Inhibition of iron(II) oxidation by arsenic(III,V) in Thiobacillus ferrooxidans: Effects on arsenopyrite bioleaching

Summary The more complex inhibitory effect of As(III) than that of As(V) on Fe(II) oxidation in a non-growing Thiobacillus ferrooxidans suspension was demonstrated. The yield of arsenic bioextraction from a chalcopyrite concentrate was not affected by arsenic inhibition due to the low sensitivity of the strain to arsenic ions, supported by a spontaneous conversion of As(III) to As(V).     

Arsenic precipitation in the bioleaching of enargite by Sulfolobus BC at 70 °C

Enargite (Cu₃AsS₄) was leached at 70°C by Sulfolobus BC in shake-flasks. The highest copper dissolution (52% after 550 h of leaching) was obtained with bacteria and 1 g l^–1 ferric ion. In the absence of ferric ion, Sulfolobus BC catalyzes the bioleaching of enargite through a direct mechanism after adhesion onto the mineral surface. In ferric bioleaching, arsenic precipitated as ferric arsenate and arsenic remained associated to the solid residues, preventing the presence of a high dissolved arsenic concentration in the leaching solution. About 90% inhibition of bacterial growth rate and activity was observed for dissolved arsenic concentrations above 600 mg l^–1 for As(III) and above 1000 mg l^–1 for As(V). Arsenic-bearing copper ores and concentrates could be leached by Sulfolobus BC in the presence of ferric iron due to the favourable precipitation of arsenic ion as ferric arsenate, avoiding significant bacterial inhibition.     

EXAFS study on arsenic species and transformation in arsenic hyperaccumulator

Synchrotron radiation extended X-ray absorption fine structure (SR EXAFS) was employed to study the transformation of coordination environment and the redox speciation of arsenic in a newly discovered arsenic hyperaccumulator, Cretan brake (Pteris cretica L. var nervosa Thunb). It showed that the arsenic in the plant mainly coordinated with oxygen, except that some arsenic coordinated with S as As-GSH in root. The complexation of arsenic with GSH might not be the predominant detoxification mechanism in Cretan brake. Although some arsenic in root presented as As(V) in Na₂HAsO₄ treatments, most of arsenic in plant presented as As(III)-O in both treatments, indicating that As(V) tended to be reduced to As(III) after it was taken up into the root, and arsenic was kept as As(III) when it was transported to the above-ground tissues. The reduction of As(V) primarily proceeded in the root.     

The influence of chemical form and concentration of arsenic on rice growth and tissue arsenic concentration

Arsenic absorption by rice (Oryza sativa, L.) in relation to the chemical form and concentration of arsenic added in nutrient solution was examined. A 4 × 3 × 2 factorial experiment was conducted with treatments consisting of four arsenic chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsenic acid, MMAA; and dimethyl arsenic acid, DMAA], three arsenic concentrations [0.05, 0.2, and 0.8 mg As L^-1], and two cultivars [Lemont and Mercury] with a different degree of susceptibility to straighthead, a physiological disease attributed to arsenic toxicity. Two controls, one for each cultivar, were also included. Arsenic phytoavailability and phytotoxicity are determined primarily by the arsenic chemical form present. Application of DMAA increased total dry matter production. While application of As(V) did not affect plant growth, both As(III) and MMAA were phytotoxic to rice. Availability of arsenic to rice followed the trend: DMAA<As(V)<MMAA<As(III). Upon absorption, DMAA was readily translocated to the shoot. Arsenic(III), As(V), and MMAA accumulated in the roots. With increased arsenic application rates the arsenic shoot/root concentration decreased for the As(III) and As(V) treatments. Monomethyl arsenic acid (MMAA), however, was translocated to the shoot upon increased application. The observed differential absorption and translocation of arsenic chemical forms by rice is possibly responsible for the straighthead disorder attributed to arsenic.     

