Chromium(VI)-resistant yeast isolated from a sewage treatment plant receiving tannery wastes

A Cr(VI)-resistant yeast, designated strain DBVPG 6502, was isolated from a sewage treatment plant receiving wastes from tannery industries in Italy. The strain was tentatively identified as a species of Candida based on morphological and physiological analyses. This strain was highly resistant to Cr(VI) when compared with eight other yeast species, growing at Cr(VI) concentrations of up to 500 micrograms/ml (10 mM). This resistance was constitutive. The Cr(VI)-resistant yeast did not reduce Cr(VI) to Cr(III) species under aerobic conditions. The yeast showed very little accumulation of Cr(VI). Consequently, the mechanism of resistance of the yeast to Cr(VI) appears to involve reduced accumulation of Cr, as has been shown in Cr(VI)-resistant bacteria.     

Chromium(VI)-resistant yeast isolated from a sewage treatment plant receiving tannery wastes.

A Cr(VI)-resistant yeast, designated strain DBVPG 6502, was isolated from a sewage treatment plant receiving wastes from tannery industries in Italy. The strain was tentatively identified as a species of Candida based on morphological and physiological analyses. This strain was highly resistant to Cr(VI) when compared with eight other yeast species, growing at Cr(VI) concentrations of up to 500 micrograms/ml (10 mM). This resistance was constitutive. The Cr(VI)-resistant yeast did not reduce Cr(VI) to Cr(III) species under aerobic conditions. The yeast showed very little accumulation of Cr(VI). Consequently, the mechanism of resistance of the yeast to Cr(VI) appears to involve reduced accumulation of Cr, as has been shown in Cr(VI)-resistant bacteria.     

Ochrobactrum tritici strain 5bvl1 - characterization of a Cr(VI)-resistant and Cr(VI)-reducing strain

Bacterial strain 5bvl1, isolated from a chromium-contaminated wastewater treatment plant and identified as Ochrobactrum tritici, was resistant to a broad range of antibiotics, to Cr(VI), Ni(II), Co(II), Cd(II), and Zn(II), and was able to grow in the presence of 5% NaCl and within the pH range 4-10. Characterization showed that strain 5bvl1 could be considered a halotolerant and alkalitolerant microorganism resistant to high concentrations of Cr(VI). This strain was able to grow aerobically in up to 10 mmolxL(-1) Cr(VI). Cr(VI) resistance was independent of sulphate concentration. Under aerobic conditions strain 5bvl1 was also able to reduce high Cr(VI) concentrations (up to 1.7 mmolxL(-1)). Increasing concentrations of Cr(VI) in the medium lowered the growth rate of strain 5bv11 but the reduction in growth rate could not be directly correlated with the amount of Cr(VI) reduced. Unlike the type strain, which was only able to reduce Cr(VI), strain 5bvl1 was resistant to Cr(VI) and able to reduce it. Moreover, in strain 5bvl1, the rate and extent of Cr(VI)-reduction were higher than in the other strains of the genus Ochrobactrum. Ochrobactrum strain 5bvl1 resists high Cr(VI) concentrations and has a high Cr(VI)-reducing ability, making it a valuable tool in bioremediation.     

Reduction of toxic hexavalent chromium by Ochrobactrum intermedium strain SDCr-5 stimulated by heavy metals

A Cr(VI) resistant bacterial strain SDCr-5, identified as Ochrobactrum intermedium on the basis of 16S rRNA gene sequencing, was tolerant to high concentrations of Cr(VI) up to 15 mg ml(-1) in acetate minimal medium. O. intermedium SDCr-5 reduced Cr(VI) under a wide range of concentrations from 100 to 1500 microg ml(-1) and reduction was optimum at 37 degrees C and pH 7. It reduced 200 and 721 microg ml(-1) Cr(VI) within 72 and 96 h, respectively. The rate of Cr(VI) reduction increased with concentration from 100 to 1500 microg ml(-1). The presence of heavy metal cations such as Cu(2+), Co(2+), Mn(2+) and Ni(2+) stimulated Cr(VI) reduction. Strain SDCr-5 might be useful for Cr(VI) detoxification under a wide range of environmental conditions.     

