Bioremediation of Heavy Metals in Liquid Media Through Fungi Isolated from Contaminated Sources

Wastewater particularly from electroplating, paint, leather, metal and tanning industries contain enormous amount of heavy metals. Microorganisms including fungi have been reported to exclude heavy metals from wastewater through bioaccumulation and biosorption at low cost and in eco-friendly way. An attempt was, therefore, made to isolate fungi from sites contaminated with heavy metals for higher tolerance and removal of heavy metals from wastewater. Seventy-six fungal isolates tolerant to heavy metals like Pb, Cd, Cr and Ni were isolated from sewage, sludge and industrial effluents containing heavy metals. Four fungi (Phanerochaete chrysosporium, Aspegillus awamori, Aspergillus flavus, Trichoderma viride) also were included in this study. The majority of the fungal isolates were able to tolerate up to 400 ppm concentration of Pb, Cd, Cr and Ni. The most heavy metal tolerant fungi were studied for removal of heavy metals from liquid media at 50 ppm concentration. Results indicated removal of substantial amount of heavy metals by some of the fungi. With respect to Pb, Cd, Cr and Ni, maximum uptake of 59.67, 16.25, 0.55, and 0.55 mg/g was observed by fungi Pb3 (Aspergillus terreus), Trichoderma viride, Cr8 (Trichoderma longibrachiatum), and isolate Ni27 (A. niger) respectively. This indicated the potential of these fungi as biosorbent for removal of heavy metals from wastewater and industrial effluents containing higher concentration of heavy metals.     

Interaction of Heavy Metals in Multimetal Biosorption by Galerina vittiformis from Soil

ABSTRACT Soil heavy metal contamination, a major threat due to industrialization, can be tackled by an efficient and economical process called bioremediation. Mushrooms are employed to accumulate heavy metals from soil due to their high metal accumulation potential and better adaptability. The bioaccumulation potential of Galerina vittiformis was already reported for individual metals. At natural conditions, since soil consists of more than one polluting metal, more focus has to be given to multimetal systems. In this study, multimetal accumulation potential was analyzed using central composite design, and the responses obtained were analyzed using response surface methodology. Heavy metals such as Cu(II), Cd(II), Cr(VI), Pb(II), and Zn(II) were subjected to biosorption at 10–250 mg/kg concentrations along with pH 5–8. The results showed that the preference of the organism for the five metals under study was in the order Pb(II) > Zn(II) > Cd(II) > Cu(II) > Cr(VI) at pH 6.5 under multimetal condition. The study also indicates that the metal interaction pattern in multimetal interaction is a property of their ionic radii. The response surface methodology clearly explains the effect of interaction of heavy metals on the accumulation potential of the organism using three-dimensional response plots. The present work suggests that the fungus Galerina vittiformis could be employed as a low-cost metal removal agent from heavy metal–polluted soil.     

Heavy metal tolerance in marine strains of Yarrowia lipolytica

Heavy metal tolerance of two marine strains of Yarrowia lipolytica was tested on solid yeast extract peptone dextrose agar plates. Based on minimum inhibitory concentration esteems, it is inferred that the two strains of Y. lipolytica were tolerant to heavy metals such as Pb(II), Cr(III), Zn(II), Cu(II), As(V), and Ni(II) ions. The impact of various heavy metal concentrations on the growth kinetics of Y. lipolytica was likewise assessed. With increased heavy metal concentration, the specific growth rate was reduced with delayed doubling time. Furthermore, biofilm development of both yeasts on the glass surfaces and in microtitre plates was assessed in presence of different heavy metals. In microtitre plates, a short lag phase of biofilm formation was noticed without the addition of heavy metals in yeast nitrogen base liquid media. A lag phase was extended over increasing metal concentrations of media. Heavy metals like Cr(VI), Cd(II), and As(V) are contrastingly influenced on biofilms’ formation of microtitre plates. Other heavy metals did not much influence on biofilms development. Thus, biofilm formation is a strategy of Y. lipolytica under stress of heavy metals has significance in bioremediation process for recovery of heavy metals from contaminated environment.

