Flexible bacterial strains that oxidize arsenite in anoxic or aerobic conditions and utilize hydrogen or acetate as alternative electron donors

Arsenic is a carcinogenic compound widely distributed in the groundwater around the world. The fate of arsenic in groundwater depends on the activity of microorganisms either by oxidizing arsenite (As^III), or by reducing arsenate (As^V). Because of the higher toxicity and mobility of As^III compared to As^V, microbial-catalyzed oxidation of As^III to As^V can lower the environmental impact of arsenic. Although aerobic As^III-oxidizing bacteria are well known, anoxic oxidation of As^III with nitrate as electron acceptor has also been shown to occur. In this study, three As^III-oxidizing bacterial strains, Azoarcus sp. strain EC1-pb1, Azoarcus sp. strain EC3-pb1 and Diaphorobacter sp. strain MC-pb1, have been characterized. Each strain was tested for its ability to oxidize As^III with four different electron acceptors, nitrate, nitrite, chlorate and oxygen. Complete As^III oxidation was achieved with both nitrate and oxygen, demonstrating the novel ability of these bacterial strains to oxidize As^III in either anoxic or aerobic conditions. Nitrate was only reduced to nitrite. Different electron donors were used to study their suitability in supporting nitrate reduction. Hydrogen and acetate were readily utilized by all the cultures. The flexibility of these As^III-oxidizing bacteria to use oxygen and nitrate to oxidize As^III as well as organic and inorganic substrates as alternative electron donors explains their presence in non-arsenic-contaminated environments. The findings suggest that at least some As^III-oxidizing bacteria are flexible with respect to electron-acceptors and electron-donors and that they are potentially widespread in low arsenic concentration environments.     

Fate of arsenate following arsenite oxidation in Agrobacterium tumefaciens GW4

The fate of arsenate (As^V) generated by microbial arsenite (As^III) oxidation is poorly understood. Agrobacterium tumefaciens wild‐type strain (GW4) was studied to determine how the cell copes with As^V generated in batch culture. GW4 grown heterotrophically with mannitol used As^III as a supplemental energy supply as reflected by enhanced growth and increased cellular levels of NADH and ATP. Under low phosphate (Pi) conditions and presence of As^III oxidation, up to ～ 50% of the resulting As^V was taken up and found associated with the periplasm, membrane or cytoplasm fractions of the cells. Arsenic was found associated with proteins and polar lipids, but not in nucleic acids or sugars. Thin‐layer chromatography and gas chromatography–mass spectrometry analysis suggested the presence of arsenolipids in membranes, presumably as part of the bilayer structure of the cell membrane and replacing Pi under Pi‐limiting conditions. The potential role of a Pi‐binding protein (PstS) for As^V uptake was assessed with the His‐tag purified protein. Intrinsic tryptophan fluorescence spectra analysis suggests that PstS can bind As^V, but with lower affinity as compared with Pi. In early stationary phase cells, the As^V : Pi ratio was approximately 4.3 and accompanied by an altered cell ultrastructure.     

Phosphate starvation response controls genes required to synthesize the phosphate analog arsenate

Environmental arsenic poisoning affects roughly 200 million people worldwide. The toxicity and mobility of arsenic in the environment is significantly influenced by microbial redox reactions, with arsenite (As^III) being more toxic than arsenate (As^V). Microbial oxidation of As^III to As^V is known to be regulated by the AioXSR signal transduction system and viewed to function for detoxification or energy generation. Here, we show that As^III oxidation is ultimately regulated by the phosphate starvation response (PSR), requiring the sensor kinase PhoR for expression of the As^III oxidase structural genes aioBA. The PhoRB and AioSR signal transduction systems are capable of transphosphorylation cross‐talk, closely integrating As^III oxidation with the PSR. Further, under PSR conditions, As^V significantly extends bacterial growth and accumulates in the lipid fraction to the apparent exclusion of phosphorus. This could spare phosphorus for nucleic acid synthesis or triphosphate metabolism wherein unstable arsenic esters are not tolerated, thereby enhancing cell survival potential. We conclude that As^III oxidation is logically part of the bacterial PSR, enabling the synthesis of the phosphate analog As^V to replace phosphorus in specific biomolecules or to synthesize other molecules capable of a similar function, although not for total replacement of cellular phosphate.     

