Oxidation and methylation status determine the effects of arsenic on the mitotic apparatus

We investigated the spindle inhibitory properties of six arsenicals differing in their methylation or oxidation state. Human lymphoblasts were exposed for 6 h to either sodium arsenate (NaAs^V), sodium arsenite (NaAs^III), monomethylarsonic acid (MMA^V), monomethylarsonous acid (MMA^III), dimethylarsinic acid (DMA^V), or dimethylarsinous acid (DMA^III). After exposure slides were prepared, and the mitotic indices (MI) were assessed. We also exposed tubulin directly to each arsenical and spectrophotometrically measured its effect on polymerization. NaAs^V caused a small but significant increase in MI. MMA^V also caused only a slight increase in MI that just reached statistical significance. In contrast, DMA^V caused a significant increase in MI, producing ∼75% the MI of demecolcine and ∼4 times the MI of the control. NaAs^III had no significant effect on MI and was quite toxic. MMA^III induced more than a twofold increase in MI compared to the control, which was about 40% that caused by demecolcine. On a micromolar basis, MMA^III was the most potent of the arsenicals tested. DMA^III gave inconsistent results. None of the pentavalent arsenicals had a substantial effect (either inhibition or enhancement) on GTP-induced polymerization of tubulin. In contrast, NaAs^III inhibited polymerization at concentrations of 1 mM and above and MMA^III and DMA^III at 10 μM and above. Taken together, these results present a complex picture of how arsenicals may affect cells. These studies demonstrate that the metabolites of arsenic are active not only as chromosome breaking and DNA damaging agents but can also interfere with cell division via tubulin disruption.     

Monomethylarsonous Acid (MMAIII) Has an Adverse Effect on the Innate Immune Response of Human Bronchial Epithelial Cells to Pseudomonas aeruginosa

Arsenic is the number one contaminant of concern with regard to human health according to the World Health Organization. Epidemiological studies on Asian and South American populations have linked arsenic exposure with an increased incidence of lung disease, including pneumonia, and chronic obstructive pulmonary disease, both of which are associated with bacterial infection. However, little is known about the effects of low dose arsenic exposure, or the contributions of organic arsenic to the innate immune response to bacterial infection. This study examined the effects on Pseudomonas aeruginosa (P. aeruginosa) induced cytokine secretion by human bronchial epithelial cells (HBEC) by inorganic sodium arsenite (iAs^III) and two major metabolites, monomethylarsonous acid (MMA^III) and dimethylarsenic acid (DMA^V), at concentrations relevant to the U.S. population. Neither iAs^III nor DMA^V altered P. aeruginosa induced cytokine secretion. By contrast, MMA^III increased P. aeruginosa induced secretion of IL-8, IL-6 and CXCL2. A combination of iAs^III, MMA^III and DMA^V (10 pbb total) reduced IL-8 and CXCL1 secretion. These data demonstrate for the first time that exposure to MMA^III alone, and a combination of iAs^III, MMA^III and DMA^V at levels relevant to the U.S. may have negative effects on the innate immune response of human bronchial epithelial cells to P. aeruginosa.     

Role of the Alkali Labile Sites,Reactive Oxygen Species and Antioxidants in DNA Damage Induced by Methylated Trivalent Metabolites of Inorganic Arsenic

In the last decade arsenic metabolism has become an important matter of discussion. Methylation of inorganic arsenic (iAs) to monomethylarsonic acid (MMA^V) and dimethylarsinic acid (DMA^V) is considered to decrease arsenic toxicity. However, in addition to these pentavalent metabolites, the trivalent metabolites monomethylarsonous (MMA^III) and dimethylarsinous acid (DMA^III) have been identified recently as intermediates in the metabolic pathway of arsenic in cultured human cells. To examine the role of oxidative damage in the generation of DNA strand breaks by methylated trivalent arsenic metabolites, we treated human lymphocytes with both metabolites at non-cytotoxic concentrations. We further tested whether these effects are sensitive to modulation by the antioxidants ascorbate (Vitamin C) and selenomethionine (Se-Met). Both trivalent metabolites produced oxidative stress related DNA damage, consisting of single strand breaks and alkali-labile sites, with MMA^III being more potent at low concentrations than DMA^III. Neither MMA^III nor DMA^III induced DNA-double strand breaks. The oxidative stress response profiles of the metabolites were parallel as determined by lipid peroxidation induction. MMA^III induced peroxidation from the lowest concentration tested, while effects of DMA^III were apparent only at concentrations above 10 μM. The antioxidant Se-Met exhibited a more pronounced inhibition of trivalent arsenic metabolite-induced oxidative-DNA damage than did vitamin C. The present findings suggest that DNA damage by methylated trivalent metabolites at non-cytotoxic concentrations may be mediated by a mix of reactive oxygen and nitrogen oxidized species.     

