Assessment of Pollution-Induced Microbial Community Tolerance to Heavy Metals in Soil Using Ammonia-Oxidizing Bacteria and Biolog Assay

This study attempted to investigate if the tolerance of soil bacterial communities in general, and autotrophic ammonia-oxidizing bacteria (AOB) in particular, evolved as a result of prolonged exposure to metals, and could be used as an indigenous bioindicator for soil metal pollution. A soil contaminated with copper, chromium, and arsenic (CCA) was mixed with an uncontaminated garden soil (GS3) to make five test soils with different metal concentrations. A modified potential ammonium oxidation assay was used to determine the metal tolerance of the AOB community. Tolerance to Cr, Cu, and As was tested at the beginning and after up to 13 months of incubation. Compared with the reference GS3 soil, the five CCA soils showed significantly higher tolerance to Cr no matter which form of Cr (Cr³⁺, CrO₄ ^2?, or Cr₂O₇ ^2?) was tested, and the Cr tolerance correlated with the total soil Cr concentration. However, the tolerance to Cu²⁺, As³⁺, and As⁵⁺ did not differ significantly between the GS3 soil and the five CCA soils. Community level physiological profiles using Biolog microtiter plates were also used to examine the chromate tolerance of the bacterial communities extracted after six months of exposure. Our results showed that the bacterial community tolerance was altered and increased as the soil Cr concentration was increased, indicating that the culturable microbial community and the AOB community responded in a similar manner.     

Isolation of hexavalent chromium-reducing anaerobes from hexavalent-chromium-contaminated and noncontaminated environments

Hexavalent chromium [Cr(VI)], is a toxic, water-soluble contaminant present in many soils and industrial effluents. Bacteria from various soils were examined for Cr(VI) resistance and reducing potential. Microbes selected from both Cr(VI)-contaminated and-noncontaminated soils and sediments were capable of catalyzing the reduction of Cr(VI) to Cr(III) a less toxic, less water-soluble form of Cr, demonstrating the utility of using a selection strategy for indigenous Cr(VI)-reducing bacteria in a bioprocess. As a result, indigenous Cr(VI)- reducing microbes from contaminated sites should provide the means for developing a bioprocess to reduce Cr(VI) to Cr(III) in nonsterile effluents such as those from soil washes. This approach also avoids the contamination problems associated with pure cultures of allochthonous microorganisms. In addition the apparent ubiquity of Cr(VI)-reducing bacteria in soil and sediments indicates potential for in situ bioremediation of Cr(VI)-contaminated soils and ground water.     

Association of microbial community composition and activity with lead,chromium, and hydrocarbon contamination

Microbial community composition and activity were characterized in soil contaminated with lead (Pb), chromium (Cr), and hydrocarbons. Contaminant levels were very heterogeneous and ranged from 50 to 16,700 mg of total petroleum hydrocarbons (TPH) kg of soil(-1), 3 to 3,300 mg of total Cr kg of soil(-1), and 1 to 17,100 mg of Pb kg of soil(-1). Microbial community compositions were estimated from the patterns of phospholipid fatty acids (PLFA); these were considerably different among the 14 soil samples. Statistical analyses suggested that the variation in PLFA was more correlated with soil hydrocarbons than with the levels of Cr and Pb. The metal sensitivity of the microbial community was determined by extracting bacteria from soil and measuring [(3)H]leucine incorporation as a function of metal concentration. Six soil samples collected in the spring of 1999 had IC(50) values (the heavy metal concentrations giving 50% reduction of microbial activity) of approximately 2.5 mM for CrO(4)2- and 0.01 mM for Pb2+. Much higher levels of Pb were required to inhibit [14C]glucose mineralization directly in soils. In microcosm experiments with these samples, microbial biomass and the ratio of microbial biomass to soil organic C were not correlated with the concentrations of hydrocarbons and heavy metals. However, microbial C respiration in samples with a higher level of hydrocarbons differed from the other soils no matter whether complex organic C (alfalfa) was added or not. The ratios of microbial C respiration to microbial biomass differed significantly among the soil samples (P < 0.05) and were relatively high in soils contaminated with hydrocarbons or heavy metals. Our results suggest that the soil microbial community was predominantly affected by hydrocarbons.     