Pathways of glucose catabolism and the origin and metabolism of pyruvate during calcium-induced conidiation ofPenicillium notatum

Experiments examined the metabolic basis of Ca²⁺-induced conidiation during the 12-h period following the addition of Ca²⁺ to 40-h vegetative cultures ofPenicillium notatum. Vegetative mycelium had enzymic capacity for three routes of glucose catabolism viz. the Embden-Meyerhof-Parnas (EMP), pentose phosphate (PP) and the Entner-Doudoroff (ED) sequences. Inhibitors of EMP enzymes restricted vegetative growth more than that associated with conidiation whilst arsenate augmented the limited capacity of lower levels of Ca²⁺ to promote conidiation. Arsenite (5.6 mmol · 1^–1) partially blocked the metabolism of pyruvate and caused its accumulation, which was also promoted by Ca²⁺ alone. Arsenite did not induce conidiation in vegetative cultures but when combined with Ca²⁺ it enhanced conidiation. Radiorespirometry and the analysis of accumulated pyruvate, promoted by arsenite, indicated that approximately 54% of carbon was catabolized via combined EMP/ED routes and 46% by the PP pathway and subsequently via a weakly functional TCA cycle. Calcium-induced cultures swung to a primarily ED (25%) and PP (75%) based catabolism with low substrate level phosphorylation, including a facility for a non-phosphorylative ED route, and further diminished oxidative TCA capacity. Pyruvate accumulation in Ca²⁺-induced cultures coincided with the decline in activity of pyruvate dehydrogenase and a reduced capacity for gluconeogenesis, with other enzymes of pyruvate metabolism showing altered activities. These changes in enzyme activities, pyruvate accumulation and its subsequent metabolism were related to growth rate and the developmental cycle, and are discussed in conjunction with the regulatory role of calcium.     

Non-enzymatic roles for the URE2 glutathione S-transferase in the response of Saccharomyces cerevisiae to arsenic

The response of Saccharomyces cerevisiae to arsenic involves a large ensemble of genes, many of which are associated with glutathione-related metabolism. The role of the glutathione S-transferase (GST) product of the URE2 gene involved in resistance of S. cerevisiae to a broad range of heavy metals was investigated. Glutathione peroxidase activity, previously reported for the Ure2p protein, was unaffected in cell-free extracts of an ure2Δ mutant of S. cerevisiae. Glutathione levels in the ure2Δ mutant were lowered about threefold compared to the isogenic wild-type strain but, as in the wild-type strain, increased 2–2.5-fold upon addition of either arsenate (As^V) or arsenite (As^III). However, lack of URE2 specifically caused sensitivity to arsenite but not to arsenate. The protective role of URE2 against arsenite depended solely on the GST-encoding 3′-end portion of the gene. The nitrogen source used for growth was suggested to be an important determinant of arsenite toxicity, in keeping with non-enzymatic roles of the URE2 gene product in GATA-type regulation.     

Mechanisms of arsenic hyperaccumulation in<Emphasis Type="Italic"> Pteris</Emphasis> species: root As influx and translocation

Several species of fern from the Pteris genus are able to accumulate extremely high concentrations of arsenic (As) in the fronds. We have conducted short-term unidirectional As influx and translocation experiments with ⁷³As-radiolabeled arsenate, and found that the concentration-dependent influx of arsenate into roots was significantly larger in two of these As-hyperaccumulating species, Pteris vittata (L.) and Pteris cretica cv. Mayii (L.), than in Nephrolepis exaltata (L.), a non-accumulating fern. The arsenate influx could be described by Michaelis-Menten kinetics and the kinetic parameter K _m was found to be lower in the Pteris species, indicating higher affinity of the transport protein for arsenate. Quantitative analysis of kinetic parameters showed that phosphate inhibited arsenate influx in a directly competitive manner, consistent with the hypothesis that arsenate enters plant roots on a phosphate-transport protein. The significantly augmented translocation of arsenic to the shoots that was seen in these As hyperaccumulator species is proposed to be due to a combination of the increased root influx and also decreased sequestration of As in the roots, as a larger fraction of As could be extracted from roots of the Pteris species than from roots of N. exaltata. This leaves a larger pool of mobile As available for translocation to the shoot, probably predominantly as arsenite.Abbreviations As ^V Arsenate - As ^III Arsenite - K _m Michaelis-Menten constant - P _i Phosphate - V _max Maximum rate of an enzyme-catalyzed reaction     