Induction of DNA damage by free radicals generated either by organic or inorganic arsenic (AsIII, MMAIII, and DMAIII) in cultures of B and T lymphocytes

The aim of this work is based in the premise that inorganic arsenic (As^III) and trivalentmethylated metabolites monomethylarsonous (MMA^III) and dimethylarsinous (DMA^III) participate in DNA damage through the generation of reactive oxygen species (ROS). We have utilized two lymphoblastic lines, Raji (B cells) and Jurkat (T cells), which were treated with the trivalent arsenic species (dose: 0–100 μM) and analyzed by two assays (comet assay and flow cytometry) in the determination of DNA damage and ROS effects in vivo. The results showed that the damage to the DNA and the generation of ROS are different in both cellular lines with respect to the dose of organic arsenic, and the order of damage is MMA^III>DMA^III>As^III. This fact suggests that the DMA^III is not always the more cytotoxic intermediary xenobiotic, as has already been reported in another study.     

Pentavalent methylated arsenicals are substrates of human AQP9

Liver aquaglyceroporin AQP9 facilitates movement of trivalent inorganic arsenite (As^III) and organic monomethylarsonous acid (MAs^III). However, the transport pathway for the two major pentavalent arsenic cellular metabolites, MAs^V and DMAs^V, remains unknown in mammals. These products of arsenic metabolism, in particular DMAs^V, are the major arsenicals excreted in the urine of mammals. In this study, we examined the uptake of the two pentavalent organic arsenicals by human AQP9 in Xenopus laevis oocytes. Xenopus laevis oocytes microinjected with AQP9 cRNA exhibited uptake of both MAs^V and DMAs^V in a pH-dependent manner. The rate of transport was much higher at acidic pH (pH5.5) than at neutral pH. Hg(II), an aquaporin inhibitor, inhibited transport of As^III, MAs^III, MAs^V and DMAs^V via AQP9. However, phloretin, which inhibits water and glycerol permeation via AQP9, can only inhibit transport of pentavalent MAs^V and DMAs^V but not trivalent As^III and MAs^III, indicating the translocation mechanisms of these arsenic species are not exactly the same. Reagents such as FCCP, valinomycin and nigericin that dissipate transmembrane proton potential or change the transmemebrane pH gradient did not significantly inhibit all arsenic transport via AQP9, suggesting the transport of pentavalent arsenic is not proton coupled. The results suggest that in addition to the initial uptake of trivalent inorganic As^III inside cells, AQP9 plays a dual role in the detoxification of arsenic metabolites by facilitating efflux from cells.     

Arsenic hazards: strategies for tolerance and remediation by plants

Arsenic toxicity has become a global concern owing to the ever-increasing contamination of water, soil and crops in many regions of the world. To limit the detrimental impact of arsenic compounds, efficient strategies such as phytoremediation are required. Suitable plants include arsenic hyperaccumulating ferns and aquatic plants that are capable of completing their life cycle in the presence of high levels of arsenic through the concerted action of arsenate reduction to arsenite, arsenite complexation, and vacuolar compartmentalization of complexed or inorganic arsenic. Tolerance can also be conferred by lowering arsenic uptake by suppression of phosphate transport activity, a major pathway for arsenate entry. In many unicellular organisms, arsenic tolerance is based on the active removal of cytosolic arsenite while limiting the uptake of arsenate. Recent molecular studies have revealed many of the gene products involved in these processes, providing the tools to improve crop species and to optimize phytoremediation; however, so far only single genes have been manipulated, which has limited progress. We will discuss recent advances and their potential applications, particularly in the context of multigenic engineering approaches.     