Dynamics of microbial community during bioremediation of phenanthrene and chromium(VI)-contaminated soil microcosms

The combined effect of phenanthrene and Cr(VI) on soil microbial activity, community composition and on the efficiency of bioremediation processes has been studied. Biometer flask systems and soil microcosm systems contaminated with 2,000 mg of phenanthrene per kg of dry soil and different Cr(VI) concentrations were investigated. Temperature, soil moisture and oxygen availability were controlled to support bioremediation. Cr(VI) inhibited the phenanthrene mineralization (CO₂ production) and cultivable PAH degrading bacteria at levels of 500–2,600 mg kg⁻¹. In the bioremediation experiments in soil microcosms the degradation of phenanthrene, the dehydrogenase activity and the increase in PAH degrading bacteria counts were retarded by the presence of Cr(VI) at all studied concentrations (25, 50 and 100 mg kg⁻¹). These negative effects did not show a correlation with Cr(VI) concentration. Whereas the presence of Cr(VI) had a negative effect on the phenanthrene elimination rate, co-contamination with phenanthrene reduced the residual Cr(VI) concentration in the water exchangeable Cr(VI) fraction (WEF) in comparison with the soil microcosm contaminated only with Cr(VI). Clear differences were found between the denaturing gradient gel electrophoresis (DGGE) patterns of each soil microcosm, showing that the presence of different Cr(VI) concentrations did modulate the community response to phenanthrene and caused perdurable changes in the structure of the microbial soil community.     

Effects of Contaminant Concentration,Aging, and Soil Properties on the Bioaccessibility of Cr(III) and Cr(VI) in Soil

Contaminated soils at numerous U.S. Department of Defense, Department of Energy, and other industrial facilities often contain huge inventories of toxic metals such as chromium. Ingestion of soil by children is often the primary risk factor that drives the need for remediation. Site assessments are typically based solely on total soil-metal concentrations and do not consider the potential for decreased bioaccessibility due to metal sequestration by soil. The objectives of this research are to investigate the effect of soil properties on the bioaccessibility of Cr(III) and Cr(VI) as a function of contaminant concentration and aging. The A and upper B horizons of two well-characterized soils, representative of Cr-contaminated soils in the southeastern United States, were treated with varying concentration of Cr(III) and Cr(VI) and allowed to age. The bioaccessibility of the contaminated soils was measured over a 200-d time period using a physiologically based extraction test (PBET) that was designed to simulate the digestive process of the stomach. The sorption of Cr(III) and Cr(VI) varied significantly as a function of soil type and horizon, and the oxidation state of the contaminant. Solid phase concentrations with Cr(III) were significantly greater than Cr(VI) for any given initial Cr concentration. This is consistent with the mechanisms of Cr(III) vs. Cr(VI) sequestration by the soils, where the formation of Cr(III)-hydroxides can result in the accumulation of large mass fractions of contaminant on mineral surfaces. Overall, Cr bioaccessibility decreased with duration of exposure for all soils and at all solid phase concentrations, with aging effects being more pronounced for Cr(III). The decrease in Cr bioaccessibility was rapid for the first 50 d and then slowed dramatically between 50 and 200 d. In general, the effects of Cr solid phase concentration on bioaccessibility was small, with Cr(III) showing the most pronounced effect; higher solid phase concentrations resulted in a decrease in bioaccessibility. Chemical extraction methods and X-ray Adsorption Spectroscopy analyses suggested that the bioaccessibility of Cr(VI) was significantly influenced by reduction processes catalyzed by soil organic carbon. Soils with sufficient organic carbon had lower Cr bioaccessibility values (～10 to 20%) due to an enhanced reduction of Cr(VI) to Cr(III). In soils where organic carbon was limited and reduction processes were minimal, the bioaccessibility of Cr(VI) dramatically increased (～60 to 70%).     