Concomitant Induction of Heme Oxygenase‐1 Attenuates the Cytotoxicity of Arsenic Species from Lumbricus Extract in Human Liver HepG2 Cells

Heme oxygenase‐1 (HO‐1) is an inducible antioxidant enzyme that degrades heme to three products, biliverdin, carbon monoxide (CO), and iron ion. The present study was originally designed to characterize the HO‐1 induction by Lumbricus extract as a potential cytoprotective mechanism. Through bioactivity‐guided fractionation, with human HepG2 cells as the cellular detector, surprisingly, we found that arsenic was enriched in the active fractions isolated from Lumbricus extract. Arsenic speciation was further carried out by liquid chromatography with inductively coupled plasma mass spectrometry (LC/ICP‐MS). Our results showed that Lumbricus extract contained two major arsenic species, arsenite (As^III; 53.7%) and arsenate (As^V; 34.2%), and six minor arsenic species. Commercial sodium arsenite (NaAsO₂) was used to verify the effects of Lumbricus extract on HO‐1 expression and related intracellular signaling pathways. Both p38 MAP kinase and NF‐E2‐related factor 2 (Nrf2) pathways were found to modulate HO‐1 induction by Lumbricus extract and NaAsO₂. The cytotoxicity of arsenite was augmented by p38 MAP kinase inhibitor SB202190 and HO‐1 inhibitor tin protoporphyrin IX (SnPP), whereas p38 MAP kinase inhibitor SB202190 also inhibited HO‐1 induction by NaAsO₂. These results suggest that arsenic‐containing compounds are responsible for HO‐1 induction by Lumbricus extract. Although the exact role of toxic arsenic compounds in the treatment of oxidative injury remains unclear, concomitant HO‐1 induction may be a key mechanism to antagonize the cytotoxicity of arsenic compounds in human cells.     

Arsenite as an Inducer of Lipid Peroxidation in Saccharomyces cerevisiae Cells

The ability of sodium arsenite at concentrations of 10^–2, 10^–4, and 10^–6 M to induce lipid peroxidation in Saccharomyces cerevisiae cells was studied. Arsenite at the concentrations 10^–2 and 10^–4 M enhanced lipid peroxidation and inhibited the growth of yeast cells. Enhanced lipid peroxidation likely induced oxidative damage to various cellular structures, which led to suppression of the metabolic activity of cells. Arsenite at the concentration 10^–6 M did not activate lipid peroxidation in cells. All of the tested arsenite concentrations inhibited the activity of -ketoglutarate dehydrogenase and pyruvate dehydrogenase in cells. The inference is made that the toxicity of arsenite may be related to its stimulating effect on intracellular lipid peroxidation.     

Evaluation of amino acid profile in contrasting arsenic accumulating rice genotypes under arsenic stress

Amino acids (AAs) play significant roles in metal binding, antioxidant defense, and signaling in plants during heavy metal stress. In the present study, the essential amino acids (EAAs), non-essential amino acids (NEAAs), as well as the enzymes of proline and cysteine biosynthetic pathways were studied in contrasting arsenic accumulating rice genotypes grown in hydroponic solutions with addition of arsenate (As^V) or arsenite (As^III). Under a mild As stress, the total AAs content significantly increased in both the rice genotypes with a greater increase in a low As accumulating rice genotype (LAARG; IET-19226) than in a high As accumulating rice genotype (HAARG; BRG-12). At the equimolar concentration (10 μM), As^III had a greater effect on EAAs than As^V. Conversely, As^V was more effective in inducing a proline accumulation than As^III. Among NEAAs, As significantly induced the accumulation of histidine, aspartic acid, and serine. In contrast, a higher As concentration (50 μM) reduced the content of most AAs, the effect being more prominent during As^III exposure. The inhibition of glutamate kinase activity was noticed in HAARG, conversely, serine acetyltransferase and cysteine synthase activities were increased which was positively correlated with the cysteine synthesis.     