Possible differences in the mechanism of malignant transformation of HaCaT cells by arsenite and its dimethyl metabolites,particularly dimethylthioarsenics

Background: As a confirmed human carcinogen, arsenic can cause skin cancer, lung cancer, etc. However, its carcinogenic mechanism is still unclear. In recent years, the oxidative stress hypothesis has become widely accepted. In mammals it has been found that arsenic can be converted to dimethylarsinous acid (DMA^III) and dimethylmonothioarsinic acid (DMMTA^V) through a series of methylation and redox reactions. DMA^III and DMMTA^V are highly toxic.Methods: Human keratinocytes (HaCaT) were exposed to different concentrations of NaAsO₂ (IAs^III), DMMTA^V and DMA^III for 24 h. Reactive oxygen species (hydrogen peroxide and superoxide), oxidative damage markers (8-hydroxydeoxyguanosine and malondialdehyde), and antioxidant markers (glutathione and superoxide dismutase) were measured. In addition, sulfane sulfurs were measured in HaCaT cells and a cell-free system.Results: In the DMMTA^V and DMA^III treatment groups, the levels of hydrogen peroxide and superoxide in HaCaT cells were higher than in the IAs^III treatment groups at the same dose. Levels of 8−OHdG and MDA in the DMMTA^V and DMA^III treatment groups were also higher than those in the IAs^III treatment groups at the same dose. However, in the DMMTA^V and DMA^III treatment groups, the levels of GSH and SOD activity were lower than that in the IAs^III treatment groups. In DMMTA^V-treated HaCaT cells, sulfane sulfurs were produced. Further, it was found that DMMTA^V could react with DMDTA^V to form persulfide in the cell-free system, which may explain the mechanism of the formation of sulfane sulfurs in DMMTA^V-treated HaCaT cells.Conclusions: DMMTA^V and DMA^III more readily induce reactive oxygen species (ROS) and cause oxidative damage in HaCaT cells than inorganic arsenic. Further, the persulfide formed by the reaction of DMMTA^V and DMDTA^V produced from the metabolism of DMMTA^V may induce a stronger reductive defense mechanism than GSH against the intracellular oxidative stress of DMMTA^V. However, the cells exposed to arsenite are transformed by the continuous nuclear translocation of Nrf2 due to oxidative stress, and the persulfide from dimethylthioarsenics may promote Nrf2 by the combination with thiol groups, especially redox control key protein, Keap1, eventually cause nuclear translocation of sustained Nrf2.     

Bioadsorption of arsenic from aqueous solution by the extremophilic bacterium Acidithiobacillus ferrooxidans DLC-5

The bioadsorption of heavy metal ions to process industrial and solid wastes is an attractive technology from an economical and environmental point of view. This study investigated the equilibrium, thermodynamics and bioadsorption characteristics of arsenite (iAs^III) and dimethylarsinate (DMA^V) by Acidithiobacillus ferrooxidans (A. ferrooxidans) DLC-5 in aqueous solution. Optimum bioadsorption conditions were determined by identifying the optimum temperature, pH, biomass dosage, initial arsenic concentration and contact time. The equilibrium data were then applied to Langmuir and Freundlich isotherm models. The results indicated that the bioadsorption processes for both iAs^III and DMA^V involved pseudo-second-order kinetics. Additionally, the bioadsorption of iAs^III and DMA^V by A. ferrooxidans DLC-5 was feasible, spontaneous and endothermic under the tested conditions. Fourier transform infrared spectroscopy (FT-IR) showed that –OH and –NH groups were involved in the bioadsorption process. A. ferrooxidans DLC-5 demonstrates potential for use in removing arsenic from aqueous solutions, especially those with very low arsenic concentrations.     

Role of AQP9 in transport of monomethyselenic acid and selenite

AQP9 is an aquaglyceroporin with a very broad substrate spectrum. In addition to its orthodox nutrient substrates, AQP9 also transports multiple neutral and ionic arsenic species including arsenic trioxide, monomethylarsenous acid (MAs^III) and dimethylarsenic acid (DMA^V). Here we discovered a new group of AQP9 substrates which includes two clinical relevant selenium species. We showed that AQP9 efficiently transports monomethylselenic acid (MSeA) with a preference for acidic pH, which has been demonstrated in Xenopus laevis oocyte following the overexpression of human AQP9. Specific inhibitors that dissipate transmembrane proton potential or change the transmembrane pH gradient, such as FCCP, valinomycin and nigericin did not significantly inhibit MSeA uptake, suggesting MSeA transport is not proton coupled. AQP9 was also found to transport ionic selenite and lactate, with much less efficiency compared with MSeA uptake. Selenite and lactate uptake via AQP9 is pH dependent and inhibited by FCCP and nigericin, but not valinomycin. The selenite and lactate uptake via AQP9 can be inhibited by different lactate analogs, indicating that their translocation share similar mechanisms. AQP9 transport of MSeA, selenite and lactate is all inhibited by a previously identified AQP9 inhibitor, phloretin, and the AQP9 substrate arsenite (As^III). These newly identified AQP9 selenium substrates imply that AQP9 play a significant role in MSeA uptake and possibly selenite uptake involved in cancer therapy under specific microenvironments.     