Bioreduction of Cr(VI) by Bacillus sp. QH-1 isolated from soil under chromium-containing slag heap in high altitude area

A chromium-reducing strain QH-1, identified as Bacillus sp., was isolated from soil under chromium-containing slag heap in Qinghai high altitude area, China. The strain was found to resist 200 mg/L Cr(VI), and Cr(VI) negatively affects the metabolic activity of the cells, as well as the cell morphology of Bacillus sp. QH-1. The reduction efficiency of Cr(VI) at concentrations of Cr(VI) 25 mg/L, 50 mg/L, 100 mg/L and 200 mg/L were 99.48 %, 65.99 %, 23.22 % and 6.99 %, respectively, decreasing with increasing initial Cr(VI) concentration. This indicates that the toxicity of Cr(VI) increased with concentration. Energy dispersive X-ray analysis revealed that there was insoluble Cr(III) generated during Cr(VI) reduction. In order to apply strain QH-1 to remove Cr(VI) from groundwater, factors of concentration of electron donors (glucose) and temperature were investigated in a synthetic medium. The results demonstrated that glucose could promote the reduction of Cr(VI) by this strain, and the general trend of Cr(VI) reduction increased with temperature within the range of 4 to 37 °C. Cr(VI) was reduced effectively at 25 °C and 37 °C, and all of Cr(VI) was reduced after 96 h at 37 °C, while the reduction was slow at 4 °C and 15 °C, and it almost ceased after about 120 h. These results could be potentially useful for the bioremediation of Cr(VI) in groundwater.     

真菌还原Cr（VI）的研究

从不同来源的样品中分离筛选出几株抗Ｃｒ（ＶＩ）的真菌,他们能在含３００￣５００ｍｇ／ＬＫ２Ｃｒ２Ｏ７的蔗糖合成培养基中生长,其中ＢＳ－１菌株抗Ｋ２Ｃｒ２Ｏ７达９００ｍｇ／Ｌ．ＢＳ－１等４株真菌在含２００ｍｇ／Ｌ Ｋ２Ｃｒ２Ｏ７的培养基中生长４￣６ｄ后,培养液中的Ｃｒ（ＶＩ）已全部消失。这些真菌经鉴定为青霉菌ＢＳ－１和ＢＳ－３,黑曲霉ＢＲ－４和黄曲霉ＢＸ－１。经紫外可见光扫描及化学分析证实,高毒的Ｃ     

Bioremediation of toxic chromium from electroplating effluent by chromate-reducing Pseudomonas aeruginosa A2Chr in two bioreactors

The chromate-reducing ability of Pseudomonas aeruginosa A2Chr was compared in batch culture, with cells entrapped in a dialysis sac, and with cells immobilized in an agarose-alginate film in conjunction with a rotating biological contactor. In all three systems, the maximum Cr(VI) reduction occurred at 10 mg Cr(VI)/l. Whereas at 50 mg Cr(VI)/l concentration, only 16% of the total Cr(VI) was reduced, five spikings with 10 mg chromate/l at 2-h intervals led to 96% reduction of the total input of 50 mg Cr(VI)/l. Thus maximum Cr(VI) reduction was achieved by avoiding Cr(VI) toxicity to the cells by respiking with lower Cr(VI) concentrations. At 10 mg Cr(VI)/l, the pattern of chromate reduction in dialysis-entrapped cells was almost similar to that of batch culture and 86% of the bacterially reduced chromium was retained inside the dialysis sac. In electroplating effluent containing 100 mg Cr(VI)/l, however, the amount of Cr(VI) reduced by the cells immobilized in agarose-alginate biofilm was twice and thrice the amount reduced by batch culture and cells entrapped in a dialysis sac, respectively.     