Ecological restoration of arsenic contaminated soil by Arundo donax L

The possible arsenic tolerance mechanisms were explored in Arundo donax L. under various supplied arsenic concentrations. The treatments included control (no metal) and five doses of arsenic trioxide i.e., 0, 50, 100, 300, 600 and 1000 μg L⁻¹ As to A. donax. The phytoextraction ability of A. donax L. plants was assessed using both the translocation and bioaccumulation factors. The transpirates were collected to analyze the arsenic concentration volatilized along-with study of anatomical characteristics of the plant parts. In general, the arsenite and arsenate accumulation linearly increased in roots, shoot and leaves with the increasing supplied arsenic levels i.e., from 2.348, 2.775 and 3.25 μg g⁻¹ at 50 μg L⁻¹ to 50, 53.125 and 64.25 μg g⁻¹ arsenite, at 1000 μg L⁻¹, from 4.075, 5.425 and 13.56 μg g⁻¹ at 50 μg L⁻¹ to 71, 62.02 and 436.219 μg g⁻¹ arsenate at 1000 μg L⁻¹, respectively. The order of arsenic accumulation in A. donax L. was: solution As(III) < Root As(III) < Shoot As(III) < Leaf As(III) < Solution As(V) < Root As(V) < Shoot As(V) < Leaf As(V). The range of arsenic volatilization by A. donax L. was 7.2–22% at higher supplied arsenic (300–1000 μg L⁻¹). Volatilization was an important mechanism to avoid toxic effects of arsenic by A. donax L. in addition to bioaccumulation.     

Toxicity of arsenic during high temperature bioleaching of gold-bearing arsenical pyrite

 A moderately thermophilic mixed culture, MT, and the thermophilic Sulfolobus acidocaldarius strain BC were studied for their response to arsenic in a defined medium and also in media containing a pyrite and an arsenical pyrite flotation concentrate. In defined medium, the individual constituents of the MT culture exhibited a high tolerance to arsenite and arsenate compared to S. acidocaldarius strain BC. When grown on increasing concentrations of the pyrite flotation concentrate, both cultures had similar specific leaching rates over the various concentrations of the mineral substrate. In contrast, S. acidocaldarius strain BC exhibited a decreasing specific leaching rate when grown on the arsenical pyrite while the MT culture was not affected. In addition, arsenic added to cultures of S. acidocaldarius strain BC growing with pyrite as a growth substrate inhibited further growth, while added arsenic had no effect on the MT culture growing on the pyrite. These data indicate that the moderately thermophilic, arsenic-resistant MT culture was able to leach arsenical pyrite more efficiently than was the S. acidocaldarius strain BC culture at high concentrations of the mineral. This emphasizes the fact that proper culture selection is an important parameter when developing commercial processes involving arsenic-containing minerals. Received: 21 June 1995/Received revision: 25 August 1995/Accepted: 7 September 1995     

4种蚯蚓肠道微生物对砷毒性的响应差异研究

蚯蚓肠道是微生物多样性的一个潜在存储库。砷对蚯蚓肠道微生物群落的影响已被证实,但砷在不同蚯蚓肠道菌群中生物转化的差异仍不清楚。为了进一步阐述土壤中广泛存在的低浓度砷(浓度为5,15,25 mg/kg)对不同种类蚯蚓肠道微生物影响的差异,将4种典型蚯蚓暴露于砷污染土壤后,测定其肠道微生物组成变化,并分析砷对不同蚯蚓肠道内砷富集、形态和砷生物转化基因的影响。结果显示,所有蚯蚓组织内均存在明显的砷富集,其富集系数由高到低依次为:安德爱胜蚓(1.93)>加州腔蚓(0.80)>通俗腔蚓(0.78)>湖北远盲蚓(0.52),蚯蚓组织和肠道内砷形态主要以无机砷为主,其中As(III)含量比例> 80%,部分蚯蚓组织内还发现少量有机砷。4种蚯蚓肠道微生物群落在门水平上主要以变形菌、厚壁菌和放线菌为主,并与周围土壤细菌群落组成存在显著差异。同时,在土壤和肠道内共检测到17个砷转化基因,其中蚯蚓肠道内As(V)还原和砷转运相关基因相对丰度较高,而砷(去)甲基化基因丰度较低。此外,低浓度砷污染对蚯蚓生长无显著影响,却能引起蚯蚓肠道微生物群落的紊乱。蚯蚓种类和砷污染是引起蚯蚓肠道微生物...     

