Volatilisation of metals and metalloids: An inherent feature of methanoarchaea?

As shown by recent studies, anaerobic members of Archaea and Bacteria are involved in processes that transform ionic species of metals and metalloids (arsenic, antimony, bismuth, selenium, tellurium and mercury) into volatile and mostly toxic derivatives (mainly methyl derivatives or hydrides). Since the fact that these transformations proceed in both environmental settings and in parts of the human body, we have to consider that these processes also interfere directly with human health. The diversity of the volatile derivatives produced and their emission rates were significantly higher in methanoarchaeal than in bacterial strains, which supports the pivotal role of methanoarchaea in transforming metals and metalloids (metal(loid)s) into their volatile derivatives. Compared with methanoarchaea, 14 anaerobic bacterial strains showed a significantly restricted spectrum of volatilised derivatives and mostly lower production rates of volatile bismuth and selenium derivatives. Since methanoarchaea isolated from the human gut (Methanosphaera stadtmanae, Methanobrevibacter smithii) showed a higher potential for metal(loid) derivatisation compared to bacterial gut isolates, we assume that methanoarchaea in the human gut are mainly responsible for the production of these volatile derivatives. The observation that trimethylbismuth ((CH(3))(3)Bi), the main volatile derivative of bismuth produced in human feces, inhibited growing cultures of Bacteroides thetaiotaomicron, a representative member of the human physiological gut flora, suggests that these volatiles exert their toxic effects on human health not only by direct interaction with host cells but also by disturbing the physiological gut microflora.     

Microbial formation and transformation of organometallic and organometalloid compounds

Abstract: Microbial formation and transformation of organometallic and organometalloid compounds comprise significant components of biogeochemical cycles for the metals mercury, lead and tin and the metalloids arsenic, selenium, tellurium and germanium. Methylated derivatives of such elements can arise as a result of chemical and biological mechanisms and this frequently results in altered volatility, solubility, toxicity and mobility. The major microbial methylating agents are methylcobalamin (CH₃CoB₁₂), involved in the methylation of mercury, tin and lead, and S -adenosylmethionine (SAM), involved in the methylation of arsenic and selenium. Evidence for the methylation of other toxic metal(loid)s is sparse. Biomethylation may result in metal(loid) detoxification since methylated derivatives may be excreted readily from cells, are often volatile and may be less toxic, e.g. organoarsenicals. However, for mercury, low yields of methylated derivatives and the existence of more efficient resistance mechanisms, e.g. reduction of Hg²⁺ to Hg⁰, suggest a lower significance in detoxification. Bioalkylation has only been characterised in detail for arsenic. Microorganisms can accumulate organometal(loid)s, a phenomenon relevant to toxicant transfer to higher organisms. As well as bioaccumulation, many microorganisms are capable of the degradation and detoxification of organometal(loid) compounds by, e.g. demethylation and dealkylation. Several organometal(loid) transformations have potential for environmental bioremediation.     

Anaerobic Respiration on Tellurate and Other Metalloids in Bacteria from Hydrothermal Vent Fields in the Eastern Pacific Ocean

This paper reports the discovery of anaerobic respiration on tellurate by bacteria isolated from deep ocean (1,543 to 1,791 m) hydrothermal vent worms. The first evidence for selenite- and vanadate-respiring bacteria from deep ocean hydrothermal vents is also presented. Enumeration of the anaerobic metal(loid)-resistant microbial community associated with hydrothermal vent animals indicates that a greater proportion of the bacterial community associated with certain vent fauna resists and reduces metal(loid)s anaerobically than aerobically, suggesting that anaerobic metal(loid) respiration might be an important process in bacteria that are symbiotic with vent fauna. Isolates from Axial Volcano and Explorer Ridge were tested for their ability to reduce tellurate, selenite, metavanadate, or orthovanadate in the absence of alternate electron acceptors. In the presence of metal(loid)s, strains showed an ability to grow and produce ATP, whereas in the absence of metal(loid)s, no growth or ATP production was observed. The protonophore carbonyl cyanide m-chlorophenylhydrazone depressed metal(loid) reduction. Anaerobic tellurate respiration will be a significant component in describing biogeochemical cycling of Te at hydrothermal vents.     