     

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.     

Heavy metal uptake capacity of fresh water algae (Oedogonium westti) from aqueous solution: A mesocosm research

The green macroalgae present in freshwater ecosystems have attracted a great attention of the world scientists for removal of heavy metals from wastewater. In this mesocosm study, the uptake rates of heavy metals such as cadmium (Cd), nickel (Ni), chromium (Cr), and lead (Pb) by Oedogonium westi (O. westti) were measured. The equilibrium adsorption capabilities of O. westti were different for Cd, Ni, Cr, and Pb (0.974, 0.418, 0.620, and 0.261 mgg^–1, respectively) at 18°C and pH 5.0. Furthermore, the removal efficiencies for Cd, Cr, Ni and Pb were observed from 55–95%, 61–93%, 59–89%, and 61–96%, respectively. The highest removal efficiency was observed for Cd and Cr from aqueous solution at acidic pH and low initial metal concentrations. However, the removal efficiencies of Ni and Pb were higher at high pH and high concentrations of metals in aqueous solution. The results summarized that O. westti is a suitable candidate for removal of selected toxic heavy metals from the aqueous solutions.     

Phytoremediation of Landfill Leachate with Colocasia esculenta,Gynerum sagittatum and Heliconia psittacorum in Constructed Wetlands

This study assessed the accumulation of Cd (II), Hg (II), Cr (VI) and Pb (II) in Gynerium sagittatum (Gs), Colocasia esculenta (Ce) and Heliconia psittacorum (He) planted in constructed wetlands treating synthetic landfill leachate. Sixteen bioreactors were operated in two experimental blocks. Metal concentrations in the influent and effluent; root, stem, branch and leaves of plants were analysed, as well as COD, N-NH₄⁺, TKN, T, pH, ORP, DO, and EC. Average removal efficiencies of COD, TKN and NH₄⁺-N were 66, 67 and 72%, respectively and heavy metal removal ranged from 92 to 98% in all units. Cr (VI) was not detected in any effluent sample. The bioconcentration factors (BCF) were 10⁰ -10². The BCF of Cr (VI) was the lowest: 0.59 and 2.5 (L kg^?1) for Gs and He respectively; whilst Cd (II) had the highest (130–135 L kg^?1) for Gs. Roots showed a higher metal content than shoots. Translocation factors (TF) were lower, He was the plant exhibiting TFs >1 for Pb (II), Cr (T) and Hg (II) and 0.4–0.9 for Cd (II) and Cr (VI). The evaluated plants demonstrate their suitability for phytoremediation of landfill leachate and all of them can be categorized as metals accumulators.     

Concurrent Removal of Mn(II) and Cr(VI) by Achromobacter sp. TY3-4

In this study, we report a bacterium, Achromobacter sp. TY3-4, capable of concurrently removing Mn (II) and Cr (VI) under oxic condition. TY3-4 reduced as much as 2.31?mM of Cr (VI) to Cr (III) in 70?h, and oxidized as much as 20?mM of Mn(II) to Mn oxides in 80?h. When 0.58?mM Cr (VI) and 10?mM Mn(II) were present together, both Cr(VI) and Mn(II) were completely removed by TY3-4 and the generated precipitates are Mn^IIIOOH, Mn^III,IV₃O₄, Mn^IVO₂ and Cr^III(OH)₃. Experiments also show that both biosroption and bioreduction of Mn(II) are the driving forces for Mn(II) removal, whereas bioreduction of Cr(VI) is the driving force for Cr(VI) removal. On the basis of these results, a possible reaction was proposed that TY3-4 concurrently reduces Cr(VI) and oxidizes Mn(II). This study is fundamental for Mn and Cr cycles. The strain shows potential for practical application.     

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.     