Chromate reduction and 16S rRNA identification of bacteria isolated from a Cr(VI)-contaminated site

A gram-positive, hexavalent chromium [chromate: Cr(VI)]-tolerant bacterium, isolated from tannery waste from Pakistan, was identified as a Microbacterium sp. by 16S rRNA gene sequence homology. The strain (designated as MP30) reduced toxic Cr(VI) only under anaerobic conditions at the expense of acetate as the electron donor. The bacterium was able to grow aerobically in L-broth supplemented with 15 mM CrO4(2-) but then did not reduce Cr(VI). At a concentration of 2.4x10(9) cells/ml, 100 microM sodium chromate was reduced within 30 h; however, the maximum specific reduction rate was obtained at lower initial cell concentrations.     

Hexavalent Chromium Reduction and Accumulation by <Emphasis Type="Italic">Acinetobacter</Emphasis> AB1 Isolated from Fez Tanneries in Morocco

Hexavalent chromium reduction and accumulation by Acinetobacter AB1 isolated from Fez tanneries effluents were tested. The effects of some environmental factors such as pH, temperature, and exposure time on Cr(VI) reduction and resistance were investigated. We found that this strain was able to resist to concentrations as high as 400 mg/l of Cr(VI). Moreover, pH 10 and the temperature 30°C constitute favourable conditions to the growth and reduction of Acinetobacter AB1. Complete reduction of Cr(VI) was observed at low initial Cr(VI) concentrations of 50 mg/l after 72 h of incubation. Furthermore, Transmission electron microscope (TEM) analysis showed morphological changes in AB1 strain due 48H exposure to 100 mg/l chromate concentration and revealed circular electron dense (dark black point) inclusion within the cell cytoplasm suggesting chromium deposition within the 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.     

Modulation of tolerance to Cr(VI) and Cr(VI) reduction by sulfate ion in a <Emphasis Type="Italic">Candida</Emphasis> yeast strain isolated from tannery wastewater

The main aim of this study was to investigate the influence of the sulfate ion on the tolerance to Cr(VI) and the Cr(VI) reduction in a yeast strain isolated from tannery wastewater and identified as Candida sp. FGSFEP by the D1/D2 domain sequence of the 26S rRNA gene. The Candida sp. FGSFEP strain was grown in culture media with sulfate concentrations ranging from 0 to 23.92 mM, in absence and presence of Cr(VI) [1.7 and 3.3 mM]. In absence of Cr(VI), the yeast specific growth rate was practically the same in every sulfate concentration tested, which suggests that sulfate had no stimulating or inhibiting effect on the yeast cell growth. In contrast, at the two initial Cr(VI) concentrations assayed, the specific growth rate of Candida sp. FGSFEP rose when sulfate concentration increased. Likewise, the greater efficiencies and volumetric rates of Cr(VI) reduction exhibited by Candida sp. FGSFEP were obtained at high sulfate concentrations. Yeast was capable of reducing 100% of 1.7 mM Cr(VI) and 84% of 3.3 mM Cr(VI), with rates of 0.98 and 0.44 mg Cr(VI)/L h, with 10 and 23.92 mM sulfate concentrations, respectively. These results indicate that sulfate plays an important role in the tolerance to Cr(VI) and Cr(VI) reduction in Candida sp. FGSFEP. These findings may have significant implications in the biological treatment of Cr(VI)-laden wastewaters.     

Bioremediation of Cr(VI) from Chromium-Contaminated Wastewater by Free and Immobilized Cells of Cellulosimicrobium cellulans KUCr3

In this report, possible utilization of a chromium-reducing bacterial strain Cellulosimicrobium cellulans KUCr3 for effective bioremediation of hexavalent chromium (Cr(VI))-containing wastewater fed with tannery effluents has been discussed. Cr(VI) reduction and bioremediation were found to be related to the growth supportive conditions in wastewater, which is indicative of cell mass dependency for Cr(VI) reduction. Cr(VI) reduction was determined by measuring the residual Cr(VI) in the cell-free supernatant using colorimetric reagent S-diphenylcarbazide. Nutrient availability and initial cell density showed a positive relation with Cr(VI) reduction, but it was inhibited with increasing concentration of Cr(VI) under laboratory condition. The optimum temperature and pH for effective Cr(VI) reduction in wastewater were found to be 35°C and 7.5, respectively. The viable cells of KUCr3 were successfully entrapped in an agarose bead that was used in continuous column and batch culture for assaying Cr(VI) reduction. In packed bed column (continuous flow) experiment, approximately 25% Cr(VI) reduction occurred after 144 h. Cr(VI) was almost 75% and 52% reduced at concentrations of 0.5 mM and 2 mM Cr(VI), respectively, after 96 h in batch culture experiment in peptone-yeast extract-glucose medium, whereas it could decrease the Cr(VI) content up to 40% from the water containing tannery waste. This study suggests that KUCr3 could be used as a candidate for possible environmental clean up operation with respect to Cr(VI) bioremediation.     