Anaerobic respiration on tellurate and other metalloids in bacteria from hydrothermal vent fields in the eastern Pacific Ocean

This paper reports the discovery of anaerobic respiration on tellurate by bacteria isolated from deep ocean (1,543 to 1,791 m) hydrothermal vent worms. The first evidence for selenite- and vanadate-respiring bacteria from deep ocean hydrothermal vents is also presented. Enumeration of the anaerobic metal(loid)-resistant microbial community associated with hydrothermal vent animals indicates that a greater proportion of the bacterial community associated with certain vent fauna resists and reduces metal(loid)s anaerobically than aerobically, suggesting that anaerobic metal(loid) respiration might be an important process in bacteria that are symbiotic with vent fauna. Isolates from Axial Volcano and Explorer Ridge were tested for their ability to reduce tellurate, selenite, metavanadate, or orthovanadate in the absence of alternate electron acceptors. In the presence of metal(loid)s, strains showed an ability to grow and produce ATP, whereas in the absence of metal(loid)s, no growth or ATP production was observed. The protonophore carbonyl cyanide m-chlorophenylhydrazone depressed metal(loid) reduction. Anaerobic tellurate respiration will be a significant component in describing biogeochemical cycling of Te at hydrothermal vents.     

Ion Exchange Properties of the Root Cell Walls Isolated from the Halophyte Plants (Suaeda altissima L.) Grown Under Conditions of Different Salinity

Metal and metalloid resistances in plant species and genotypes/accessions are becoming increasingly better understood at the molecular and physiological level. Much of the recent focus into metal resistances has been on hyperaccumulators as these are excellent systems to study resistances due to their very abnormal metal(loid) physiology and because of their biotechnological potential. Advances into the mechanistic basis of metal(loid) resistances have been made through the investigation of metal(loid) transporters, the construction of mutants with altered metal(loid) transport and metabolism, a better understanding of the genetic basis of resistance and hyperaccumulation and investigations into the role of metal(loid) ion chelators. This review highlights these recent advances.     

Influence of arsenic on antimony methylation by the aerobic yeast<Emphasis Type="Italic"> Cryptococcus humicolus</Emphasis>

The anamorphic basidomycetous yeast Cryptococcus humicolus was shown by hydride generation-gas chromatography-atomic absorption spectrometry to methylate inorganic antimony compounds to mono-, di-, and trimethylantimony species under oxic growth conditions. Methylantimony levels were positively correlated with initial substrate concentrations up to 300 mg Sb l^–1 as potassium antimony tartrate (K-Sb-tartrate). Increasing concentrations of K-Sb-tartrate increased the ratio of di- to trimethylantimony species, indicating that methylation of dimethylantimony was rate limiting. Antimony methylation capability in C. humicolus was developed after the exponential growth phase and was dependent upon protein synthesis in the early stationary phase. Inclusion of inorganic arsenic (III) or (V) species alongside antimony in culture incubations enhanced antimony methylation. Pre-incubation of cells with inorganic arsenic (III) further induced antimony methylation capability, whereas pre-incubation with inorganic antimony (III) did not. Exposure of cells to inorganic arsenic—either through pre-incubation or provision during cultivation—influenced the antimony speciation; involatile trimethylantimony species was the sole methylated antimony species detected, i.e. mono- and dimethylantimony species were not detected. Competitive inhibition of antimony methylation was observed at high arsenic loadings. These data indicate that antimony methylation is a fortuitous process, catalysed at least in part by enzymes responsible for arsenic methylation.     