Biogeochemical Elimination of Chromium (VI) from Contaminated Water

Ferrous iron [Fe(II)] reductively transforms heavy metals in contaminated groundwater, and the bacterial reduction of indigenous ferric iron [Fe(III)] to Fe(II) has been proposed as a means of establishing redox reactive barriers in the subsurface. The reduction of Fe(III) to Fe(II) can be accomplished by stimulation of indigenous dissimilatory metal-reducing bacteria (DMRB) or injection of DMRB into the subsurface. The microbially produced Fe(II) can chemically react with contaminants such as Cr(VI) to form insoluble Cr(III) precipitates. The DMRB Shewanella algae BrY reduced surface-associated Fe(III) to Fe(II), which in batch and column experiments chemically reduced highly soluble Cr(VI) to insoluble Cr(III). Once the chemical Cr(VI) reduction capacity of the Fe(II)/Fe(III) couple in the experimental systems was exhausted, the addition of S. algae BrY allowed for the repeated reduction of Fe(III) to Fe(II), which again reduced Cr(VI) to Cr(III). The research presented herein indicates that a biological process using DMRB allows the establishment of a biogeochemical cycle that facilitates chromium precipitation. Such a system could provide a means for establishing and maintaining remedial redox reactive zones in Fe(III)-bearing subsurface environments.     

Evaluation of Cr(VI) Exposed and Unexposed Plant Parts of Tradescantia pallida (Rose) D. R. Hunt. for Cr Removal from Wastewater by Biosorption

Phytoremediation is an efficient method for the removal of heavy metals from contaminated systems. A productive disposal of metal accumulating plants is a major concern in current scenario. In this work, Cr(VI) accumulating Tradescantia pallida plant parts were investigated for its reuse as a biosorbent for the removal of Cr(VI) ions. The effect of pH, contact time, sorbent dosage, Cr(VI) concentration and temperature was examined to optimize these process parameters. Results showed that Cr(VI) exposed/unexposed T. pallida leaf biomass could remove 94% of chromium with a sorption capacity of 64.672 mg g^?1. Whereas the kinetics of Cr(VI) biosorption was well explained by the pseudo second-order kinetic model, the Langmuir model better described the data on Cr(VI) sorption isotherm compared with the Freundlich model. The changes in the free energy (ΔG°), entropy (ΔS°) and enthalpy (ΔH°) were found to be ?5.276 kJ mol^?1, 0.391 kJ mol^?1 K^?1 and 11.346 kJ mol^?1, respectively, which indicated the process to be spontaneous, feasible and endothermic in nature. FTIR spectra of T. pallida leaf biomass revealed the active participation of ligands, such as ?NH, amide, hydroxyl and sulphonate groups present in the biomass for Cr(VI) binding, SEM analysis revealed a porous structure of the biosorbent for an easy uptake of Cr(VI).     

Spontaneous Cr(VI) and Cd(II) biosorption potential of native pinnae tissue of Pteris vittata L., a tropical invasive pteridophyte

Heavy metal pollution is a prevalent and critical environmental concern. Its rampancy is attributed to indiscriminate anthropogenic activities. Several technologies including biosorption have been continuously researched upon to overcome the limitations of the conventional method of treatments in removal of heavy metals. Biosorption technology involves the application of a biomass in its nonliving form. Pteris vittata L., a pteridophyte, considered as an invasive weed was investigated in the present study as a potential decontaminant of toxic metals, Cr(VI) and Cd(II). The adsorption capacity of the biosorbent for Cr(VI) and Cd(II) under equilibrium conditions was investigated. The morphology, elemental composition, functional groups, and thermal stability of the biosorbent before and after metal loading were evaluated. At 303?K and an equilibrium time of 120?min, the maximum loading of Cr(VI) on the biosorbent was estimated to be 166.7?mg/g at pH 2 and Cd(II) to be 31.3?mg/g at pH 6. Isotherm models, kinetic studies, and thermodynamic studies indicated the mechanisms, chemisorption, ion exchange and intraparticle diffusion, controlling the Cr(VI) and Cd(II) uptake, respectively. The interactive effect of multi-metal ions in binary component systems was synergistic for Cd(II) uptake. The results validate the toxic metal removal potency of the biosorbent.     