Soil chromium bioremediation: Synergic activity of actinobacteria and plants

The aim of this work was to evaluate a strategy to reduce the bioavailable chromium fraction in soil, using a Cr(VI) resistant microorganism, Streptomyces sp. MC1, under non sterile conditions, with maize plants as bioindicator and/or bioremediator.Soil samples were contaminated with 100, 200 and 400 mg kg⁻¹ of Cr(VI) or Cr(III). Bioavailable chromium (35%) was only detected in samples with Cr(VI). Soil samples with Cr(VI) 200 mg kg⁻¹ were inoculated with Streptomyces sp. MC1, and bioavailable chromium decreased up to 73%.Zea mays seedlings were planted in soil samples contaminated with chromium. Plantlets accumulated chromium mainly as Cr(III), and biomass decreased up to 88%. Streptomyces sp. MC1 was inoculated in soil samples contaminated with 200 mg kg⁻¹ of Cr(VI) and Z.mays seedlings were planted.Streptomyces sp. MC1 caused Z.mays biomass increase (57%), chromium accumulation and bioavailable chromium decreased up to 46% and 96%, respectively.This work constitutes the first contribution of cooperative action between actinobacteria and Z.mays in the bioremediation of Cr(VI) contaminated soil. The large removal capacity of bioavailable chromium by Streptomyces sp. MC1 and Z.mays infers that they could be successfully applied together in bioremediation of soils contaminated with Cr(VI).     

Rhizoreduction of arsenate and chromate in Australian native grass, shrub and tree vegetation

Background and aims

Rhizosphere influences the dynamics of nutrients and contaminants through increased microbial activity, release of root exudates and alteration of pH. The objective of this study was to evaluate the rhizosphere-induced reduction (i.e. rhizoreduction) and redistribution of arsenate [As(V)] and chromate [Cr(VI)] in Australian native vegetation in relation to their bioavailability.

Methods

The reduction of As(V) and Cr(VI) was examined using rhizosphere soils from a number of Australian native vegetation (Acacia pubescens, Eucalyptus camaldulensis, Enchylaena tomentosa, Templetonia retusa, Dichantheum sericeum and Austrodanthonia richardsonii). Naturally contaminated As and Cr soils were used to examine the effect of Dichantheum sericeum on the redistribution and bioavailability of these metal(loid)s.

Results

The rhizosphere soil contained higher levels of microbial activity, dissolved organic carbon and organic acid content than the non-rhizosphere soil. The rhizosphere soil caused up to 2.4 and 5.1 fold increases in the rate of reduction of As(V) and Cr(VI), respectively. There was a significant relationship between rhizosphere-induced increases in microbial activity (Δ basal respiration) and As(V) and Cr(VI) reduction (Δ rate of reduction), indicating the role of increased microbial activity in rhizosphere soil on metal(loid) reduction. In the plant growth experiment, Dichantheum sericeum enhanced the reduction of metal(loid)s in the naturally contaminated soils, thereby increasing the bioavailability of As but decreasing that of Cr.

Conclusions

Depending on the nature of metal(loid)s present in soil, the rhizosphere-induced reduction by plant species such as Dichantheum sericeum and Templetonia retusa has implications to both their bioavailability to higher plants and microorganisms, and remediation of contaminated soils. While rhizoreduction decreases Cr bioavailability it increases that of As.     

Association of Microbial Community Composition and Activity with Lead, Chromium, and Hydrocarbon Contamination

Microbial community composition and activity were characterized in soil contaminated with lead (Pb), chromium (Cr), and hydrocarbons. Contaminant levels were very heterogeneous and ranged from 50 to 16,700 mg of total petroleum hydrocarbons (TPH) kg of soil⁻¹, 3 to 3,300 mg of total Cr kg of soil⁻¹, and 1 to 17,100 mg of Pb kg of soil⁻¹. Microbial community compositions were estimated from the patterns of phospholipid fatty acids (PLFA); these were considerably different among the 14 soil samples. Statistical analyses suggested that the variation in PLFA was more correlated with soil hydrocarbons than with the levels of Cr and Pb. The metal sensitivity of the microbial community was determined by extracting bacteria from soil and measuring [³H]leucine incorporation as a function of metal concentration. Six soil samples collected in the spring of 1999 had IC₅₀ values (the heavy metal concentrations giving 50% reduction of microbial activity) of approximately 2.5 mM for CrO₄²⁻ and 0.01 mM for Pb²⁺. Much higher levels of Pb were required to inhibit [¹⁴C]glucose mineralization directly in soils. In microcosm experiments with these samples, microbial biomass and the ratio of microbial biomass to soil organic C were not correlated with the concentrations of hydrocarbons and heavy metals. However, microbial C respiration in samples with a higher level of hydrocarbons differed from the other soils no matter whether complex organic C (alfalfa) was added or not. The ratios of microbial C respiration to microbial biomass differed significantly among the soil samples (P < 0.05) and were relatively high in soils contaminated with hydrocarbons or heavy metals. Our results suggest that the soil microbial community was predominantly affected by hydrocarbons.     