Bioremediation of chromium by the yeast Pichia guilliermondii: toxicity and accumulation of Cr (III) and Cr (VI) and the influence of riboflavin on Cr tolerance

A comparative study has been made on the sensitivity of the yeast Pichia guilliermondii to Cr (III) and Cr (VI) as well as on the Cr uptake potential at growth-inhibitory concentrations of chromium. The strains used in the study were either isolated from natural sources or obtained from a laboratory strain collection. The results show that most of the natural strains were more tolerant to chromium and were able to grow in the presence of 5 mM Cr (III) or 0.5 mM Cr (VI), that is at concentrations which substantially inhibited the growth of laboratory strains. The cellular Cr content after treatment was similar for both strain types and ranged from 1.2-4.0 mg/g d.w. and 0.4-0.9 mg/g d.w., for Cr (III) and Cr (VI) forms, respectively, however, in one case of a natural strain it reached the value of 10 mg Cr (III)/g dry mass. Natural-source strains were grouped into four groups based on the yeasts' differential response to Cr (III) and Cr (VI). Hexavalent Cr-resistant mutants of a P. giuilliermondii laboratory strain, which revealed markedly changed capabilities of chromium accumulation, were obtained by means of UV-induced mutagenesis. Cr (VI) treatment triggered oversynthesis of riboflavin and the addition of exogenous riboflavin increased P. guilliermondii resistance to both Cr (III) and Cr (VI). Electrophoretic protein profiles revealed the induction and/or suppression of several proteins in response to toxic Cr (VI) levels.     

Reduction of hexavalent chromium by Pseudomonas fluorescens LB300 in batch and continuous cultures

Batch and continuous cultures of Pseudomonas fluorescens LB300 were shown to reduce hexavalent chromium, Cr(VI), aerobically at neutral pH (pH 7.0) with citrate as carbon and energy source. The product of Cr(VI) reduction was previously shown and confirmed in this work to be trivalent chromium, Cr(III), by quantitative reoxidation to Cr(VI) with KMnO₄. In separate batch cultures (100 ml) containing initial Cr(VI) concentrations of 314.0, 200.0 and 112.5 mg Cr(VI) L^–1, the organism reduced 61%, 69% and 99.7% of the Cr(VI), respectively. In a comparison of stationary and shaken cultures, the organism reduced 81% of Cr(VI) in 147 h in stationary culture and 80% in 122 h in shaken culture. In continuous culture, the organism lowered the influent Cr(VI) concentration by 28% with an 11.7-h residence time, by 39% with a 20.8-h residence time and by 57% with a 38.5-h residence time. A mass balance of chromium in a continuous culture at steady state showed an insignificant uptake of chromium by cells of P. fluorescens LB300. Correspondence to: P. C. DeLeo     

Bio-reduction of Cr(VI) by exopolysaccharides (EPS) from indigenous bacterial species of Sukinda chromite mine, India

Chrome mining activity has contributed intensively towards pollution of hexavalent chromium around Sukinda Valley, Orissa, India. In an attempt to study the specific contribution of exopolysaccharides (EPS) extracted from indigenous isolates towards Cr(VI) reduction, three chromium (VI) tolerant strains were isolated from the effluent mining sludge. Based on the tolerance towards Cr(VI) and EPS production capacity, one of them was selected for further work. The taxonomic identity of the selected strain was confirmed to be Enterobacter cloacae (showing 98% similarity in BLAST search to E. cloacae) through 16S rRNA analysis. The EPS production was observed to increase with increasing Cr(VI) concentration in the growth medium, highest being 0.078 at 100?mg/l Cr(VI). The extracted EPS from Enterobacter cloacae SUKCr1D was able to reduce 31.7% of Cr(VI) at 10?mg/l concentration, which was relevant to the prevailing natural concentrations at Sukinda mine effluent sludge. The FT-IR spectral studies confirmed the surface chemical interactions of hexavalent chromium with EPS.     