Role of extracellular polymeric substance (EPS) in toxicity response of soil bacteria Bacillus sp. S3 to multiple heavy metals

Heavy metal resistant bacteria are of great interest because of their potential use in bioremediation. Understanding the survival and adaptive strategies of these bacteria under heavy metal stress is important for better utilization of these bacteria in remediation. The objective of this study was to investigate the role of bacterial extracellular polymeric substance (EPS) in detoxifying against different heavy metals in Bacillus sp. S3, a new hyper antimony-oxidizing bacterium previously isolated from contaminated mine soils. The results showed that Bacillus sp. S3 is a multi-metal resistant bacterial strain, especially to Sb(III), Cu(II) and Cr(VI). Toxic Cd(II), Cr(VI) and Cu(II) could stimulate the secretion of EPS in Bacillus sp. S3, significantly enhancing the adsorption and detoxification capacity of heavy metals. Both Fourier transform infrared spectroscopy (FTIR) and three-dimensional excitation–emission matrix (3D-EEM) analysis further confirmed that proteins were the main compounds of EPS for metal binding. In contrast, the EPS production was not induced under Sb(III) stress. Furthermore, the TEM–EDX micrograph showed that Bacillus sp. S3 strain preferentially transported the Sb(III) to the inside of the cell rather than adsorbed it on the extracellular surface, indicating intracellular detoxification rather than extracellular EPS precipitation played an important role in microbial resistance towards Sb(III). Together, our study suggests that the toxicity response of EPS to heavy metals is associated with difference in EPS properties, metal types and corresponding environmental conditions, which is likely to contribute to microbial-mediated remediation.

     

Aquatic macrophytes mediated remediation of toxic metals from moderately contaminated industrial effluent

The present study assessed Zn, Cr, Cd, and Pb removal efficiency of Colocasia esculenta, Hydrilla verticillata, Phragmitis australis, Typha latifolia, and Spirodella polyrhiza from sewage-mixed industrial effluent. The fresh/dry weight and relative growth rate of each macrophyte decreased with increasing effluent concentration. H. verticillata and C. esculenta exhibited better growth at 50% effluent over control. The maximum Zn, Cd, and Pb accumulation (1008.23, 28.03, and 483.55 mg/kg dry wt., respectively) was recorded in C. esculenta, whereas Cr (114.48 mg/kg dry wt.) in H. verticillata at 100% effluent. Metal accumulation in roots of all plants species was higher (≥50%) initially with increasing effluent concentration and later transferred to shoots. All plants exhibited BCF >1.0 for all heavy metals, highest being for Zn (91.2) and Cd (75.2) in H. verticillata, for Cr (97.9) and Pb (103) in C. esculenta. Except S. polyrhhiza, all other plants exhibited TF <1.0. Maximum removal efficiency of Zn was 82.8% by H. verticillata, whilst that of Cr, Cd, and Pb by C. esculenta at 50% effluent, demonstrating wide applicability of H. verticillata and C. esculenta for treatment of mixed industrial effluent having heavy metals.     

As(V) Reduction,As(III) Oxidation,and Cr(VI) Reduction by Multi-metal-resistant Bacillus subtilis,Bacillus safensis,and Bacillus cereus Species Isolated from Wastewater Treatment Plant