Oxidation of As(III) by the Bacterial Community of a Marine Sediment Monitored by Microcalorimetry

Oxidation of arsenic(III) by the bacterial community of a contaminated sediment (from the Estaque marina, Marseille, France) was studied using microcalorimetry. A low, but reproducible, heat output was detectable during microbial As(III) oxidation. The heat produced was of the same order of magnitude as the heat value calculated from the standard molar enthalpy change for the As(III) oxidation by oxygen. Parameters associated with the biogeochemical cycles of arsenic, iron and carbon were studied in parallel. Amendment with arsenite delayed CO₂ production and increased the rate of Fe(II) oxidation in the sediment. These results suggest a correlation between arsenic and iron biogeochemical cycles and mineralization of organic matter.

Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.     

Effect of Fluoride on Arsenic Uptake from Arsenic-Contaminated Groundwater using Pteris vittata L.

High-arsenic groundwater in inland basins usually contains high concentrations of fluoride. In the present study, the effects of fluoride on arsenic uptake by Pteris vittata and on arsenic transformation in growth media were investigated under greenhouse conditions. After P. vittata was hydroponically exposed to 66.8 μM As (V) in the presence of 1.05 mM F^? in the form of NaF, KF, or NaF+KF for 10 d, no visible toxicity symptoms were observed, and there were not significant differences in the dry biomass among the four treatments. The results showed that P. vittata tolerated F^? concentrations as high as 1.05 mM but did not accumulate fluoride in their own tissues. Arsenic uptake was inhibited in the presence of 1.05 mM F^?. However, in hydroponic batches with 60 μM As (III) or 65 μM As (V), it was found that 210.6 and 316.0 μM F^? promoted arsenic uptake. As(III) was oxidized to As(V) in the growth media in the presence and absence of plants, and F^? had no effect on the rate of As(III) transformation. These experiments demonstrated that P. vittata was a good candidate to remediate arsenic-contaminated groundwater in the presence of fluoride. Our results can be used to develop strategies to remediate As-F-contaminated water using P. vittata.     

Arsenic Resistance and Bioaccumulation of an Indigenous Bacterium Isolated from Aquifer Sediments of Datong Basin,Northern China

To obtain bacteria with arsenic accumulation potential that can be used to remove arsenic from contaminated waters, experiments were made to investigate the tolerance and accumulation to arsenic of an indigenous bacterium XZM002 isolated from aquifer sediments of Datong Basin, northern China. The results showed that strain XZM002 belongs to the genus Bacillus and has evolved defense mechanisms to reduce arsenic injury: the change of cellular shape from initial rod to oval and then to round with increment of arsenic toxicity. The effect of arsenate or arsenite on the bacterial growth was also investigated. Results showed that growth of the strain was inhibited under As(III) and high concentration As(V) (over 1200 μg l^?1) conditions in the first 2 days and promoted under low concentration As(V) (under 400 μg l^?1) condition. Its arsenic bioaccumulation potential was surveyed by monitoring the concentration changes of total arsenic and arsenic speciation in the medium and in the cytoplasm, and those of total arsenic on the membrane. Methylated arsenic species were not detected throughout the experiment. The results indicated that 11.5% of arsenic was removed from liquid medium into the bacterial cells and 9.22% of As(V) in the medium was transformed gradually to As(III) during 4 d of incubation. Approximately 80% of the total accumulated arsenic was adsorbed onto the membrane instead of into cytoplasm; and the arsenic accumulation almost approached saturation after incubation for 72 h.     

