Rapid Biotransformation of Arsenate into Oxo-Arsenosugars by a Freshwater Unicellular Green Alga,Chlamydomonas reinhardtii

We examined the short-term metabolic processes of arsenate for 24 h in a freshwater unicellular green alga, Chlamydomonas reinhardtii wild-type strain CC-125. The arsenic species in the algal extracts were identified by high-performance liquid chromatography/inductively coupled plasma mass spectrometry after water extraction using a sonicator. Speciation analyses of arsenic showed that the levels of arsenite, arsenate, and methylarsonic acid in the cells rapidly increased for 30 min to 1 h, and those of dimethylarsinic acid and oxo-arsenosugar-glycerol also tended to increase continuously for 24 h, while that of oxo-arsenosugar-phosphate was quite low and fluctuated throughout the experiment. These results indicate that this alga can rapidly biotransform arsenate into oxo-arsenosugar-glycerol for at least 10 min and then oxo-arsenosugar-phosphate through both reduction of incorporated arsenate to arsenite and methylation of arsenite and/or arsenate retained in the cells to dimethylarsinic acid via methylarsonic acid as an possible intermediate.     

ars1, an Arabidopsis mutant exhibiting increased tolerance to arsenate and increased phosphate uptake

Arsenic is one of the most toxic pollutants at contaminated sites, yet little is known about the mechanisms by which certain plants survive exposure to high arsenic levels. To gain insight into the mechanisms of arsenic tolerance in plants, we developed a genetic screen to isolate Arabidopsis thaliana mutants with altered tolerance to arsenic. We report here on the isolation of a mutant arsenic resisant 1 (ars1) with increased tolerance to arsenate. ars1 germinates and develops under conditions that completely inhibit growth of wild-type plants and shows a semi-dominant arsenic resistance phenotype. ars1 accumulates levels of arsenic similar to that accumulated by wild-type plants, suggesting that ars1 plants have an increased ability to detoxify arsenate. However, ars1 plants produce phytochelatin levels similar to levels produced by the wild type, and the enhanced resistance of ars1 is not abolished by the gamma-glutamylcysteine synthetase inhibitor l-buthionine sulfoxime (BSO). Furthermore, ars1 plants do not show resistance to arsenite or other toxic metals such as cadmium and chromium. However, ars1 plants do show a higher rate of phosphate uptake than that shown by wild-type plants, and wild-type plants grown with an excess of phosphate show increased tolerance to arsenate. Traditional models of arsenate tolerance in plants are based on the suppression of phosphate uptake pathways and consequently on the reduced uptake of arsenate. Our data suggest that arsenate tolerance in ars1 could be due to a new mechanism mediated by increased phosphate uptake in ars1. Models discussing how increased phosphate uptake could contribute to arsenate tolerance are discussed.     

Genomic potential for arsenic efflux and methylation varies among global Prochlorococcus populations

The globally significant picocyanobacterium Prochlorococcus is the main primary producer in oligotrophic subtropical gyres. When phosphate concentrations are very low in the marine environment, the mol:mol availability of phosphate relative to the chemically similar arsenate molecule is reduced, potentially resulting in increased cellular arsenic exposure. To mediate accidental arsenate uptake, some Prochlorococcus isolates contain genes encoding a full or partial efflux detoxification pathway, consisting of an arsenate reductase (arsC), an arsenite-specific efflux pump (acr3) and an arsenic-related repressive regulator (arsR). This efflux pathway was the only previously known arsenic detox pathway in Prochlorococcus. We have identified an additional putative arsenic mediation strategy in Prochlorococcus driven by the enzyme arsenite S-adenosylmethionine methyltransferase (ArsM) which can convert inorganic arsenic into more innocuous organic forms and appears to be a more widespread mode of detoxification. We used a phylogenetically informed approach to identify Prochlorococcus linked arsenic genes from both pathways in the Global Ocean Sampling survey. The putative arsenic methylation pathway is nearly ubiquitously present in global Prochlorococcus populations. In contrast, the complete efflux pathway is only maintained in populations which experience extremely low PO₄:AsO₄, such as regions in the tropical and subtropical Atlantic. Thus, environmental exposure to arsenic appears to select for maintenance of the efflux detoxification pathway in Prochlorococcus. The differential distribution of these two pathways has implications for global arsenic cycling, as their associated end products, arsenite or organoarsenicals, have differing biochemical activities and residence times.     