Complete genome sequence of Desulfurispirillum indicum strain S5(T)

Desulfurispirillum indicum strain S5(T) is a strictly anaerobic bacterium isolated from river sediment in Chennai, India. D. indicum belongs to the deep branching phylum of Chrysiogenetes, which currently only includes three other cultured species. Strain S5(T) is the type strain of the species and it is capable of growth using selenate, selenite, arsenate, nitrate or nitrite as terminal electron acceptors. The 2,928,377 bp genome encodes 2,619 proteins and 49 RNA genes, and the information gained from its sequence will be relevant to the elucidation of microbially-mediated transformations of arsenic and selenium, in addition to deepening our knowledge of the underrepresented phylum of Chrysiogenetes.     

Prediction of heavy-metal concentrations in mature plants by chemical analysis of seedlings

Five plant species were cultivated on a soil from the Neckar alluvial fan near Heidelberg (FRG) polluted by the emissions of a cement plant. Thallium, cadmium and lead concentrations in seedlings and mature plants were determined by atomic absorption analysis. AdditionallyBrassica napus L.napus was grown on soils containing 5 different concentrations of heavy metals, achieved by mixing two similar soils, from the same area but with different metal concentrations. Thallium and cadmium were shown to be taken up by roots whilst lead which was also absorbed, was deposited mainly on the plant surface. However during cultivation in the winter months, a remarkable deposit of lead via the roots was found. Thallium in the soil from a anthrorogen source was more available to plants than thallium of geological origin. During the lifetime of a plant concentrations of thallium and cadmium were always highest in the seedling. The decrease in metal concentration with maturity depended on the plant species and the element, but was not a function of the metal concentration in the soil.     

Contamination of Floodplain Soils along the Wupper River,Germany, with As,Co, Cu,Ni, Sb,and Zn and the Impact of Pre-definite Redox Variations on the Mobility of These Elements

This study demonstrates that floodplain soils of the River Wupper, Germany, are seriously contaminated with metal(loid)s. We used an automated biogeochemical microcosm system allowing controlled variation of redox potential (E_H) to assess the impact of pre-definite redox conditions on the dynamics of arsenic (As), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), antimony (Sb), and zinc (Zn) in homogenized soil material taken from an acid floodplain soil. The concentrations of Co, Cu, Mn, Ni, Sb, and Zn in soil solution were low at low E_H, possibly due to the precipitation of metal sulfides, and increased with rising E_H, presumably caused by their association with dissolved organic carbon (DOC). A significant positive correlation between metal/DOC-ratio and E_H indicated that the binding of the metals to DOC shifted from stronger to weaker when E_H rose. Decreasing As concentrations with increasing E_H in soil solution indicated co-precipitation with Fe(hydr)oxides and/or oxidation of more soluble As(III) to less soluble As(V) during oxidation. The other studied elements seemed not to co-precipitate with newly formed Fe(hydr)oxides when E_H rose, possibly due to the prevailing low pH. In the future, the specific role of DOC and sulfur chemistry on metal(loid) dynamics should be elucidated more fully, and similar studies should be conducted with additional frequently flooded soils worldwide to verify these results.     

Azospirillum affects Eh and potential denitrification in a soil

The effect of inoculation withAzospirillum brasilense (strain 7001) on soil Eh under anaerobic conditions (N₂ flux) was examined during 144 h at 26°C and 230 h at 18°C. The Eh values of the control (not inoculated) soil decreased to approximately 80 or 140 mV in both cases, whereas after 24 h of anaerobic incubation, the Eh of theAzospirillum-inoculated soil remained at higher values. After 144 and 230 h of anaerobic incubation, the denitrifying activity (measured in anaerobiosis with excess of e^- acceptor and donor) in the inoculated soil was seven and three times lower respectively, than in the non-inoculated soil. This indicates thatAzospirillum may affect the soil Eh and consequently any highly Eh-dependent microbial activity, such as denitrification.     

Draft genome sequence of plant growth-promoting rhizobium Mesorhizobium amorphae, isolated from zinc-lead mine tailings

Here, we describe the draft genome sequence of Mesorhizobium amorphae strain CCNWGS0123, isolated from nodules of Robinia pseudoacacia growing on zinc-lead mine tailings. A large number of metal(loid) resistance genes, as well as genes reported to promote plant growth, were identified, presenting a great future potential for aiding phytoremediation in metal(loid)-contaminated soil.     

