Enhancing arsenate metabolism in Microcystis aeruginosa and relieving risks of arsenite and microcystins by nano-Fe2O3 under dissolved organic phosphorus conditions.

Little information is available on how nano-Fe2O3 substituted iron ions as a possible iron source impacting on algal growth and arsenate (As(V)) metabolism under dissolved organic phosphorus (DOP) (D-glucose-6-phosphate (GP)) conditions. We investigated the growth of Microcystis aeruginosa and As(V) metabolism together with their metabolites in As(V) aquatic environments with nano-Fe2O3 and GP as the sole iron and P sources, respectively. Results showed that nano-Fe2O3 showed inhibitory effects on M. aeruginosa growth and microcystin (MCs) release under GP conditions in As(V) polluted water. There was little influence on As species changes in GP media under different nano-Fe2O3 concentrations except for obvious total As (TAs) removal in 100.0 mg L-1 nano-Fe2O3 levels. As(V) metabolism dominated with As(V) biotransformation in algal cells was facilitated and arsenite (As(III)) releasing risk was relieved clearly by nano-Fe2O3 under GP conditions. The dissolved organic matter (DOM) in media exhibited more fatty acid analogs containing -CO, -CH2 =CH2, and -CH functional groups with increasing nano-Fe2O3 concentrations, but the fluorescent analogs were relatively reduced especially for the fluorescent DOM dominated by aromatic protein-like tryptophan which was significantly inhibited by nano-Fe2O3. Thus, As methylation that was facilitated in M. aeruginosa by nano-Fe2O3 in GP environments also caused more organic substances to release that absorb infrared spectra while reducing the release risks of As(III) and MCs as well as protein-containing tryptophan fractions. From 1H-NMR analysis, this might be caused by the increased metabolites of aromatic compounds, organic acid/amino acid, and carbohydrates/glucose in algal cells. The findings are vital for a better understanding of nano-Fe2O3 role-playing in As bioremediation by microalgae and the subsequent potential aquatic ecological risks.

The combined impacts of selenium and phosphorus on the fate of arsenic in rice seedlings (Oryza sativa L.).

Although the single role of selenium (Se) or phosphorus (P) in regulating the As contamination of rice plants has been reported in some studies, the combined impacts of Se and P on the fate of As and the underlying mechanisms are poorly understood. To address this knowledge gap, the uptake, translocation, and biotransformation of As mediated by Se were investigated in rice (Oryza sativa L.) seedlings hydroponically cultured with P-normal and P-deficient conditions. The results showed Se addition stimulated the uptake of arsenite and arsenate by 15.6% and 30.7%, respectively in P-normal condition, and such effect was more profound in P-deficient condition with the value of 43.8% and 70.8%. However, regardless of Se addition, P-deficiency elevated the As uptake by 47.0%-92.1% for arsenate but had no obvious effects for arsenite. Accompanying with the As transfer factorShoot/Root reduced by 74.5%-80.2% and 71.1%-85.7%, Se addition decreased the shoot As content by 65.8%-69.7% and 59.6%-73.1%, respectively, in the arsenite- and arsenate-treated rice plants. Relative to the corresponding treatments of P-normal condition, P-deficiency reduced the As transfer factorShoot/Root by 38.9%-52.5% and thus decreasing the shoot As content by 35.2%-42.5% in the arsenite-treated plants; while the opposite impacts were observed in the arsenate-treated plants, in which the shoot As content was increased by 22.4%-83.7%. The analysis results of As species showed As(III) was dominant in both shoots (68.9%-75.1%) and roots (94.9%-97.2%), and neither Se addition nor P-deficiency had obvious impacts on the interconversion between As(III) and As(V). Our results demonstrate the regulating roles of Se in As accumulation mainly depend on P regimes and the specific rice tissues, but the effects of P-deficiency on the fate of As were influenced by the form of As added to the culture.

Selenium Increased Arsenic Accumulation by Upregulating the Expression of Genes Responsible for Arsenic Reduction, Translocation, and Sequestration in Arsenic Hyperaccumulator Pteris vittata.