Large arrays of tandemly repeated DNA sequences in the green alga Chlamydomonas reinhardtii

We describe the characterization of tandemly repeated DNA sequences, which resemble the satellite DNA sequences of multicellular eucaryotes, in the unicellular green alga Chlamydomonas reinhardtii. Restriction enzymes that cleave C. reinhardtii DNA relatively frequently produce a number of high molecular weight DNA fragments in addition to the bulk of low molecular weight DNA fragments. pTANC 1.5 contains a 1.5 kb Sau3A fragment cloned from one of these large bands. pTANC 1.5 hybridized to at least three large arrays (200 to 700 kb) of tandemly repeated DNA sequences in the cell-wall-deficient strain cw1.5. These arrays are composed of repeat units that are each cleaved once by BamHl into bands of 1.5, 1.9, 2.0 and 2.5 kb in size. The copy numbers of the 1.5, 1.9, 2.0 and 2.5 kb Bamhl bands vary between different C. reinhardtii strains. Chlamydomonas smithii and a number of C. reinhardtii strains are deficient in all four BamHl bands. Genetic analysis of wild-type strain 137c, which is deficient in the 2.0 kb BamHl band, indicates that the 1.5, 1.9 and 2.5 kb BamHl bands derive from at least five loci. The 1.5, 1.9 and 2.5 kb repeat units are not extensively interspersed with each other in strain 137c. Pulsed-field gel electrophoresis of intact C. reinhardtii chromosomes indicates that TANC arrays are present on more than one chromosome.     

Effects of arsenate and phosphate on their accumulation by an arsenic-hyperaccumulator Pteris vittata L.

Arsenate and phosphate interactions are important for better understanding their uptake and accumulation by plant due to their similarities in chemical behaviors. The present study examined the effects of arsenate and phosphate on plant biomass and uptake of arsenate and phosphate by Chinese brake (Pteris vittata L.), a newly-discovered arsenic hyperaccumulator. The plants were grown for 20 weeks in a soil, which received the combinations of 670, 2670, or 5340 mol kg^–1 arsenate and 800, 1600, or 3200 mol kg^–1 phosphate, respectively. Interactions between arsenate and phosphate influenced their availability in the soil, and thus plant growth and uptake of arsenate and phosphate. At low and medium arsenate levels (670 and 2670 mol kg^–1), phosphate had slight effects on arsenate uptake by and growth of Chinese brake. However, phosphate substantially increased plant biomass and arsenate accumulation by alleviating arsenate phytotoxicity at high arsenate levels (5340 mol kg^–1). Moderate doses of arsenate increased plant phosphate uptake, but decreased phosphate concentrations at high doses because of its phytotoxicity. Based on our results, the minimum P/As molar ratios should be at least 1.2 in soil solution or 1.0 in fern fronds for the growth of Chinese brake. Our findings suggest that phosphate application may be an important strategy for efficient use of Chinese brake to phytoremediate arsenic contaminated soils. Further study is needed on the mechanisms of interactive effects of arsenate and phosphate on Chinese brake in hydroponic systems.     