Aerobic Reduction of Hexavalent Chromium in Soil by Indigenous Microorganisms

Surface soil containing 25,100 mg/kg total Cr [12,400 mg/kg Cr(VI)] obtained from a Superfund site was used in laboratory microcosm studies to evaluate the potential for aerobic reduction of Cr(VI) by the indigenous soil microbial community. Hexavalent chromium in soil was reduced by as much as 33% (from 1840 to 1240 mg/L) within 21 days under enrichment conditions. Reduction of Cr(VI) in this system was biologically mediated and depended on the availability of usable energy sources. Mass balance studies suggested that the microbial populations removed Cr(VI) from the soil solutions by reduction. Indigenous microbial soil communities even with no history of Cr(VI) contamination were capable of mediating this process. However, Cr(VI) removal was not observed when microbial populations from a sewage sludge sample were added to the soil microcosms. The results suggest that Cr(VI)-reducing microbial populations are widespread in soil, and thus the potential exists for in situ remediation of environmentally significant levels of Cr( VI) contamination.     

Soil microbial community responses to additions of organic carbon substrates and heavy metals (Pb and Cr)

Microcosm experiments were conducted with soils contaminated with heavy metals (Pb and Cr) and aromatic hydrocarbons to determine the effects of each upon microbial community structure and function. Organic substrates were added as a driving force for change in the microbial community. Glucose represented an energy source used by a broad variety of bacteria, whereas fewer soil species were expected to use xylene. The metal amendments were chosen to inhibit the acute rate of organic mineralization by either 50% or 90%, and lower mineralization rates persisted over the entire 31-day incubation period. Significant biomass increases were abolished when metals were added in addition to organic carbon. The addition of organic carbon alone had the most significant impact on community composition and led to the proliferation of a few dominant phylotypes, as detected by PCR-denaturing gradient gel electrophoresis of bacterial 16S rRNA genes. However, the community-wide effects of heavy metal addition differed between the two carbon sources. For glucose, either Pb or Cr produced large changes and replacement with new phylotypes. In contrast, many phylotypes selected by xylene treatment were retained when either metal was added. Members of the Actinomycetales were very prevalent in microcosms with xylene and Cr(VI); gene copy numbers of biphenyl dioxygenase and phenol hydroxylase (but not other oxygenases) were elevated in these microcosms, as determined by real-time PCR. Much lower metal concentrations were needed to inhibit the catabolism of xylene than of glucose. Cr(VI) appeared to be reduced during the 31-day incubations, but in the case of glucose there was substantial microbial activity when much of the Cr(VI) remained. In the case of xylene, this was less clear.     

Impact of chromium-contaminated wastewaters on the microbial community of a river

The influence of chromium on the microbial community structure was analyzed in a river system subjected to long-term chromium contamination, by plating and by sequencing 16S rRNA genes cloned from DNA extracted from the river sediments. We also analyzed the influence of chromium on the ability of the microbial community to resist and reduce Cr(VI) and on its resistance to antibiotics. Shifts in the microbial community structure were analyzed by amplified ribosomal DNA restriction analysis fingerprinting. The isolates obtained were phylogenetically related to Actinobacteria, Firmicutes, Bacteroidetes and Proteobacteria, whereas Acidobacteria and Deltaproteobacteria were only revealed by clone analyses. Cr(VI)-resistant and Cr(VI)-reducing strains were isolated in all sites examined. However, each sample site had a microbial community with a different antibiotic resistance pattern. Our study seems to indicate that in this river ecosystem chromium influenced the microbial communities, altering some of their functional characteristics, such as the percentage of the microbial community able to resist or to reduce Cr(VI) and the phylogenetic groups isolated, but it did not affect the structural diversity. Furthermore, the concentration of Cr(VI) in the sediments could not be correlated with a lower number of bacteria or lower index of generic diversity, neither with the ability of the microbial community to resist or to reduce higher Cr(VI) concentrations.     