Selenate enhances arsenic (As) accumulation in As-hyperaccumulator Pteris vittata, but the associated molecular mechanisms are unclear. Here, we investigated the mechanisms of selenate-induced arsenic accumulation by exposing P. vittata to 50 µM arsenate (AsV50) and 1.25 (Se1.25) or 5 µM (Se5) selenate in hydroponics. After 2 weeks, plant biomass, plant As and Se contents, As speciation in plant and growth media, and important genes related to As detoxification in P. vittata were determined. These genes included P transporters PvPht1;3 and PvPht1;4 (AsV uptake), arsenate reductases PvHAC1 and PvHAC2 (AsV reduction), and arsenite (AsIII) antiporters PvACR3 and PvACR3;2 (AsIII translocation) in the roots, and AsIII antiporters PvACR3;1 and PvACR3;3 (AsIII sequestration) in the fronds. The results show that Se1.25 was more effective than Se5 in increasing As accumulation in both P. vittata roots and fronds, which increased by 27 and 153% to 353 and 506 mg kg-1. The As speciation analyses show that selenate increased the AsIII levels in P. vittata, with 124-282% more AsIII being translocated into the fronds. The qPCR analyses indicate that Se1.25 upregulated the gene expression of PvHAC1 by 1.2-fold, and PvACR3 and PvACR3;2 by 1.0- to 2.5-fold in the roots, and PvACR3;1 and PvACR3;3 by 0.6- to 1.1-fold in the fronds under AsV50 treatment. Though arsenate enhanced gene expression of P transporters PvPht1;3 and PvPht1;4, selenate had little effect. Our results indicate that selenate effectively increased As accumulation in P. vittata, mostly by increasing reduction of AsV to AsIII in the roots, AsIII translocation from the roots to fronds, and AsIII sequestration into the vacuoles in the fronds. The results suggest that selenate may be used to enhance phytoremediation of As-contaminated soils using P. vittata.

Selenium alleviates arsenic induced stress by modulating growth, oxidative stress, antioxidant defense and thiol metabolism in rice seedlings.

This study aims to investigate the potentiality of selenium in modulating arsenic stress in rice seedlings. Arsenate accumulation along with its transformation to arsenite was enhanced in arsenate exposed seedlings. Arsenite induced oxidative stress and severely affected the growth of the seedlings. Arsenate exposure caused an elevation in ascorbate and glutathione levels along with the activities of their metabolizing enzymes viz., ascorbate peroxidase, glutathione reductase, glutathione-S-transferase, and glutathione peroxidase. Phytochelatins content was increased under arsenic stress to subdue the toxic effects in the test seedlings. Co-application of arsenate and selenate in rice seedlings manifested pronounced alteration of oxidative stress, antioxidant defense, and thiol metabolism as compared to arsenate treatment only. ANOVA analysis (Tukey's HSD test) demonstrated the relevance of using selenate along with arsenate to maintain the normal growth and development of rice seedlings. Thus, exogenous supplementation of selenium will be a beneficial approach to cultivate rice seedlings in arsenic polluted soil.

Arsenic toxicity in the environment is a global concern, causes chronic signs of poisoning to plants and humans, leads to ecological imbalance. Selenium is known for its antagonistic characteristics and has been found to be effective in combating the adversities of arsenic at low concentrations (5 µM). The present study was performed to explore the comparative responses of rice seedlings during the joint application of selenium and arsenic in terms of growth, generation of oxidative stress, antioxidant defense, and thiol metabolism. Although the molecular basis of arsenic­selenium interaction is widely known a small number of reports were listed about the physio-chemical role of selenium against arsenic stress. Thus, we investigated the influence of selenium to alleviate arsenic-induced toxic effects by modulating the activities of antioxidant enzymes and reducing the levels of oxidative stress markers. It has been noted that selenium regulates thiol metabolism which is known to play a key role in growth preservation by restriction of arsenic translocation. The outcome from the study would be useful in field trials for sustainable agriculture in arsenic-contaminated soil.

Regulation, Diversity and Evolution of Boron Transporters in Plants.