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.     

Heavy Metal/Metalloid Indexing and Balances in Agricultural Soils: Methodological Approach for Research

Heavy metal(loid) accumulation in agricultural soils is a threat to the soil capacity, quality, and productivity. It also increases human exposure to heavy metal(loid)s via consumption of contaminated plant-based foods. The detrimental effects of soil contamination also deteriorate the environment of plants and animals. For sustainable agriculture, therefore, the soil must be protected from toxic levels of heavy metal(loid)s. Studies on heavy metal(loid) balances in agricultural soils are important in predicting future risks to sustainable production from agro-ecological zones and human exposure to heavy metal(loid)s. The latest and continuous indexing of the problem seems a prerequisite for sustainable agriculture. This review provides some background information and then summarizes key methodological approaches for studies on indexing and balance of heavy metal(loid)s in agricultural soils. In the end, important soil and health indices are explained that may be useful in understanding the extent of the problem. The provided information would contribute to sustainable heavy metal(loid) management in the agricultural soils, high crop production, better soil protection, and ultimately to human health.     

Bacterial respiration of arsenic and selenium

Oxyanions of arsenic and selenium can be used in microbial anaerobic respiration as terminal electron acceptors. The detection of arsenate and selenate respiring bacteria in numerous pristine and contaminated environments and their rapid appearance in enrichment culture suggest that they are widespread and metabolically active in nature. Although the bacterial species that have been isolated and characterized are still few in number, they are scattered throughout the bacterial domain and include Gram-positive bacteria, beta, gamma and epsilon Proteobacteria and the sole member of a deeply branching lineage of the bacteria, Chrysiogenes arsenatus. The oxidation of a number of organic substrates (i.e. acetate, lactate, pyruvate, glycerol, ethanol) or hydrogen can be coupled to the reduction of arsenate and selenate, but the actual donor used varies from species to species. Both periplasmic and membrane-associated arsenate and selenate reductases have been characterized. Although the number of subunits and molecular masses differs, they all contain molybdenum. The extent of the environmental impact on the transformation and mobilization of arsenic and selenium by microbial dissimilatory processes is only now being fully appreciated.     

Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance. The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils. If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.     

柽柳根际一株盐单胞菌Bachu 26的分离、鉴定及其盐胁迫下的促生作用研究

盐渍化是世界性的土壤问题,植物促生根际细菌(plant growth-promoting rhizobacteria,PGPR)在盐碱地改良和促进植物生长方面具有独特优势。柽柳是典型的盐生植物,筛选其根际微生物并研究其促生效果与促生机制,以此开发微生物菌肥,具有重要的应用价值。【目的】筛选耐盐碱植物柽柳的根际微生物,对其基本特性、耐盐碱能力、促生功能及促生效果进行评估。【方法】从新疆巴楚境内野生柽柳根际土壤中筛选出一株耐盐碱细菌菌株Bachu 26;通过形态学观察、生理生化特性测定和16S rRNA基因序列分析,对该菌株进行鉴定;利用不同盐浓度(0%–20%)和不同pH(7.0–13.0),对菌株Bachu 26的耐盐耐碱能力进行测定;采用多种功能鉴定培养基测定其促生功能,并对生长素吲哚乙酸(indole-3-acetic acid, IAA)进行定量测定;通过二分格培养皿实验验证菌株产生挥发性酸性物质的能力;在普通培养皿上将拟南芥幼苗与菌株Bachu 26共培养,分析菌株对拟南芥幼苗的促生作用;在二分格培养皿上将拟南芥与Bachu 26隔离培养,分析菌株产生的挥发性酸性物质对拟南芥幼...     