Genomic Responses to Arsenic in the Cyanobacterium Synechocystis sp. PCC 6803

Arsenic is a ubiquitous contaminant and a toxic metalloid which presents two main redox states in nature: arsenite [As^III] and arsenate [As^V]. Arsenic resistance in Synechocystis sp. strain PCC 6803 is mediated by the arsBHC operon and two additional arsenate reductases encoded by the arsI1 and arsI2 genes. Here we describe the genome-wide responses to the presence of arsenate and arsenite in wild type and mutants in the arsenic resistance system. Both forms of arsenic produced similar responses in the wild type strain, including induction of several stress related genes and repression of energy generation processes. These responses were transient in the wild type strain but maintained in time in an arsB mutant strain, which lacks the arsenite transporter. In contrast, the responses observed in a strain lacking all arsenate reductases were somewhat different and included lower induction of genes involved in metal homeostasis and Fe-S cluster biogenesis, suggesting that these two processes are targeted by arsenite in the wild type strain. Finally, analysis of the arsR mutant strain revealed that ArsR seems to only control 5 genes in the genome. Furthermore, the arsR mutant strain exhibited hypersentivity to nickel, copper and cadmium and this phenotype was suppressed by mutation in arsB but not in arsC gene suggesting that overexpression of arsB is detrimental in the presence of these metals in the media.     

Speciation of Arsenic under Dynamic Conditions

In periodically flooded soils, reductive conditions can occur, which favor the dissolution of Fe (hydr)oxides. Fe (hydr)oxides such as goethite are important sorbents for arsenate (As^V), which is the dominant As species in soils under aerobic conditions. Hence, the dissolution of Fe (hydr)oxides under reductive conditions can result in the mobilization and reduction of As^V and, thus, in an increase in the bioavailability of arsenic. The temporal dynamics of these processes and possible re‐sorption or precipitation of arsenite (As^III) formed are poorly understood. Under controlled laboratory conditions, the temporal change in the redox potential and arsenic speciation with time after a simulated flooding event in a quartz‐goethite organic matter substrate, spiked with As^V, was examined. During a period of 6 weeks, substrate solutions were sampled weekly using micro‐suction cups and analyzed for pH, As^III and As^V, Fe, Mn and P concentrations. Redox potentials and matric potentials were determined in situ in the substrate‐bearing cylinders. The redox potential and the ratio between As^III and As^V concentrations remained unchanged during the experiment without organic matter application. With organic matter applied, the redox potential decreased and the As^III concentrations in the substrate solution increased while the total As concentrations in the substrate solution strongly decreased. An addition of goethite (1 g/kg) per se led to a decrease of the total As in the substrate solution (almost 50 %). In respect to the potential As availability for plants, and consequently, the transfer into the food chain, the results are difficult to evaluate. The lower the total As concentrations in the substrate solution, determined with decreasing redox potential, the least plant As uptake will occur. This effect may however be compensated by a shift of the molar P/As^V ratio in the solution in favor of As^V which is expected to increase the As uptake.     