Boron (B) is an essential trace element in plants, and borate cross-linking of pectic polysaccharide rhamnogalacturonan-II (RG-II) in cell walls is required for normal cell growth. High concentrations of B are toxic to cells. Therefore, plants need to control B transport to respond to B conditions in the environment. Over the past two decades, genetic analyses of Arabidopsis thaliana have revealed that B transport is governed by two types of membrane transport molecules: NIPs (nodulin-26-like intrinsic proteins), which facilitate boric acid permeation, and BORs, which export borate from cells. In this article, we review recent findings on the (i) regulation at the cell level, (ii) diversity among plant species and (iii) evolution of these B transporters in plants. We first describe the systems regulating these B transporters at the cell level, focusing on the molecular mechanisms underlying the polar localization of proteins and B-dependent expression, as well as their physiological significance in A. thaliana. Then, we examine the presence of homologous genes and characterize the functions of NIPs and BORs in B homeostasis, in a wide range of plant species, including Brassica napus, Oryza sativa and Zea mays. Finally, we discuss the evolutionary aspects of NIPs and BORs as B transporters, and the possible relationship between the diversification of B transport and the occurrence of RG-II in plants. This review considers the sophisticated systems of B transport that are conserved among various plant species, which were established to meet mineral nutrient requirements.

Studies of selenium and arsenic mutual protection in human HepG2 cells.

Hundreds of millions of people worldwide are exposed to unacceptable levels of carcinogenic inorganic arsenic. Animal models have shown that selenium and arsenic are mutually protective through the formation and elimination of the seleno-bis(S-glutathionyl) arsinium ion [(GS)2AsSe]-. Consistent with this, human selenium deficiency in arsenic-endemic regions is associated with arsenic-induced disease, leading to the initiation of human selenium supplementation trials. In contrast to the protective effect observed in vivo, in vitro studies have suggested that selenite increases arsenite cellular retention and toxicity. This difference might be explained by the rapid conversion of selenite to selenide in vivo. In the current study, selenite did not protect the human hepatoma (HepG2) cell line against the toxicity of arsenite at equimolar concentrations, however selenide increased the IC50 by 2.3-fold. Cytotoxicity assays of arsenite + selenite and arsenite + selenide at different molar ratios revealed higher overall mutual antagonism of arsenite + selenide toxicity than arsenite + selenite. Despite this protective effect, in comparison to 75Se-selenite, HepG2 cells in suspension were at least 3-fold more efficient at accumulating selenium from reduced 75Se-selenide, and its accumulation was further increased by arsenite. X-ray fluorescence imaging of HepG2 cells also showed that arsenic accumulation, in the presence of selenide, was higher than in the presence of selenite. These results are consistent with a greater intracellular availability of selenide relative to selenite for protection against arsenite, and the formation and retention of a less toxic product, possibly [(GS)2AsSe]-.

Abundant and diverse arsenic-metabolizing microorganisms in peatlands treating arsenic-contaminated mining wastewaters.

Mining operations produce large quantities of wastewater. At a mine site in Northern Finland, two natural peatlands are used for the treatment of mining-influenced waters with high concentrations of sulphate and potentially toxic arsenic (As). In the present study, As removal and the involved microbial processes in those treatment peatlands (TPs) were assessed. Arsenic-metabolizing microorganisms were abundant in peat soil from both TPs (up to 108 cells gdw -1 ), with arsenate respirers being about 100 times more abundant than arsenite oxidizers. In uninhibited microcosm incubations, supplemented arsenite was oxidized under oxic conditions and supplemented arsenate was reduced under anoxic conditions, while little to no oxidation/reduction was observed in NaN3 -inhibited microcosms, indicating high As-turnover potential of peat microbes. Formation of thioarsenates was observed in anoxic microcosms. Sequencing of the functional genemarkers aioA (arsenite oxidizers), arrA (arsenate respirers) and arsC (detoxifying arsenate reducers) demonstrated high diversity of the As-metabolizing microbial community. The microbial community composition differed between the two TPs, which may have affected As removal efficiencies. In the present situation, arsenate reduction is likely the dominant net process and contributes substantially to As removal. Changes in TP usage (e.g. mine closure) with lowered water tables and heightened oxygen availability in peat might lead to re-oxidation and re-mobilization of bound arsenite.