The formation,vacuolar localization,and tonoplast transport of salicylic acid glucose conjugates in tobacco cell suspension cultures

The metabolism of salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv. KY 14) cell suspension cultures was examined by adding [7–¹⁴C]SA to the cell cultures for 24 h and identifying the metabolites through high performance liquid chromatography analysis. The three major metabolites of SA were SA 2-O--D-glucose (SAG), methylsalicylate 2-O--D-glucose (MeSAG) and methylsalicylate. Studies on the intracellular localization of the metabolites revealed that all of the SAG associated with tobacco protoplasts was localized in the vacuole. However, the majority of the MeSAG was located outside the vacuole. The tobacco cells contained an SA inducible SA glucosyltransferase (SAGT) enzyme that formed SAG. The SAGT enzyme was not associated with the vacuole and appeared to be a cytoplasmic enzyme. The vacuolar transport of SAG was characterized by measuring the uptake of [¹⁴C]SAG into tonoplast vesicles isolated from tobacco cell cultures. SAG uptake was stimulated eightfold by the addition of MgATP. The ATP-dependent uptake of SAG was inhibited by bafilomycin A₁ (a specific inhibitor of the vacuolar H⁺-ATPase) and dissipation of the transtonoplast H⁺-electrochemical gradient. Vanadate was not an inhibitor of SAG uptake. Several -glucose conjugates were strong inhibitors of SAG uptake, whereas glutathione and glucuronide conjugates were only marginally inhibitory. The SAG uptake exhibited Michaelis–Menten type saturation kinetics with a K_m and V_max value of 11 M and 205 pmol min^–1 mg^–1, respectively, for SAG. Based on the transport characteristics it appears as if the vacuolar uptake of SAG in tobacco cells occurs through an H⁺-antiport-type mechanism.     

Proposal for novel metabolic pathway of highly toxic dimethylated arsenics accompanied by enzymatic sulfuration,desulfuration and oxidation

The International Agency for Research on Cancer (IARC) has concluded that dimethylarsinic acid [(CH₃)₂AsO(OH), DMA^V], a main metabolite of inorganic arsenic, is responsible for carcinogenesis in urinary bladder and lung in rodents, and various modes of carcinogenic action have been proposed. One theory concerning the mode of action is that the biotransformation of dimethylarsinous acid [(CH₃)₂AsOH, DMA^III] from DMA^V plays an important role in the carcinogenesis by way of reactive oxygen species (ROS) production. Furthermore, dimethylmonothioarsinic acid [(CH₃)₂AsS(OH), DMMTA^V], a metabolite of DMA^V, has also been noted because of its higher toxicity. However, the metabolic mechanisms of formation and disappearance of DMA^III and DMMTA^V, and their toxicity are not fully understood. Thus, the purpose of the present study was to clarify the mechanism of metabolic formation of DMMTA^V and DMA^V from DMA^III. The in vitro transformation of arsenicals by treatment with liver homogenate from rodents and sulfur transferase was detected by HPLC-ICP-MS and HPLC-tandem MS. DMMTA^V is produced from DMA^III but not DMA^V by cellular fractions from mouse liver homogenates and by rhodanese from bovine liver in the presence of thiosulfate, a sulfur donor. Not only DMMTA^V thus produced but also DMA^III are re-converted into DMA^V by an in vitro addition of S9 mix. These findings indicate that the metabolic process not only of DMA^III to DMA^V or DMMTA^V but also of DMMTA^V to DMA^V consists of a complicated mode of interaction between monooxygenase including cytochrome P450 (CYP) and/or sulfur transferase.     