Analysis of the Anaerobic Microbial Community Capable of Degrading p-Toluene Sulfonate

Three strains of Clostridium sp., 14 (VKM B-2201), 42 (VKM B-2202), and 21 (VKM B-2279), two methanogens, Methanobacterium formicicum MH (VKM B-2198) and Methanosarcina mazei MM (VKM B-2199), and one sulfate-reducing bacterium, Desulfovibrio sp. SR1 (VKM B-2200), were isolated in pure cultures from an anaerobic microbial community capable of degrading p-toluene sulfonate. Strain 14 was able to degrade p-toluene sulfonate in the presence of yeast extract and bactotryptone and, like strain 42, to utilize p-toluene sulfonate as the sole sulfur source with the production of toluene. p-Toluene sulfonate stimulated the growth of Ms. mazei MM on acetate. The sulfate-reducing strain Desulfovibrio sp. SR1 utilized p-toluene sulfonate as an electron acceptor. The putative scheme of p-toluene sulfonate degradation by the anaerobic microbial community is discussed.     

Microbial Methylation of Metalloids: Arsenic, Antimony, and Bismuth

A significant 19th century public health problem was that the inhabitants of many houses containing wallpaper decorated with green arsenical pigments experienced illness and death. The problem was caused by certain fungi that grew in the presence of inorganic arsenic to form a toxic, garlic-odored gas. The garlic odor was actually put to use in a very delicate microbiological test for arsenic. In 1933, the gas was shown to be trimethylarsine. It was not until 1971 that arsenic methylation by bacteria was demonstrated. Further research in biomethylation has been facilitated by the development of delicate techniques for the determination of arsenic species. As described in this review, many microorganisms (bacteria, fungi, and yeasts) and animals are now known to biomethylate arsenic, forming both volatile (e.g., methylarsines) and nonvolatile (e.g., methylarsonic acid and dimethylarsinic acid) compounds. The enzymatic mechanisms for this biomethylation are discussed. The microbial conversion of sodium arsenate to trimethylarsine proceeds by alternate reduction and methylation steps, with S-adenosylmethionine as the usual methyl donor. Thiols have important roles in the reductions. In anaerobic bacteria, methylcobalamin may be the donor. The other metalloid elements of the periodic table group 15, antimony and bismuth, also undergo biomethylation to some extent. Trimethylstibine formation by microorganisms is now well established, but this process apparently does not occur in animals. Formation of trimethylbismuth by microorganisms has been reported in a few cases. Microbial methylation plays important roles in the biogeochemical cycling of these metalloid elements and possibly in their detoxification. The wheel has come full circle, and public health considerations are again important.     

Microbial methylation of metalloids: arsenic, antimony, and bismuth.

A significant 19th century public health problem was that the inhabitants of many houses containing wallpaper decorated with green arsenical pigments experienced illness and death. The problem was caused by certain fungi that grew in the presence of inorganic arsenic to form a toxic, garlic-odored gas. The garlic odor was actually put to use in a very delicate microbiological test for arsenic. In 1933, the gas was shown to be trimethylarsine. It was not until 1971 that arsenic methylation by bacteria was demonstrated. Further research in biomethylation has been facilitated by the development of delicate techniques for the determination of arsenic species. As described in this review, many microorganisms (bacteria, fungi, and yeasts) and animals are now known to biomethylate arsenic, forming both volatile (e.g., methylarsines) and nonvolatile (e.g., methylarsonic acid and dimethylarsinic acid) compounds. The enzymatic mechanisms for this biomethylation are discussed. The microbial conversion of sodium arsenate to trimethylarsine proceeds by alternate reduction and methylation steps, with S-adenosylmethionine as the usual methyl donor. Thiols have important roles in the reductions. In anaerobic bacteria, methylcobalamin may be the donor. The other metalloid elements of the periodic table group 15, antimony and bismuth, also undergo biomethylation to some extent. Trimethylstibine formation by microorganisms is now well established, but this process apparently does not occur in animals. Formation of trimethylbismuth by microorganisms has been reported in a few cases. Microbial methylation plays important roles in the biogeochemical cycling of these metalloid elements and possibly in their detoxification. The wheel has come full circle, and public health considerations are again important.