Immobilizing unicellular microalga on pellet-forming filamentous fungus: Can this provide new insights into the remediation of arsenic from contaminated water?

Response surface methodology was employed to investigate the effects of nitrogen (X1), phosphorus (X2), and glucose (X3) on arsenic removal by fungal-algal pellets. X1, X3, and X1X3 had significant effects. Arsenic accumulation and transformation were compared among Chlorella vulgaris, Aspergillus oryzae, and fungal-algal pellets under different arsenate and phosphorus concentrations. Fungal-algal pellets had the highest removal rate and was best able to accumulate arsenate in all treatments. The reduction of arsenate to arsenite was found in all tested organisms, while arsenic methylation was only identified in C. vulgaris. The biomass of fungal-algal pellets was not inhibited by arsenate. SEM micrographs showed that arsenic led to a change in mycelial structure from compact to loose pellets. FT-IR spectra showed that four functional groups might be involved in arsenate adsorption. Arsenic tolerance and accumulation in fungal-algal pellets opens the way to its potential application in the remediation of arsenic from contaminated water.

Single-cell analysis by ICP-MS/MS as a fast tool for cellular bioavailability studies of arsenite.

Single-cell inductively coupled plasma mass spectrometry (SC-ICP-MS) has become a powerful and fast tool to evaluate the elemental composition at a single-cell level. In this study, the cellular bioavailability of arsenite (incubation of 25 and 50 µM for 0-48 h) has been successfully assessed by SC-ICP-MS/MS for the first time directly after re-suspending the cells in water. This procedure avoids the normally arising cell membrane permeabilization caused by cell fixation methods (e.g. methanol fixation). The reliability and feasibility of this SC-ICP-MS/MS approach with a limit of detection of 0.35 fg per cell was validated by conventional bulk ICP-MS/MS analysis after cell digestion and parallel measurement of sulfur and phosphorus.

A novel biodegradable arsenic adsorbent by immobilization of iron oxyhydroxide (FeOOH) on the root powder of long-root Eichhornia crassipes.

In this study, FeOOH was immobilized on the biodegradable root powder, abbreviated as RP, of long-root Eichhornia crassipes, a kind of waste biomass, to improve the adsorption performances for aqueous arsenic contaminants. The adsorption kinetics and thermodynamics experiments showed that the adsorption rates and capacities of the root powder for arsenate (As(V)) and arsenite (As(III)) were both enhanced markedly after modification with FeOOH. The adsorption of As(V) and As(III) by the modified root powder, abbreviated as MRP, could arrive at equilibrium in 50 min and the saturated adsorption capacities reached up to 8.67-9.43 mg/g for As(V) and 5.21-5.65 mg/g for As(V) at temperature of 10-50 °C, respectively. Besides, the effect of pH and ionic strength on adsorption was investigated and the results showed that the optimum pH for the arsenic adsorption using the MRP was 9.0 and the As(V) adsorption was more sensitive to ionic strength. Furthermore, the complexation of hydratable hydroxyls on FeOOH with arsenic contaminants was concluded as the adsorption force according FTIR and XPS analyses. The MRP used could be regenerated via 0.4 mol/L NaOH solution and no apparent adsorption capacity losses appeared after 6 cyclic utilizations.

Selenate mitigates arsenite toxicity in rice (Oryza sativa L.) by reducing arsenic uptake and ameliorates amino acid content and thiol metabolism.

