factors considered were pH, temperature, and inoculum size at two markedly different levels. All three dependent variables have significant effect on Cr(VI) reduction. Optimal Cr(VI) removal by the bacterial consortium occurred at pH 9, temperature 37°C, and inoculum size OD = 3. Analysis of variance (ANOVA) showed a high coefficient of determination (R²) value of 0.984, thus ensuring a satisfactory adjustment of the second-order regression model with the experimental data. In addition, the effect of bioaugmentation of Cr(VI)-polluted soil microcosms with the bacterial consortium was investigated using the best factor levels. Contaminated soil by 20 and 60 mg/Kg of Cr(VI) showed reductions of 83% and 65% of initial Cr(VI) by the bacterial consortium, suggesting that this bacterial consortium might diminish phytoavailable Cr(VI) in soil and be useful for cleaning up chromium-contaminated sites.     

Mechanisms of bacterial resistance to chromium compounds

Chromium is a non-essential and well-known toxic metal for microorganisms and plants. The widespread industrial use of this heavy metal has caused it to be considered as a serious environmental pollutant. Chromium exists in nature as two main species, the trivalent form, Cr(III), which is relatively innocuous, and the hexavalent form, Cr(VI), considered a more toxic species. At the intracellular level, however, Cr(III) seems to be responsible for most toxic effects of chromium. Cr(VI) is usually present as the oxyanion chromate. Inhibition of sulfate membrane transport and oxidative damage to biomolecules are associated with the toxic effects of chromate in bacteria. Several bacterial mechanisms of resistance to chromate have been reported. The best characterized mechanisms comprise efflux of chromate ions from the cell cytoplasm and reduction of Cr(VI) to Cr(III). Chromate efflux by the ChrA transporter has been established in Pseudomonas aeruginosa and Cupriavidus metallidurans (formerly Alcaligenes eutrophus) and consists of an energy-dependent process driven by the membrane potential. The CHR protein family, which includes putative ChrA orthologs, currently contains about 135 sequences from all three domains of life. Chromate reduction is carried out by chromate reductases from diverse bacterial species generating Cr(III) that may be detoxified by other mechanisms. Most characterized enzymes belong to the widespread NAD(P)H-dependent flavoprotein family of reductases. Several examples of bacterial systems protecting from the oxidative stress caused by chromate have been described. Other mechanisms of bacterial resistance to chromate involve the expression of components of the machinery for repair of DNA damage, and systems related to the homeostasis of iron and sulfur.     

Developmental regulation of NAD+ kinase in Neurospora crassa

The specific activity of NAD⁺ kinase (ATP:NAD⁺ 2-phosphotransferase, EC 2.7.1.23) from Neurospora crassa shows sharp peaks when the organism enters a new developmental stage of the asexual life cycle: the peaks are observed during hydration and germination of conidia, at the transition from exponential to stationary growth and at the photostimulated conidiation. As stimulation of NAD⁺ kinase activity by light in conidiating mycelium is not sensitive to translation inhibitors, the activiation of pre-existing molecules, rather than induction of protein synthesis de novo may be supposed. Enzyme electrophoresis revealed the presence of four forms of NAD⁺ kinase having different apparent molecular weights (I=333,000; II=306,000; III=229,000 and IV=203,000). Manifestation of the activity of individual forms of NAD⁺ kinase is developmentally controlled: form III is most abundant during vegetative growth, forms I and II prevail in conidia. At the conidial germination the increase of NAD⁺ kinase activity is associated with the activation of form III, whereas during photostimulation of conidiation form II is the most activated one. Therefore, certain molecular forms of the enzyme may be regarded as biochemical markers for different developmental stages of N. crassa.     

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.