Reducing the Leachability and Bioaccessibility of Arsenic in Soils using Supported Nano Titanium Dioxide

We herein report the development and testing of a novel material, namely activated carbon-supported nano titanium dioxide (ACTD) for the immobilization of arsenic in soil. This material, which was prepared using a sol-gel method, effectively reduced the toxicity characteristic leaching procedure (TCLP) leachability and physiologically based extraction test (PBET) bioaccessibility of As(III) in soil samples. Upon processing the soils for 56 d at an ACTD dosage of 0.25 mmol g^?1, the TCLP leachability of As(III) was reduced by 82.7–97.7%, while the bioaccessibility was lowered by 58.6–81.2%. In addition, sequential extraction resulted in an 11.5–96.0% decrease in the mobile-As(III) and the mobilizable-As(III) fractions, but an increase in the residual-As(III) fraction upon treatment with ACTD. These observations indicate that the application of ACTD could result in an 80% reduction in As(III) environmental leaching, thereby confirming that ACTD appears suitable for the treatment of arsenic-contaminated soil.     

The antibiotic action of methylarsenite is an emergent property of microbial communities

Arsenic is the most ubiquitous environmental toxin. Here, we demonstrate that bacteria have evolved the ability to use arsenic to gain a competitive advantage over other bacteria at least twice. Microbes generate toxic methylarsenite (MAs(III)) by methylation of arsenite (As(III)) or reduction of methylarsenate (MAs(V)). MAs(III) is oxidized aerobically to MAs(V), making methylation a detoxification process. MAs(V) is continually re‐reduced to MAs(III) by other community members, giving them a competitive advantage over sensitive bacteria. Because generation of a sustained pool of MAs(III) requires microbial communities, these complex interactions are an emergent property. We show that reduction of MAs(V) by Burkholderia sp. MR1 produces toxic MAs(III) that inhibits growth of Escherichia coli in mixed culture. There are three microbial mechanisms for resistance to MAs(III). ArsH oxidizes MAs(III) to MAs(V). ArsI degrades MAs(III) to As(III). ArsP confers resistance by efflux. Cells of E. coli expressing arsI, arsH or arsP grow in mixed culture with Burkholderia sp. MR1 in the presence of MAs(V). Thus MAs(III) has antibiotic properties: a toxic organic compound produced by one microbe to kill off competitors. Our results demonstrate that life has adapted to use environmental arsenic as a weapon in the continuing battle for dominance.     

Optimization of arsenic phytoremediation using Eichhornia crassipes

This study aimed to evaluate the pH, phosphate, and nitrate in the process of arsenic absorption by Eichhornia crassipes (water hyacinth), using the surface response methodology, in order to optimize the process. The plants were exposed to a concentration of arsenic of 0.5 mg L^?1 (NaAsO₂) over a period of 10 days. The results indicated optimal levels for the absorption of arsenic by E. crassipes at pH equal to 7.5, absence of phosphate, and minimum nitrate level of 0.0887 mmol L^?1. For the tested concentration, E. crassipes was able to accumulate 498.4 mg kg^?1 of As (dry base) in its plant tissue and to reduce 83% of the initial concentration present in the aqueous medium where it was cultivated. The concentration of phosphorus in solution linearly increased the phosphorus content in the plants and negatively influenced the absorption of arsenic. The concentration of 0.5 mg L^?1 of As did not significantly affect the relative growth rate (RGR) and the tolerance index (TI). 94% of As (III) initially solubilized in water was converted by the end of the experiment period into As (V). The water hyacinth was important in the phytoremediation of arsenic when cultivated under optimal conditions for its removal.     

Conserved cysteine residues determine substrate specificity in a novel As(III) S‐adenosylmethionine methyltransferase from Aspergillus fumigatus

Methylation of inorganic arsenic is a central process in the organoarsenical biogeochemical cycle. Members of every kingdom have ArsM As(III) S‐adenosylmethionine (SAM) methyltransferases that methylates inorganic As(III) into mono‐ (MAs(III)), di‐ (DMAs(III)) and tri‐ (TMAs(III)) methylarsenicals. Every characterized ArsM to date has four conserved cysteine residues. All four cysteines are required for methylation of As(III) to MAs(III), but methylation of MAs(III) to DMAs(III) requires only the two cysteines closest to the C‐terminus. Fungi produce volatile and toxic arsines, but the physiological roles of arsenic methylation and the biochemical basis is unknown. Here they demonstrate that most fungal species have ArsM orthologs with only three conserved cysteine residues. The genome of Aspergillus fumigatus has four arsM genes encoding ArsMs with only the second, third and fourth conserved cysteine residues. AfArsM1 methylates MAs(III) but not As(III). Heterologous expression of AfarsM1 in an Escherichia coli conferred resistance to MAs(III) but not As(III). The existence of ArsMs with only three conserved cysteine residues suggest that the ability to methylate MAs(III) may be an evolutionary step toward enzymes capable of methylating As(III), the result of a loss of function mutation in organisms with infrequent exposure to inorganic As(III) or as a resistance mechanism for MAs(III).     