Effects of phosphate on arsenate and arsenite sensitivity in two rice (Oryza sativa L.) cultivars of different sensitivity

Arsenate and arsenite sensitivity and arsenate influx tests were conducted for two rice cultivars of different arsenic sensitivity, Azucena and Bala. These were to establish if the mechanism of reduced arsenic sensitivity is achieved through an altered phosphate uptake system, as shown for Holcus lanatus. High phosphate treatments (≥50 μM) provided protection against both arsenate and arsenite. Unlike the H. lanatus tolerance mechanism, in the less sensitive cultivar Bala, arsenate influx did not decrease with phosphate treatment and phosphate transporters appeared to be constitutively upregulated; V_max for arsenate influx remain similar when Bala was grown in the presence or absence of phosphate (V_max - 0.90 and 0.63 nmol g⁻¹ f.wt min⁻¹ respectively). Although mean K_m appear different, Bala did not show lower affinity to arsenate than Azucena in the absence of phosphate (K_m - Azucena, 0.30 mM and Bala, 0.18), while in phosphate treatment, Bala arsenate affinity was half that observed for Azucena (K_m - Azucena, 0.14 and Bala, 0.36 mM). These were low compared to a 4 and 6 fold decrease seen for similar studies on H. lanatus in the absence and presence of phosphate. Phosphate-induced arsenic protection was observed but the mechanism does not resemble that of H. lanatus. Alternative mechanisms were discussed.     

Toxicological properties of the thiolated inorganic arsenic and arsenosugar metabolite thio-dimethylarsinic acid in human bladder cells

Thio-dimethylarsinic acid (thio-DMA^V) has recently been identified as human metabolite after exposure toward both the human carcinogen inorganic arsenic and arsenosugars, which are the major arsenical constituents of marine algae. This study aims to get further insight in the toxic modes of action of thio-DMA^V in cultured human urothelial cells. Among others effects of thio-DMA^V on eight cell death related endpoints, cell cycle distribution, genotoxicity, cellular bioavailability as well as for the first time its impact on DNA damage induced poly(ADP-ribosyl)ation were investigated and compared to effects induced by arsenite. The data indicate that thio-DMA^V exerts its cellular toxicity in a similar or even lower concentration range, however most likely via different mechanisms, than arsenite. Most interestingly, thio-DMA^V decreased damage-induced cellular poly(ADP-ribosyl)ation by 35,000-fold lower concentrations than arsenite. The inhibition of this essential DNA-damage induced and DNA-repair related signaling reaction might contribute to inorganic arsenic induced toxicity, at least in the bladder. Therefore, and also because thio-DMA^V is to date by far the most toxic human metabolite identified after arsenosugar intake, thio-DMA^V should contemporary be fully (also in vivo) toxicologically characterized, to assess risks to human health related to inorganic arsenic but especially arsenosugar dietary intake.     

Microbial oxidation of arsenite in a subarctic environment: diversity of arsenite oxidase genes and identification of a psychrotolerant arsenite oxidiser

Background  

Arsenic is toxic to most living cells. The two soluble inorganic forms of arsenic are arsenite (+3) and arsenate (+5), with arsenite the more toxic. Prokaryotic metabolism of arsenic has been reported in both thermal and moderate environments and has been shown to be involved in the redox cycling of arsenic. No arsenic metabolism (either dissimilatory arsenate reduction or arsenite oxidation) has ever been reported in cold environments (i.e. < 10°C).     

Substrate regulation of ascorbate transport activity in astrocytes

Astrocytes possess a concentrativel-ascorbate (vitamin C) uptake mechanism involving a Na⁺-dependentl-ascorbate transporter located in the plasma membrane. The present experiments examined the effects of deprivation and supplementation of extracellularl-ascorbate on the activity of this transport system. Initial rates ofl-ascorbate uptake were measured by incubating primary cultures of rat astrocytes withl-[¹⁴C]ascorbate for 1 min at 37°C. We observed that the apparent maximal rate of uptake (V _max) increased rapidly (<1 h) when cultured cells were deprived ofl-ascorbate. In contrast, there was no change in the apparent affinity of the transport system forl-[¹⁴C]ascorbate. The increase inV _max was reversed by addition ofl-ascorbate, but notD-isoascorbate, to the medium. The effects of external ascorbate on ascorbate transport activity were specific in that preincubation of cultures withl-ascorbate did not affect uptake of 2-deoxy-D-[³H(G)]glucose. We conclude that the astroglial ascorbate transport system is modulated by changes in substrate availability. Regulation of transport activity may play a role in intracellular ascorbate homeostasis by compensating for regional differences and temporal fluctuations in external ascorbate levels.     