Stress response of yeast candida intermedia to Cr(VI)

Stress response of yeast Candida intermedia ZIM 156 exposed to chromium(VI) was investigated. Yeast cells were treated with Cr(VI) in concentrations of 50, 100, 300 and 500 microM in the mid-exponential growth phase. Monitoring of some bioprocess parameters during growth, specifically pO(2), showed that Cr(VI) addition, specifically in concentration of 100 and partially 50 micromol/L, increased metabolism intensity, which is connected to induced stress responses. Furthermore, oxidation of 2',7'-dichlorofluorescin indicated increased intracellular oxidant level, specifically at 100 microM Cr(VI) concentration. Antioxidant defense systems were further investigated. Catalase and superoxide dismutase activity was not increased in the cells exposed to the both Cr(VI) concentrations, which indicate that catalase and superoxide dismutase do not participate in cell defense systems. In contrast intracellular glutathione content in reduced form increased significantly in the cells exposed to 100 micromol Cr(VI)/L. Therefore, we demonstrated that glutathione plays an important role in the stress response of C. intermedia to Cr(VI).     

耐盐菌Staphylococcus sp. YZ-1和Bacillus cereus CC-1的Cr(VI)脱毒特性与机理

[背景]高盐含铬废水的去除过程中,Cr(Ⅵ)还原菌是研究者关注的重点,但目前对耐盐菌株的Cr(Ⅵ)脱毒特性及机理的分析仍较少。[目的]比较两株耐盐菌株的Cr(Ⅵ)移除特性,并区分Cr(Ⅵ)耐受机制的差异;通过基因组测序分析,从基因层面推测铬耐受相关基因;构建铬还原菌的混菌体系,考察两者对去除污染物的协同作用。[方法]从青海茶卡盐湖分离耐盐菌Staphylococcus sp.YZ-1,与Bacillus cereus CC-1进行基础特性和Cr(Ⅵ)去除性能的比较,并通过全基因组序列的分析验证特性测试的结果。[结果]两株菌都具有铬移除特性,但CC-1的铬移除效率更高,在初始Cr(Ⅵ)浓度为0.1 mmol/L情况下,CC-1能在12h内移除95.3%的Cr(Ⅵ),而YZ-1只能移除40.1%。在进一步实验中发现YZ-1只能对Cr(Ⅵ)进行还原,将其转化为可溶的有机态Cr(Ⅲ),而CC-1能同时对Cr(Ⅵ)进行还原和吸附。全基因组分析发现YZ-1具有编码外排泵蛋白的基因和编码NAD(P)H氧化还原酶的基因,而CC-1具有编码铬转运蛋白ChrA和细胞色素C氧化还原酶的基因。两株菌的混菌体系在处理含Cr(Ⅵ)、Te(Ⅳ)的废水时,菌群能将还原产物聚集成团并沉淀到底部。[结论]菌株YZ-1和CC-1均为耐盐铬还原菌,但YZ-1中的铬还原酶为诱导型酶,CC-1则为组成型酶。基因组数据分析鉴别出两者可能同时存在多种铬耐受机制相关编码基因。混合菌群可以结合YZ-1的自絮凝特性和两者均有的Te(Ⅳ)/Cr(Ⅵ)还原活性,具有潜在的实用价值。     

Preliminary studies on continuous chromium(VI) biological removal from wastewater by anaerobic-aerobic activated sludge process