Arsenic (As) is a toxic element with the potential to cause health effects in humans. Besides rice is a source of both amino acids (AAs) and mineral nutrients, it is undesired source of As for billions of people consuming rice as the staple food. Selenium (Se) is an essential metalloid, which can regulate As toxicity by strengthening antioxidant potential. The present study was designed to investigate As(III) stress mitigating effect of Se(VI) in rice. The level of As, thiolic ligands and AAs was analyzed in rice seedlings after exposure to As(III)/Se(VI) alone and As(III)+Se(VI) treatments. Selenate supplementation (As(III) 25µM+Se(VI) 25µM) decreased total As accumulation in both root and shoot (179 & 144%) as compared to As(III) alone treatment. The As(III)+Se(VI) treatment also induced the levels of non-protein thiols (NPTs), glutathione (GSH) and phytochelatins (PCs) as compared to As(III) alone treatment and also modulated the activity of enzymes of thiol metabolism. The content of amino acids (AAs) was significantly altered with Se(VI) supplementation. Importantly, essential amino acids (EAAs) were enhanced in As(III)+Se(VI) treatment as compared to As(III) alone treatment. In contrast, stress related non-essential amino acids (NEAAs) like GABA, Glu, Gly, Pro and Cys showed enhanced levels in As(III) alone treatment. In conclusion, rice supplemented with Se(VI) tolerated As toxicity with reduced As accumulation and increased the nutrition quality by increasing EAAs.

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.

Arbuscular mycorrhizal symbiosis influences arsenic accumulation and speciation in Medicago truncatula L. in arsenic-contaminated soil.

In two pot experiments, wild type and a non-mycorrhizal mutant (TR25:3-1) of Medicago truncatula were grown in arsenic (As)-contaminated soil to investigate the influences of arbuscular mycorrhizal fungi (AMF) on As accumulation and speciation in host plants. The results indicated that the plant biomass of M. truncatula was dramatically increased by AM symbiosis. Mycorrhizal colonization significantly increased phosphorus concentrations and decreased As concentrations in plants. Moreover, mycorrhizal colonization generally increased the percentage of arsenite in total As both in shoots and roots, while dimethylarsenic acid (DMA) was only detected in shoots of mycorrhizal plants. The results suggested that AMF are most likely to get involved in the methylating of inorganic As into less toxic organic DMA and also in the reduction of arsenate to arsenite. The study allowed a deeper insight into the As detoxification mechanisms in AM associations. By using the mutant M. truncatula, we demonstrated the importance of AMF in plant As tolerance under natural conditions.

Effects of nitrogen and phosphorus on arsenite accumulation, oxidation, and toxicity in Chlamydomonas reinhardtii.

We studied arsenite (iAs(III)) accumulation, oxidation, and toxicity in the freshwater green alga Chlamydomonas reinhardtii under nutrient-enriched (+NP), phosphorus-limited (-P), and nitrogen-limited (-N) conditions. The -P alga (55.1 µM) had a Michaelis constant (Kd) for uptake approximately one tenth of the +NP (419 µM) and -N (501 µM) cells, indicating iAs(III) uptake inhibition by extracellular phosphate. This conclusion was supported by the hyperbolic reduction in iAs(III) uptake rate (V) from 9.2 to 0.8 µmol/g-dw/h when the extracellular phosphate concentration went up from 0 to 250 µM. The maximal iAs(III) uptake rate (Vmax) of the -N alga (24.3 µmol/g-dw/h) was twice as much as that of the +NP (12 µmol/g-dw/h) and -P (8.1 µmol/g-dw/h) cells. It implies that more arsenic transporters were synthesized under the -N condition. Once accumulated, iAs(III) was oxidized and a higher proportion of arsenate (iAs(V)) was observed at lower [As]dis or under nutrient-limited conditions. Nevertheless, iAs(III) oxidation mainly occurred outside the cells with the extent of oxidation reciprocal to [As]dis. Based on the logistic modeling of the concentration-response curves in the +NP, -P, and -N toxicity tests, iAs(III) had an [As]dis-based EC50 of 1763, 13.1, and 1208 µM and an intracellular arsenic concentration based EC50 of 35.6, 28.8, and 195 µmol/g-dw, respectively. Higher iAs(III) toxicity to the -P cells occured because of their increased iAs(III) accumulation, whereas the underlying mechanisms why the -N alga was more tolerant need to be further revealed. Overall, both N and P had remarkable effects on the behavior and effects of iAs(III), which cannot be disregarded in the biogeochemical cycling research of arsenic.

[Distribution of arsenic species and its DNA damage in subchronic arsenite-exposed mice].