Diversity of bacterial communities inhabiting soil and groundwater of arsenic contaminated areas in West Bengal,India

Soil and water contaminated with arsenic (As) through natural or anthropogenic inputs are commonly considered as native source of tolerant bacterial strains. The present study was successful in characterizing 12 hyper-tolerant bacteria, satisfying maximum tolerable concentration (MTC) for arsenate (As⁵⁺) ≥ 300 mM and arsenite (As³⁺) ≥ 30 mM, isolated from As affected North 24 Parganas and South 24 Parganas districts of West Bengal, India. Most of the bacteria showing higher level of tolerance to As⁵⁺ and As³⁺ were found as gram-positive and bacilli in shape. Positive responses to different biochemical tests indicated that some of these bacteria could be potent sources of various biotechnologically important enzymes. Some of the hyper-tolerant bacteria could reduce As⁵⁺ to As³⁺ while all others could oxidise As³⁺ to As⁵⁺. Phylogenetic analysis revealed that those hyper-tolerant bacterial strains were distributed among three phyla such as Actinobacteria, Firmicutes, and γ-Proteobacteria. The Firmicutes were well represented in this study with more than half of the hyper-tolerant strains corresponding to members of this group. Moreover, majority of the isolates except SR10 belonging to this phylum were affiliated to different species of the genus Bacillus and showed different tolerance capability to As³⁺ and As⁵⁺. We present the first report of the genus Paenibacillus as being involved in arsenite oxidation with hyper-tolerance property to As. Four isolates named as SDe5, SDe12, SDe13, and SDe15 belonging to genera Bacillus and Rhodococcus exhibited highest tolerance to As and therefore represented as good candidates for bioremediation processes of native polluted soil and ground water.     

Relative toxicity of arsenite and arsenate on germination and early seedling growth of rice (Oryza sativa L.)

Elevated soil arsenic levels resulting from long-term use of arsenic contaminated ground for irrigation in Bangladesh may inhibit seed germination and seedling establishment of rice, the country's main food crop. A germination study on rice seeds and a short-term toxicity experiment with different concentrations of arsenite and arsenate on rice seedlings were conducted. Percent germination over control decreased significantly with increasing concentrations of arsenite and arsenate. Arsenite was found to be more toxic than arsenate for rice seed germination. There were varietal differences among the test varieties in response to arsenite and arsenate exposure. The performance of the dry season variety Purbachi was the best among the varieties. Germination of Purbachi was not inhibited at all up to 4 mg l^–1 arsenite and 8 mg l^–1 arsenate treatment. Root tolerance index (RTI) and relative shoot height (RSH) for rice seedlings decreased with increasing concentrations of arsenite and arsenate. Reduction of RTI caused by arsenate was higher than that of arsenite. In general, dry season varieties have more tolerance to arsenite or arsenate than the wet season varieties.     

Arsenite oxidizing multiple metal resistant bacteria isolated from industrial effluent: their potential use in wastewater treatment

Arsenite oxidizing bacteria, isolated from industrial wastewater, showed high resistance against arsenite (40 mM) and other heavy metals (10 mM Pb; 8 mM Cd; 6 mM Cr; 10 mM Cu and 26.6 mM As⁵⁺). Bacterial isolates were characterized, on the basis of morphological, biochemical and 16S rRNA ribotyping, as Bacillus cereus (1.1S) and Acinetobacter junii (1.3S). The optimum temperature and pH for the growth of both strains were found to be 37 °C and 7. Both the strains showed maximum growth after 24 h of incubation. The predominant form of arsenite oxidase was extracellular in B. cereus while in A. junii both types of activities, intracellular and extracellular, were found_. The extracellular aresenite oxidase activity was found to be 730 and 750 µM/m for B. cereus and A. junii, respectively. The arsenite oxidase from both bacterial strains showed maximum activity at 37 °C, pH 7 and enhanced in the presence of Zn²⁺. The presence of two protein bands with molecular weight of approximately 70 and 14 kDa in the presence of arsenic points out a possible role in arsenite oxidation. Arsenite oxidation potential of B. cereus and A. junii was determined up to 92 and 88 % in industrial wastewater after 6 days of incubation. The bacterial treated wastewater improved the growth of Vigna radiata as compared to the untreated wastewater. It indicates that these bacterial strains may find some potential applications in wastewater treatment systems to transform toxic arsenite into less toxic form, arsenate.     