Visualisation of gradients in arsenic concentrations around individual roots of Zea mays L. using agar-immobilized bioreporter bacteria

The classical concept of arsenic transfer into plants through arsenate uptake via phosphate transporters, reduction to arsenite, complexation and compartmentation within vacuoles is challenged by recent identification of bidirectional transporters for arsenite and their potential role in plant As status regulation. Soil-based studies with chemical analysis of soil solution require root mat formation amplifying root effects on their surroundings and additionally denying investigations along individual roots differing in age and function. We tried to overcome these shortcomings by using bioreporter bacteria to visualise the spatial distribution of inorganic arsenic along roots and to characterize inorganic arsenic gradients in the rhizosphere concurrent with root age and branching. Therefore we developed an agar-based carrier element ensuring intimate contact between bioreporters and root-soil system and enabling fast and easy reporter output analysis. We show that inorganic arsenic distribution is related to root development with the highest bioreporter signal induction around lateral roots, which are known to show the highest expression of transporters responsible for bidirectional arsenite flux. Since there is so far no evidence for an arsenate efflux mechanism this is a strong indicator that we observed rather arsenite than arsenate efflux. No signal was detected along the distal region of young adventitious roots, i.e. the region of extension growth and root hair formation. The novel bioreporter assay may thus complement conventional measurements by providing information on the spatial distribution of inorganic arsenic on mm to cm-scale.     

Toxicity and metabolism of subcytotoxic inorganic arsenic in human renal proximal tubule epithelial cells (HK-2)

Arsenic is an environmental toxicant and a human carcinogen. The kidney, a known target organ of arsenic toxicity, is critical for both in vivo arsenic biotransformation and elimination. This study investigates the potential of an immortalized human proximal tubular epithelial cell line, HK-2, to serve as a representative model for low level exposures of the human kidney to arsenic. Subcytotoxic concentrations of arsenite (< or = 10 micromol/L) and arsenate (< 100 micromol/L) were determined by leakage of LDH from cells exposed for 24 h. Threshold concentrations of arsenite (between 1 and 10 micromol/L) and arsenate (between 10 and 25 micromol/L) were found to affect MTT processing by mitochondria. Biotransformation of subcytotoxic arsenite or arsenate was determined using HPLC-ICP-MS to detect metabolites in cell culture media and cell lysates. Following 24 h, analysis of media revealed that arsenite was minimally oxidized to arsenate and arsenate was reduced to arsenite. Only arsenite was detected in cell lysates. Pentavalent methylated arsenicals were not detected in media or lysates following exposure to either inorganic arsenical. The activities of key arsenic biotransformation enzymes--MMAV reductase and AsIII methyltransferase--were evaluated to determine whether HK-2 cells could reduce and methylate arsenicals. When compared to the activities of these enzymes in other animal tissues, the specific activities of HK-2 cells were indicative of a robust capacity to metabolize arsenic. It appears this human renal cell line is capable of biotransforming inorganic arsenic compounds, primarily reducing arsenate to arsenite. In addition, even at low concentrations, the mitochondria are a primary target for toxicity.     

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.     

The effect of arsenate and arsenite on photosynthesis in Scenedesmus obliquus. A Potentiometric study in a closed CO2-system

Abstract

A Potentiometric titration method was used to study the adverse effect of arsenate (As(V)) and arsenite (As(III)) on inorganic carbon uptake in suspensions of the green alga Scenedesmus obliquus. The measurements were performed in a closed CO₂-system with diluted synthetic seawater (1‰ salinity) as ionic medium. Usually, the algal chlorophyll concentration was 0.4 mg dm^?3, while the arsenate- and arsenite-concentrations were varied within the limits 0.1 to 200 μmol dm^?3. In some experiments arsenate toxicity was studied in the presence of 1 to 100 μmol dm^?3 of phosphate (P(V)).

With concentrations of arsenate or arsenite less than 0.1 μmol dm^?3 no toxic effects were observed. However, at As-concentrations of 200 μmol dm^?3, the algal carbon uptake was reduced by 41% with arsenate and 29% with arsenite, i.e., arsenate is more toxic to Scenedesmus obliquus than arsenite. The toxicity of arsenate was negligible in the presence of a ten fold excess of phosphate. This is probably due to chemical similarities between arsenate and phosphate causing competition between the ions for the binding sites.

The importance of taking the speciation as well as the buffer capacity of the algal system into account, when calculating the carbon uptake, is also discussed.