OBJECTIVE: To study the arsenic distribution, speciation, its effects on the balance of other elements and the DNA damage by subchronic arsenite exposure in mice. METHODS: The 8-week-old C57BL/6N mice were matched by weight and divided into control group and supplementation group, which were given 0 or 10 microg/ml of sodium arsenite in the drinking water, and continuous exposed for 6 months. RESULTS: Arsenic was found in various tissues and organs. The highest ones were in the kidney, lung and liver, reached (563.9 +/- 222.5), (458.6 +/- 191.0) and (279.8 +/- 81.2) ng/g, respectively while the lowest in the blood and brain, reached (82.2 +/- 26.7) ng/ml and (101.8 +/- 30.1) ng/g, respectively. Arsenic exists mainly in the form of dimethylarsinous acid (DMA). Compared to the control group, there was a significant difference (P < 0.05) between arsenic and chromium, copper, zinc, selenium, lead in some organs of arsenic exposed group, but not cadmium. Furthermore, the 8-hydroxydeoxyguanosine (8-OHdG) level of the exposed group was (149.1 +/- 1.0) ng/ml, which was significantly higher than the control group of (76.4 +/- 27.9) ng/ml. CONCLUSION: Arsenic accumulated in various tissues and organs mainly in the form of DMA, which affected the balance of chromium, copper, zinc, selenium and lead in the body, and led to DNA damage after subchronic exposure.

Effect of silicate supplementation on the alleviation of arsenite toxicity in 93-11 (Oryza sativa L. indica).

Chronic exposure to arsenic (As) in rice has raised many health and environmental problems. As reported, great variation exists among different rice genotypes in As uptake, translocation, and accumulation. Under hydroponic culture, we find that the Chinese wild rice (Oryza rufipogon; acc. 104624) takes up the most arsenic among tested genotypes. Of the cultivated rice, the indica cv. 93-11 has the lowest arsenic translocation factor value but accumulates the maximum concentration of arsenic followed by Nipponbare, Minghui 86, and Zhonghua 11. Higher level of arsenite concentration (50 µM) can induce extensive photosynthesis and root growth inhibition, and cause severe oxidative stress. Interestingly, external silicate (Si) supplementation has significantly increased the net photosynthetic rate, and promoted root elongation, as well as strongly ameliorated the oxidative stress by increasing the activities of antioxidant enzymes superoxide dismutase, ascorbate peroxidase, and peroxidase in roots and/or leaves of 93-11 seedlings. Notably, 1.873 mM concentration of Si considerably decreases the total As uptake and As content in roots, but significantly increases the As translocation from roots to shoots. In contrast, Si supplementation with 1.0 mM concentration significantly increases the total As uptake and As concentrations in roots and shoots of 93-11 seedlings after 50 µM arsenite treatment for 6 days.

Changes in the synaptic structure of hippocampal neurons and impairment of spatial memory in a rat model caused by chronic arsenite exposure.

Many epidemiological studies and in vitro experiments have found that chronic arsenic exposure may influence memory formation. The goal of this study was to create an animal model of memory impairment induced by chronic arsenite exposure and to study the underlying mechanisms. Sixty male Sprague-Dawley (SD) male rats were randomly divided into a control group, a low-dose sodium arsenite exposure group and a high-dose sodium arsenite exposure group. Sodium arsenite was administered by adding it to drinking water for 3 months. Then, the spatial memory of the rats was examined with Morris water maze and Y maze. The concentration of arsenic in the blood and the brain was determined by an atomic fluorescence absorption spectrometer. The ultra-structure of hippocampal neurons was observed by an electron microscope. Timm staining was used for observing mossy fibers. We found that the concentration of arsenic in the blood and the brain increased in a dose-response manner (P<0.05). The performance of rats in the arsenite exposed group (15 mg/kg) was significantly impaired in the Morris water maze and Y maze tasks than those in the control group (P<0.05). Sodium arsenite exposure resulted in abnormal structural changes in the myelin sheaths of nerve fibers and decreases in the terminals of mossy fibers. Together, chronic sodium arsenite exposure through drinking water results in detrimental changes in the neuronal synapses, which may contribute to the arsenite-induced impairment of spatial memory.

Characterization of As efflux from the roots of As hyperaccumulator Pteris vittata L.