The metabolic acclimation of Arabidopsis thaliana to arsenate is sensitized by the loss of mitochondrial LIPOAMIDE DEHYDROGENASE2, a key enzyme in oxidative metabolism

Mitochondrial lipoamide dehydrogenase is essential for the activity of four mitochondrial enzyme complexes central to oxidative metabolism. The reduction in protein amount and enzyme activity caused by disruption of mitochondrial LIPOAMIDE DEHYDROGENASE2 enhanced the arsenic sensitivity of Arabidopsis thaliana. Both arsenate and arsenite inhibited root elongation, decreased seedling size and increased anthocyanin production more profoundly in knockout mutants than in wild‐type seedlings. Arsenate also stimulated lateral root formation in the mutants. The activity of lipoamide dehydrogenase in isolated mitochondria was sensitive to arsenite, but not arsenate, indicating that arsenite could be the mediator of the observed phenotypes. Steady‐state metabolite abundances were only mildly affected by mutation of mitochondrial LIPOAMIDE DEHYDROGENASE2. In contrast, arsenate induced the remodelling of metabolite pools associated with oxidative metabolism in wild‐type seedlings, an effect that was enhanced in the mutant, especially around the enzyme complexes containing mitochondrial lipoamide dehydrogenase. These results indicate that mitochondrial lipoamide dehydrogenase is an important protein for determining the sensitivity of oxidative metabolism to arsenate in Arabidopsis.     

Arsenate, arsenite and dimethyl arsinic acid (DMA) uptake and tolerance in maize (Zea mays L.)

The influx of arsenate, arsenite and dimethyl arsinic acid (DMA) were studied in 7-day-old excised maize roots (Zea mays L.), and then related to arsenate, arsenite and DMA toxicity. Arsenate, arsenite and DMA influx was all found concentration dependent with significant genotypic differences for arsenite and DMA. Arsenate influx in phosphate starved plants best fitted the four-parameter Michaelis–Menten model corresponding to an additive high and low affinity uptake system, while the uptake of phosphate replete plants followed the two parameter model of Michaelis–Menten kinetics. Arsenite influx was well described by the two parameter model of ‘Michaelis–Menten’ kinetics. DMA influx was comprised of linear phase and a hyperbolic phase. DMA influx was much lower than that for arsenite and arsenate. Arsenate and DMA influx decreased when phosphate was given as a pre-treatment as opposed to phosphate starved plants. The +P treatment tended to decrease influx by 50% for arsenate while this figure was 90% for DMA. Arsenite influx increasing slightly at higher arsenite concentrations in P starved plants but at lower arsenite concentrations, there was little or no difference in arsenite uptake. Low toxicity was found for DMA on maize compared with arsenate and arsenite and the relative toxicity of arsenic species was As(V) > As(III) >> DMA.     

A novel pathway of arsenate detoxification

Microorganisms have evolved various mechanisms to detoxify arsenic, an ubiquitous environmental toxin. Known mechanisms include arsenite efflux, arsenate reduction followed by arsenite efflux and arsenite methylation. In this issue, Chen et al. describe a novel mechanism for arsenate detoxification via synergistic interaction of glyceraldehyde‐3‐phosphate dehydrogenase (GAPDH) and a major facilitator superfamily protein (ArsJ). They propose that GAPDH catalyzes the formation of 1‐arseno‐3‐phosphoglycerate, which is then extruded out of the cell by ArsJ. The significance of this pathway and questions for further research are discussed.     

Bacterial Community Structure and Functional arrA Gene Diversity Associated with Arsenic Reduction and Release in an Industrially Contaminated Soil