Soil Microbial Community Responses to Additions of Organic Carbon Substrates and Heavy Metals (Pb and Cr)

Microcosm experiments were conducted with soils contaminated with heavy metals (Pb and Cr) and aromatic hydrocarbons to determine the effects of each upon microbial community structure and function. Organic substrates were added as a driving force for change in the microbial community. Glucose represented an energy source used by a broad variety of bacteria, whereas fewer soil species were expected to use xylene. The metal amendments were chosen to inhibit the acute rate of organic mineralization by either 50% or 90%, and lower mineralization rates persisted over the entire 31-day incubation period. Significant biomass increases were abolished when metals were added in addition to organic carbon. The addition of organic carbon alone had the most significant impact on community composition and led to the proliferation of a few dominant phylotypes, as detected by PCR-denaturing gradient gel electrophoresis of bacterial 16S rRNA genes. However, the community-wide effects of heavy metal addition differed between the two carbon sources. For glucose, either Pb or Cr produced large changes and replacement with new phylotypes. In contrast, many phylotypes selected by xylene treatment were retained when either metal was added. Members of the Actinomycetales were very prevalent in microcosms with xylene and Cr(VI); gene copy numbers of biphenyl dioxygenase and phenol hydroxylase (but not other oxygenases) were elevated in these microcosms, as determined by real-time PCR. Much lower metal concentrations were needed to inhibit the catabolism of xylene than of glucose. Cr(VI) appeared to be reduced during the 31-day incubations, but in the case of glucose there was substantial microbial activity when much of the Cr(VI) remained. In the case of xylene, this was less clear.     

Responses of the Anaerobic Bacterial Community to Addition of Organic C in Chromium(VI)- and Iron(III)-Amended Microcosms

Chromium (VI) is toxic to microorganisms and can inhibit the biodegradation of organic pollutants in contaminated soils. We used microcosms amended with either glucose or protein (to drive bacterial community change) and Fe(III) (to stimulate iron-reducing bacteria) to study the effect of various concentrations of Cr(VI) on anaerobic bacterial communities. Microcosms were destructively sampled based on microbial activity (measured as evolution of CO₂) and analyzed for the following: (i) dominant bacterial community by PCR-denaturing gradient gel electrophoresis (DGGE) of the 16S rRNA gene; (ii) culturable Cr-resistant bacteria; and (iii) enrichment of iron-reducing bacteria of the Geobacteraceae family by real-time PCR. The addition of organic C stimulated the activities of anaerobic communities. Cr(VI) amendment resulted in lower rates of CO₂ production in glucose microcosms and a slow mineralization phase in protein-amended microcosms. Glucose and protein amendments selected for different bacterial communities. This selection was modified by the addition of Cr(VI), since some DGGE bands were intensified and new bands appeared in Cr(VI)-amended microcosms. A second dose of Cr(VI), added after the onset of activity, had a strong inhibitory effect when higher levels of Cr were added, indicating that the developing Cr-resistant communities had a relatively low tolerance threshold. Most of the isolated Cr-resistant bacteria were closely related to previously studied Cr-resistant anaerobes, such as Pantoea, Pseudomonas, and Enterobacter species. Geobacteraceae were not enriched during the incubation. The studied Cr(VI)-contaminated soil contained a viable anaerobic bacterial community; however, Cr(VI) altered its composition, which could affect the soil biodegradation potential.     