Industrial progress has resulted in threatening concentrations of toxic metals in various areas of the world. Bioremediation is an economical alternative to chemical methods. Bacteria resistant to As(III), As(V), Cr, Co, Cu, Cd, Hg, Ni, Pb, Se, and Zn were isolated from the wastewater treatment plant of Kasur, Pakistan. Highest resistance against all metals was exhibited by MX-1, MX-3, MX-4, and MX-5. The isolates possessed dual ability to oxidize as well as to reduce As. Highest As(V) reduction (454 µM) was exhibited by MX-1, while most As(III) oxidation was shown by MX-3 (170 µM). The isolates were also capable of reducing Cr(VI), and maximum Cr(VI) reduction (500 µM) was exhibited by MX-3. Transformation of DH5α with MX-1 plasmid showed that resistance genes for As(III), As(V), Cr, Cd, Se, Hg, and Ni were plasmid borne, while in case of MX-3, resistance genes for As(III), As(V), Co, Cu, Se, Pb, Zn, and Ni were present on plasmid. MX-1 and MX-3 were also positive for auxin production (37 and 32.96 µg ml^?1, respectively). MX-3 was also found to produce hydrogen cyanide (HCN) and solubilized phosphate. These isolates promoted plant (Vigna radiata) growth both in the presence and absence of the metals. MX-1, MX-3, and MX-5 were identified as Bacillus subtilis, Bacillus safensis, and Bacillus cereus through 16S rRNA gene sequencing, respectively. Such bacteria having multiple traits of resisting multiple metals, dual ability to oxidize/reduce As, and reduce Cr(VI) along with the ability to support plant growth are good tools for remediation of metal-contaminated sites and its cultivation.     

Evaluation of Lupinus Species to Accumulate Heavy Metals From W aste Waters

Several Lupinus species, for example, Lupinus albus, Lupinus luteus, Lupinus angustifolius, and Lupinus hispanicus were used to accumulate Mn(II), Cd(II), Pb(II), Cr(III), Cr(VI), Hg²⁺, and CH₃Hg⁺ from waste waters. The influence of different species concentrations (50 and 100 mg L^-1) and pH on growing behavior as well as the resulting distribution of metals in the plants were investigated. The results obtained showed that lupins were able to germinate and to grow in the presence of the metals mentioned above, even when they were present at levels as high as 50 mg L^-1. Accumulation of Pb(II), Cr(III), and Cd(II) was higher in roots than in shoots. As far as mercury is concerned, the highest CH₃Hg and Hg²⁺ accumulation was detected in roots, but fast transport toward the leaves was noticed. In contrast to mercury, the uptake of chromium seems to be influenced by the chemical form of the analyte, remaining Cr(VI) in solution. No differences in growing behavior and accumulation were observed for the four Lupinus species studied. Even though plants were exposed only a relatively short time to the metal solutions, metal concentrations of approximately 2 g/kg of dry matter were detected in the young lupins plants. The feasibility of utilizing Lupinus plants for the removal of heavy metals from wastewater was also investigated. Lupins were able to grow under extreme conditions (wastewater, pH lower than 2) and to remove 98% of the initial amount of toxic metals present in the sample.     

Heavy metal accumulation in rice (Oryza sativa) near electronic waste dumps and related human health risk assessment

This research was conducted to assess heavy metal contamination in the environment and within Oryza sativa. The translocation factors (TFs) and bioaccumulation factors (BAFs) for heavy metals in O. sativa and estimated daily intake (EDI) and health risk index (HRI) were measured. The samples were analyzed for heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES). Pb and Cr concentrations in water samples within and near the electronic-waste dumping area exceeded water quality standards for surface water sources from the Pollution Control Department in the Ministry of Natural Resources and Environment of Thailand (PCD). The Pb concentration in soil samples within the area also exceeded soil quality standards for habitat and agriculture from PCD. Most of the metals were highly concentrated in roots, except for Mn which has the highest concentration in leaves. Pb concentrations in rice grains exceeded the FAO/WHO standard (0.2 mg/kg). The average TF values for heavy metals from the soil to roots, roots to stems, stems to leaves, and stems to grains were Mn > Pb > Ni > Cr, Mn > Cr > Ni > Pb, Ni > Pb > Mn > Cr, and Pb > Ni > Cr > Mn, respectively. The average BAF values in O. sativa were Mn > Ni > Pb > Cr. The EDI for Cr, Pb, Mn, and Ni via O. sativa consumption were 6.19, 6.02, 370.57, and 3.80 µg/kg/day, respectively. The HRI for Cr, Pb, Mn, and Ni via O. sativa consumption were 0.30, 1.50, 2.60, and 0.002, respectively.     