Arsenic-resistant <Emphasis Type="BoldItalic">Pseudomonas</Emphasis> spp. and <Emphasis Type="BoldItalic">Bacillus</Emphasis> sp. bacterial strains reducing As(V) to As(III), isolated from Alps soils,Italy

Five arsenic-resistant bacterial strains (designated MP1400, MP1400a, MP1400d, APSLA3, and BPSLA3) were isolated from soils collected at the Alps region (Italy), which showed no contamination by arsenic. Phylogenetic analysis of the 16S rRNA gene sequences assigned them to the genera Pseudomonas and Bacillus. Bacillus sp. strain 1400d and Pseudomonas spp. strains APSLA3 and MP1400 showed higher tolerance to As(III), as indicated by minimum inhibitory concentrations of 10 mmol/L. Pseudomonas sp. strain MP1400 exhibited higher tolerance to As(V) (minimum inhibitory concentration of 135 mmol/L). The isolated arsenic-resistant strains were able to reduce As(V) to As(III), especially Pseudomonas sp. strain MP1400 reducing 2 mmol/L of As(V) to As(III) within 24 h. The results suggest that the isolated bacterial strains play a role in the arsenic biogeochemical cycle of arsenic-poor soils in the Alps mount area.     

Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal‐reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.     

<Emphasis Type="Italic">Caenorhabditis elegans gcs-1</Emphasis> Confers Resistance to Arsenic-Induced Oxidative Stress

Gamma-glutamylcysteine synthetase (γ-GCS) catalyzes the first, rate-limiting step in the biosynthesis of glutathione (GSH). To evaluate the protective role of cellular GSH against arsenic-induced oxidative stress in Caenorhabditis elegans (C. elegans), we examined the effect of the C. elegans ortholog of GCS(h), gcs-1, in response to inorganic arsenic exposure. We have evaluated the responses of wild-type and gcs-1 mutant nematodes to both inorganic arsenite (As(III)) and arsenate (As(V)) ions and found that gcs-1 mutant nematodes are more sensitive to arsenic toxicity than that of wild-type animals. The amount of metal ion required to kill half of the population of worms falls in the order of wild-type/As(V)>gcs-1/As(V)> wild-type/As(III)>gcs-1/As(III). gcs-1 mutant nematodes also showed an earlier response to the exposure of As(III) and As(V) than that of wild-type animals. Pretreatment with GSH significantly raised the survival rate of gcs-1 mutant worms compared to As(III)- or As(V)-treated worms alone. These results indicate that GCS-1 is essential for the synthesis of intracellular GSH in C. elegans and consequently that the intracellular GSH status plays a critical role in protection of C. elegans from arsenic-induced oxidative stress.     

Continuous microbial leaching of a pyritic concentrate by Leptospirillum-like bacteria

Summary Continuous leaching of a pyritic flotation concentrate by mixed cultures of acidophilic bacteria was studied in a laboratory scale airlift reactor. Enrichment cultures adapted to the flotation concentrate contained Thiobacillus ferrooxidans and Thiobacillus thiooxidans. During the late stationary growth phase of these thiobacilli growth of Leptospirillum-like bacteria was observed, too. In discontinuous cultivation no significant influence of Leptospirillum-like bacteria on leaching rates could be detected. During continuous leaching at pH 1.5 Leptospirillum-like bacteria displaced Thiobacillus ferrooxidans. The iron leaching rate achieved by Leptospirillum-rich cultures was found to be up to 3.9 times higher than that by Leptospirillum-free cultures.