Metal Transport and Bioavailability in Soil Contaminated with CCA-Treated Wood Leachates

A laboratory study was conducted to investigate metal transport and accumulation within soils contaminated with As, Cr, and Cu from CCA-treated wood leachates. New blocks of CCA-treated wood were leached using synthetic rainwater. Soil columns were constructed and filled with three different soils, including a sandy soil, an organic soil and a clay soil. The leachate was applied intermittently until 80 pore volumes were eluted through each column. Metal concentrations (Cu, Cr, and As) were measured in the leachate before passage through the columns as well as in each elutriate fraction collected. Chemical analysis was complemented with toxicity testing using Ceriodaphnia dubia, Selenastrum capricornutum, and MetPLATE?. Following application of 80 pore volumes of leachate, the columns were dissected and the profile of the metal concentrations within each column was determined. A comparison of the arsenic, chromium and copper leaching patterns found arsenic to be the most mobile, with copper the most retained in the soil columns (As < Cr < Cu). Transport patterns of As differed in the three soil types, with observed mobility highest in the sandy soil and lowest in the clay soil. The three metals accumulated in the top layer of soil. Arsenic posed the greatest risk when soil concentrations were compared to risk-based target levels. Although metals were detected in soil elutriates, no toxicity was detected in any of the soil column elutriates using any of the three toxicity assays.     

Responses of the anaerobic bacterial community to addition of organic C in chromium(VI)- and iron(III)-amended microcosms

Chromium (VI) is toxic to microorganisms and can inhibit the biodegradation of organic pollutants in contaminated soils. We used microcosms amended with either glucose or protein (to drive bacterial community change) and Fe(III) (to stimulate iron-reducing bacteria) to study the effect of various concentrations of Cr(VI) on anaerobic bacterial communities. Microcosms were destructively sampled based on microbial activity (measured as evolution of CO2) and analyzed for the following: (i) dominant bacterial community by PCR-denaturing gradient gel electrophoresis (DGGE) of the 16S rRNA gene; (ii) culturable Cr-resistant bacteria; and (iii) enrichment of iron-reducing bacteria of the Geobacteraceae family by real-time PCR. The addition of organic C stimulated the activities of anaerobic communities. Cr(VI) amendment resulted in lower rates of CO2 production in glucose microcosms and a slow mineralization phase in protein-amended microcosms. Glucose and protein amendments selected for different bacterial communities. This selection was modified by the addition of Cr(VI), since some DGGE bands were intensified and new bands appeared in Cr(VI)-amended microcosms. A second dose of Cr(VI), added after the onset of activity, had a strong inhibitory effect when higher levels of Cr were added, indicating that the developing Cr-resistant communities had a relatively low tolerance threshold. Most of the isolated Cr-resistant bacteria were closely related to previously studied Cr-resistant anaerobes, such as Pantoea, Pseudomonas, and Enterobacter species. Geobacteraceae were not enriched during the incubation. The studied Cr(VI)-contaminated soil contained a viable anaerobic bacterial community; however, Cr(VI) altered its composition, which could affect the soil biodegradation potential.     

Microbial Community Analysis of Soils Contaminated with Lead, Chromium and Petroleum Hydrocarbons

The impact on the microbial community of long-term environmental exposure to metal and organic contamination was investigated. Twenty-four soil samples were collected along a transect dug in soils contaminated with road paint and paint solvents, mainly toluene. Chemical analysis along the transect revealed a range from high to low concentrations of metals (lead and chromium) and organic solvent compounds. Principal components analysis of microbial community structure based on denaturing gradient gel electrophoresis of the V3 region of the 16S rRNA gene and fatty acid methyl esters derived from phospholipids (phospholipid fatty acid analysis) showing samples with similar fingerprints also had similar contaminant concentrations. There was also a weak positive correlation between microbial biomass and the organic carbon concentration. Results indicated that microbial populations are present despite some extreme contaminant levels in this mixed-waste contaminated site. Nucleotide sequence determination of the 16S rRNA gene indicated the presence of phylogenetically diverse bacteria belonging to the α-, β-, γ-, and δ-Proteobacteria, the high and low G + C Gram-positive bacteria, green nonsulfur, OP8, and others that did not group within a described division. This indicates that soils contaminated with both heavy metals and hydrocarbons for several decades have undergone changes in community composition, but still contain a phylogenetically diverse group of bacteria (including novel phylotypes) that warrant further investigation.     