The long-term continuous chromium(VI) removal from synthetic wastewater affected by influent hexavalent chromium (Cr(VI)) and glucose concentrations were studied with an anaerobic-aerobic activated sludge process. It was observed that before activated sludge was acclimated, the chromium in the effluent increased immediately as the influent chromium increased. However, both Cr(VI) and total chromium (TCr) in the effluent significantly decreased after acclimation. In the acclimated activated sludge, the chromium removal efficiency was 100% Cr(VI) and 98.56% TCr at influent Cr(VI) levels of 20 mg/day, 100% Cr(VI) and 98.92% TCr at influent Cr(VI) levels of 40 mg/day, and 98.64% Cr(VI) and 97.16% TCr at influent Cr(VI) levels of 60 mg/day. The corresponding effluent Cr(VI) and TCr concentrations were 0 and 0.012 mg/l, 0 and 0.018 mg/l, and 0.034 mg/l and 0.071 mg/l, respectively. When the influent glucose increased from 1125 to 1500 mg/l at influent Cr(VI) dosage of 60 mg/day, the Cr(VI) and TCr removal efficiency with the acclimated activated sludge improved from 98.64% and 97.16% to 100% and 98.48%, respectively, and the chromium concentration in the effluent decreased from 0.034 mg/l of Cr(VI) and 0.071 mg/l of TCr to 0 (Cr(VI)) and 0.038 mg/l (TCr). The effluent COD and turbidity was around 40 mg/l and 0, respectively, after the activated sludge was acclimated. Further studies showed that after the activated sludge was acclimated, its specific dehydrogenases activity (SDA) and protein contents increased. The SDA and protein increased respectively 15% and 10% when influent Cr(VI) increased from 20 to 60 mg/day.     

Bacterial reduction of hexavalent chromium by Enterobacter cloacae strain HO1 grown on sucrose

Chromium(VI) was reduced by Enterobacter cloacae strain HO1 grown with sucrose as a carbon source and nitrate as the initial terminal electron acceptor. Under excess substrate conditions, the Cr(VI) concentration, initially at 5 and 10 mg/l, was reduced to less than 100 mg/l.     

Cr(Ⅵ)还原菌Microbacterium sp.BD6的分离鉴定及还原特性

【目的】从电镀厂下水道的淤泥中分离筛选Cr(Ⅵ)高效还原菌,并对其生长和还原特性进行研究,以期为Cr(Ⅵ)污染的生物修复提供优质的菌种资源和应用参考。【方法】采用富集培养法从淤泥中分离、筛选出Cr(Ⅵ)还原菌,通过生理生化及16S rRNA基因序列分析进行初步鉴定。采用单因素实验确定菌株的最佳培养条件和抵抗胁迫环境的能力,利用外加电子供体改善菌株的Cr(Ⅵ)还原能力,筛选出最佳电子供体研究对菌株还原的影响。【结果】经分离筛选得到1株Cr(Ⅵ)耐受还原菌,初步鉴定为微杆菌属(Microbacterium sp.),命名为BD6。菌株BD6适宜在中温、偏碱性的环境条件下生长,能耐受50.0 g/L NaCl的高盐环境。Mn^2+对菌种的生长表现出较高的抑制,Ni^2+、Zn^2+、Cd^2+的抑制作用较小,Cu^2)产生了一定的促进作用。Cr(Ⅵ)对BD6的最低抑菌浓度为1700 mg/L。添加甘油、果糖、乳糖、葡萄糖、丙酮酸钠作为电子供体促进了菌株对Cr(Ⅵ)的还原。选择甘油作为菌株还原Cr(Ⅵ)的最佳电子供体,无电子供体添加时菌株96 h内对100 mg/L Cr(Ⅵ)的还原率仅为69.63%,添加2 g/L的甘油菌株在36 h内的还原率达到了100%。通过加大甘油的添加量可以促进菌株对初始浓度较高Cr(Ⅵ)的还原,但要受到Cr(Ⅵ)的毒性限制。菌株的最适还原条件和最适生长条件吻合,在50.0 g/L NaCl的高盐条件和50 mg/L Cd^2+的毒性环境中,添加2 g/L的甘油,菌株对100 mg/L Cr(Ⅵ)的还原率分别为72 h 96.79%、54 h 99.86%。【结论】分离筛选得到的Microbacterium sp.BD6是一株潜在的可用于Cr(Ⅵ)污染生物还原修复的候选菌株。