This study aimed at evaluating potential arsenic (As) mobility in an industrially contaminated soil (64 mg/kg of As) of the Meuse River basin, and at identifying key bacterial groups that drive soil As dynamics. Both speciation and release of As from this soil was followed under anaerobic conditions using a laboratory batch experiment. In the presence of exogenous carbon sources, As^V initially present in the soil matrix and/or adsorbed on synthetic hydrous ferric oxides were solubilized and mainly reduced to As^III by indigenous soil microflora. After a 1-month incubation period in these biotic conditions, As^III accounted for 80–85% of the total dissolved As and more than 60% of the solid-phase As. Bacterial community structure (i.e., 16S rDNA-based capillary electrophoresis single-strand conformation polymorphism profiles) changed with incubation time and As amendment. The detection of distantly related arsenate respiratory reductase genes (arrA), as functional markers of As^V respirers, indicates that novel dissimilatory As^V-reducing bacteria may be involved in As biotransformation and mobility in anoxic soils. Since As and iron were concomitantly released, a crucial role of indirect As-mobilizing bacteria on As behavior was also revealed. Our results show that the majority of As within the soil matrix was bioavailable and bioaccessible for heterotrophic As^V reduction to As^III, which may increase As toxicity and mobility in the contaminated soils.     

Isolation and Characterization of Arsenate-Reducing Bacteria from Arsenic-Contaminated Sites in New Zealand

Two environmental sites in New Zealand were sampled (e.g., water and sediment) for bacterial isolates that could use either arsenite as an electron donor or arsenate as an electron acceptor under aerobic and anaerobic growth conditions, respectively. These two sites were subjected to widespread arsenic contamination from mine tailings generated from historic gold mining activities or from geothermal effluent. No bacteria were isolated from these sites that could utilize arsenite or arsenate under the respective growth conditions tested, but a number of chemoheterotrophic bacteria were isolated that could grow in the presence of high concentrations of arsenic species. In total, 17 morphologically distinct arsenic-resistant heterotrophic bacteria isolates were enriched from the sediment samples, and analysis of the 16S rRNA gene sequence of these bacteria revealed them to be members of the genera Exiguobacterium, Aeromonas, Bacillus, Pseudomonas, Escherichia, and Acinetobacter. Two isolates, Exiguobacterium sp. WK6 and Aeromonas sp. CA1, were of particular interest because they appeared to gain metabolic energy from arsenate under aerobic growth conditions, as demonstrated by an increase in cellular growth yield and growth rate in the presence of arsenate. Both bacteria were capable of reducing arsenate to arsenite via a non-respiratory mechanism. Strain WK6 was positive for arsB, but the pathway of arsenate reduction for isolate CA1 was via a hitherto unknown mechanism. These isolates were not gaining an energetic advantage from arsenate or arsenite utilization, but were instead detoxifying arsenate to arsenite. As a subsidiary process to arsenate reduction, the external pH of the growth medium increased (i.e., became more alkaline), allowing these bacteria to grow for extended periods of time.     

Synergistic interaction of glyceraldehydes‐3‐phosphate dehydrogenase and ArsJ,a novel organoarsenical efflux permease,confers arsenate resistance

Microbial biotransformations are major contributors to the arsenic biogeocycle. In parallel with transformations of inorganic arsenic, organoarsenicals pathways have recently been recognized as important components of global cycling of arsenic. The well‐characterized pathway of resistance to arsenate is reduction coupled to arsenite efflux. Here, we describe a new pathway of arsenate resistance involving biosynthesis and extrusion of an unusual pentavalent organoarsenical. A number of arsenic resistance (ars) operons have two genes of unknown function that are linked in these operons. One, gapdh, encodes the glycolytic enzyme glyceraldehyde‐3‐phosphate dehydrogenase. The other, arsJ, encodes a major facilitator superfamily (MFS) protein. The two genes were cloned from the chromosome of Pseudomonas aeruginosa. When expressed together, but not alone, in Escherichia coli, gapdh and arsJ specifically conferred resistance to arsenate and decreased accumulation of As(V). Everted membrane vesicles from cells expressing arsJ accumulated As(V) in the presence of purified GAPDH, D‐glceraldehylde 3‐phosphate (G3P) and NAD⁺. GAPDH forms the unstable organoarsenical 1‐arseno‐3‐phosphoglycerate (1As3PGA). We propose that ArsJ is an efflux permease that extrudes 1As3PGA from cells, where it rapidly dissociates into As(V) and 3‐phosphoglycerate (3PGA), creating a novel pathway of arsenate resistance.