In some plant species, various arsenic (As) species have been reported to efflux from the roots. However, the details of As efflux by the As hyperaccumulator Pteris vittata remain unknown. In this study, root As efflux was investigated for different phosphorus (P) supply conditions during or after a 24-h arsenate uptake experiment under hydroponic growth conditions. During an 8-h arsenate uptake experiment, P-supplied (P+) P. vittata exhibited much greater arsenite efflux relative to arsenate uptake when compared with P-deprived (P-) P. vittata, indicating that arsenite efflux was not proportional to arsenate uptake. In the As efflux experiment following 24 h of arsenate uptake, arsenate efflux was also observed with arsenite efflux in the external solution. All the results showed relatively low rates of arsenate efflux, ranging from 5.4 to 16.1% of the previously absorbed As, indicating that a low rate of arsenate efflux to the external solution is also a characteristic of P. vittata, as was reported with arsenite efflux. In conclusion, after 24 h of arsenate uptake, both P+ and P- P. vittata loaded/effluxed similar amounts of arsenite to the fronds and the external solution, indicating a similar process of xylem loading and efflux for arsenite, with the order of the arsenite concentrations being solution ≪ roots ≪ fronds.

The mechanism of the polynucleotide phosphorylase-catalyzed arsenolysis of ADP.

Using ADP and arsenate (AsV), polynucleotide phosphorylase (PNPase) catalyzes the apparent arsenolysis of ADP to AMP-arsenate and inorganic phosphate, with the former hydrolyzing rapidly into AMP and AsV. However, in the presence of glutathione, AMP-arsenate may also undergo reductive decomposition, yielding AMP and arsenite (AsIII). In order to clarify the mechanism of ADP arsenolysis mediated by Escherichia coli PNPase, we analyzed the time course of the reaction in the presence of increasing concentrations of ADP, with or without polyadenylate (poly-A) supplementation. These studies revealed that increasing supply of ADP enhanced the consumption of ADP but inhibited the production of both AMP and AsIII. Formation of these products was amplified by adding trace amount of poly-A. Furthermore, AMP and AsIII production accelerated with time, whereas ADP consumption slowed down. These observations collectively suggest that PNPase does not catalyze the arsenolysis of ADP directly (in a single step), but in two separate consecutive steps: the enzyme first converts ADP into poly-A, then it cleaves the newly synthesized poly-A by arsenolysis. It is inferred that one active site of PNPase can catalyze only one of these reactions at a time and that high ADP concentrations favor poly-A synthesis, thereby inhibiting the arsenolysis.

Effects of cultivation conditions on the uptake of arsenite and arsenic chemical species accumulated by Pteris vittata in hydroponics.

The physiological responses of the arsenic-hyperaccumulator, Pteris vittata, such as arsenic uptake and chemical transformation in the fern, have been investigated. However, a few questions remain regarding arsenic treatment in hydroponics. Incubation conditions such as aeration, arsenic concentration, and incubation period might affect those responses of P. vittata in hydroponics. Arsenite uptake was low under anaerobic conditions, as previously reported. However, in an arsenite uptake experiment, phosphorous (P) starvation-dependent uptake of arsenate was observed under aerobic conditions. Time course-dependent analysis of arsenite oxidation showed that arsenite was gradually oxidized to arsenate during incubation. Arsenite oxidation was not observed in any of the control conditions, such as exposure to a nutrient solution or to culture medium only, or with the use of dried root; arsenite oxidation was only observed when live root was used. This result suggests that sufficient aeration allows the rhizosphere system to oxidize arsenite and enables the fern to efficiently take up arsenite as arsenate. X-ray absorption near edge structure (XANES) analyses showed that long-duration exposure to arsenic using a hydroponic system led to the accumulation of arsenate as the dominant species in the root tips, but not in the whole roots, partly because up-regulation of arsenate uptake by P starvation of the fern was caused and retained by long-time incubation. Analysis of concentration-dependent arsenate uptake by P. vittata showed that the uptake switched from a high-affinity transport system to a low-affinity system at high arsenate concentrations, which partially explains the increased arsenate abundance in the whole root.