武汉市蔬菜重金属污染现状的调查与评价

通过对武汉市6种蔬菜的供食部位及所对应的土壤中重金属Hg(汞)、Cu(铜)、Pb(铅)、Cd(镉)、Cr(铬)、Zn(锌)、As(砷)含量及分布特征进行了检测,结果表明:Pb、Cr是武汉市蔬菜中主要的污染元素;Hg、Cd只在个别叶菜类上超标;Zn、Cu、As无超标样。在所时应的土样中,普遍存在污染的是Cd,其它6种重金属元素均未超标。     

Fe(III), Cr(VI), and Fe(III) mediated Cr(VI) reduction in alkaline media using a <Emphasis Type="Italic">Halomonas</Emphasis> isolate from Soap Lake,Washington

Hexavalent chromium is one of the most widely distributed environmental contaminants. Given the carcinogenic and mutagenic consequences of Cr(VI) exposure, the release of Cr(VI) into the environment has long been a major concern. While many reports of microbial Cr(VI) reduction are in circulation, very few have demonstrated Cr(VI) reduction under alkaline conditions. Since Cr(VI) exhibits higher mobility in alkaline soils relative to pH neutral soils, and since Cr contamination of alkaline soils is associated with a number of industrial activities, microbial Cr(VI) reduction under alkaline conditions requires attention. Soda lakes are the most stable alkaline environments on earth, and contain a wide diversity of alkaliphilic organisms. In this study, a bacterial isolate belonging to the Halomonas genus was obtained from Soap Lake, a chemically stratified alkaline lake located in central Washington State. The ability of this isolate to reduce Cr(VI) and Fe(III) was assessed under alkaline (pH = 9), anoxic, non-growth conditions with acetate as an electron donor. Metal reduction rates were quantified using Monod kinetics. In addition, Cr(VI) reduction experiments were carried out in the presence of Fe(III) to evaluate the possible enhancement of Cr(VI) reduction rates through electron shuttling mechanisms. While Fe(III) reduction rates were slow compared to previously reported rates, Cr(VI) reduction rates fell within range of previously reported rates.     

砷矿区农田土壤微生物群落碳源代谢多样性

采用Biolog方法研究了砷(As)矿区农田土壤微生物碳源利用多样性及其与土壤化学性质的关系.结果表明： 3种土壤的N、P、K、有机质(OM)、Cu和Zn全量依次为中As > 高As > 低As土壤.中As和高As土壤微生物平均吸光度以及多样性指数(H′、D、U)显著高于低As土壤.主成分分析与生理碳代谢指纹图谱分析表明,中As和高As土壤微生物对糖类、氨基酸类等碳源的利用程度显著高于低As土壤.典范对应分析显示,影响土壤微生物群落碳源代谢的主要有全P、OM、全Pb、全Zn、全N和pH,全As并不是最主要的影响因子.可见,养分是影响长期污染土壤微生物群落结构和功能多样性的主要因素.     

Bioaccumulation and biosorption of chromium by Aspergillus niger MTCC 2594

Chromium toxicity is of prime concern due to chrome tanning processes in the leather sector. Chrome tanning results in the discharge of toxic levels of chromium causing pollution hazards. Chromium levels of Cr(III) and Cr(VI) were high above permissible limits in chrome samples after chrome tanning. The potential of Aspergillus niger MTCC 2594 to accumulate chromium as well as its biosorption capacity is investigated in this study. Bioaccumulation of Cr(III) and Cr(VI) in the spent chrome liquor has resulted in a 75-78% reduction of the initial Cr content in 24-36 h. A. niger biomass is found to be very effective in the biosorption of Cr(III) and Cr(VI) in spent chrome liquor. Maximum adsorption of 83% for biosorption of Cr(III) at 48 h and 79% of Cr(VI) at 36 h in spent chrome liquor is observed. The biosorption characteristics fit well with Langmuir and Freundlich isotherms and the adsorption parameters are evaluated. The biosorption of Cr also follows Lagergren kinetics. A. niger biomass is effectively used for the biosorption of chromium with 79-83% Cr removal in 36-48 h.