Characterization of PGP Traits of a Hexavalent Chromium Resistant Raoultella sp. Isolated from the Rice Field near Industrial Sewage of Burdwan District,WB, India

Chromium (Cr) is the most toxic at its hexavalent state. Widespread use of chromium for various anthropogenic activities causing rapid decline of the agricultural productivity is now a major global concern. The purpose of this study was to isolate the plant growth promoting (PGP) chromium-resistant bacteria and characterize it before being applied for bioremediation. A potent Cr-resistant rhizobacterium (CrS2) was isolated from the rice field near an industrial sewage and identified as Raoultella sp. based on 16S rDNA sequence homology with some phenotypic characteristics. The strain exhibited Cr(VI) resistance up to 25 mM and also possesses some important PGP traits. The selected CrS2 strain has varied degrees of resistance to other toxic heavy metals/metalloids like arsenic, cadmium, and lead. The removal capacity of chromium was studied in broth cultures. The appropriate growth media for the strain is peptone yeast glucose media with glucose (0.5%) and peptone (1%) as carbon and nitrogen sources, respectively. The strain removed substantial amount of chromium after media optimization. The chromate reductase (EC.1.6.5.2) activity was constitutive in nature of this strain. Thus, the strain CrS2 may be exploited for bioremediation of Cr(VI) in Cr-contaminated agricultural soil, where it might also enhance plant growth promotion.     

Lead removal by Spirulina platensis biomass

In this investigation, we report on the biosorption of Pb (II) from aqueous solutions by the nonliving biomass of the micro-alga (cyanobacterium) Spirulina platensis. Propagation of the micro-alga was carried out in outside oblong raceway ponds. The biomass was cleaned, dried and used for the investigation. The effects of pH, adsorbent dose, temperature, initial concentration of Pb (II), and contact time on the adsorption of lead by the dry biomass were studied. The experiments were carried out in 250 ml conical flasks containing 100 ml of test solutions using an orbital incubator at 150 rpm. Concentrations of the metal before and after the experiments were measured using Atomic Absorption Spectrophotometer. Very high levels of Pb (II) removal (>91%) were obtained. The optimum conditions for maximal adsorption by S. platensis were found to be pH 3; 2 g of adsorbent dose; incubation at 26°C; 100 mg/l of lead initial concentration and 60 minutes of contact time. The experimental data fitted well with Freundlich isotherm equation with R² values greater than 0.97. Based on our results, we recommend the utilization of S. platensis biomass for heavy metal removal from aqueous solutions.     

Differential effects of Zn concentrations on Cr(VI) uptake by two Salvinia species: involvement of thiol compounds

Abstract

Floating ferns of the genus Salvinia have great potential for phytoremediation of heavy metals. To date, the effect of essential metals on the accumulation and transport of toxic metals by aquatic ferns has not been suitably established. The aim of this study was to compare the ability of floating leaves of Salvinia minima and Salvinia rotundifolia species to accumulate Cr from Cr(VI solutions containing very low (0.02?mg L^?1) and low (5?mg L^?1) Zn concentrations. After 7-day metal-exposure period, results showed that Zn increased Cr accumulation in S. minima leaves whereas in S. rotundifolia decreased significantly. Contrarily Zn accumulation did not show great differences between species. This fact may indicate that Zn interfere Cr(VI) uptake by S. rotundifolia. Bioconcentration factor (BCF) and translocation factor (TF) were affected differently by Zn in both Salvinia species. Membrane stability index (MSI) of both Salvinia species was decreased significantly by 5?mg L^?1 Zn concentration. Zn ions also increased hydrogen peroxide accumulation in fronds of Salvinia species. Total thiols (TT), non-protein thiols (NPT) and protein-bound thiols (PBT) were differentially affected by Cr(VI) and Zn ions. This study provides evidences on the involvement of different mechanisms against Cr(VI)/Zn toxicity in S. minima and S rotundifolia species.     

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.