Characterization of As(III) oxidizing <Emphasis Type="Italic">Achromobacter</Emphasis> sp. strain N2: effects on arsenic toxicity and translocation in rice

Achromobacter sp. strain N2 was isolated from a pyrite-cinder-contaminated soil and presented plant growth promoting traits (ACC deaminase activity, production of indole-3-acetic and jasmonic acids, siderophores secretion, and phosphate solubilization) and arsenic transformation abilities. Achromobacter sp. strain N2 was resistant to different metals and metalloids, including arsenate (100 mM) and arsenite (5 mM). The strain was resistant to ionic stressors (i.e., arsenate and NaCl), whereas bacterial growth was impaired by osmotic stress. Strain N2 was able to oxidize 1.0 mmol L^?1 of arsenite to arsenate in 72 h. This evidence was supported by the retrieval of an arsenite oxidase AioA gene highly homologous to arsenite oxidases of Achromobacter and Alcaligenes species. Rice seeds of Oryza sativa (var. Loto) were bio-primed with ACCD-induced and non-induced cells in order to evaluate the effect of inoculation on rice seedlings growth and arsenic uptake. The bacterization with ACCD-induced cells significantly improved seed germination and seedling heights if compared with the seeds inoculated with non-induced cells and non-primed seeds. Enhanced arsenic uptake was evidenced in the presence of ACCD-induced cells, suggesting a role of ACCD activity on the mitigation of the toxicity of arsenic accumulated by the plant. This kind of responses should be taken into account when proposing PGP strains for improving plant growth in arsenic-rich soils.     

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.     

Rhizosphere colonization and arsenic translocation in sunflower (Helianthus annuus L.) by arsenate reducing Alcaligenes sp. strain Dhal-L

In the present study, six arsenic-resistant strains previously isolated were tested for their plant growth promoting characteristics and heavy metal resistance, in order to choose one model strain as an inoculum for sunflower plants in pot experiments. The aim was to investigate the effect of arsenic-resistant strain on sunflower growth and on arsenic uptake from arsenic contaminated soil. Based on plant growth promoting characteristics and heavy metal resistance, Alcaligenes sp. strain Dhal-L was chosen as an inoculum. Beside the ability to reduce arsenate to arsenite via an Ars operon, the strain exhibited 1-amino-cyclopropane-1-carboxylic acid deaminase activity and it was also able to produce siderophore and indole acetic acid. Pot experiments were conducted with an agricultural soil contaminated with arsenic (214 mg kg^?1). A real time PCR method was set up based on the quantification of ACR3(2) type of arsenite efflux pump carried by Alcaligenes sp. strain Dhal-L, in order to monitor presence and colonisation of the strain in the bulk and rhizospheric soil. As a result of strain inoculation, arsenic uptake by plants was increased by 53 %, whereas ACR3(2) gene copy number in rhizospheric soil was 100 times higher in inoculated than in control pots, indicating the colonisation of strain. The results indicated that the presence of arsenate reducing strains in the rhizosphere of sunflower influences arsenic mobilization and promotes arsenic uptake by plant.     

Arsenate transport and reduction in the hyper-tolerant fungus Aspergillus sp. P37

Aspergillus sp. P37 is able to grow at arsenate concentrations of 0.2 M--more than 20-fold higher than that withstood by reference microorganisms such Escherichia coli, Saccharomyces cerevisiae and Aspergillus nidulans. This paper examines the transport of arsenate and phosphate and the reduction of arsenate in Aspergillus sp. P37. These properties were compared with the corresponding properties of the archetype strain Aspergillus nidulans TS1. Both uptake and efflux of arsenate were inhibited by carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, suggesting that the transport system(s) is(are) membrane-potential dependent. As uptake of arsenate and phosphate are higher in Aspergillus sp. P37 than in A. nidulans, the increase in arsenate resistance cannot be accounted for by a change in uptake. Cells of both strains loaded with arsenic slowly released the oxyanion. Speciation of the arsenic in the medium showed an enhanced level of arsenate reduction in Aspergillus sp. P37. These data suggest that increased arsenate reduction is at least in part responsible for the hyper-tolerant phenotype of this fungus.     

Effects of Phosphate on Arsenate Uptake and Translocation in Nonmetallicolous and Metallicolous Populations of Pteris Vittata L. Under Solution Culture

An arsenic hyperaccumulator, Pteris vittata L., is common in nature and could occur either on As-contaminated soils or on uncontaminated soils. However, it is not clear whether phosphate transporter play similar roles in As uptake and translocation in nonmetallicolous and metallicolous populations of P. vittata. Five populations were used to investigate effects of phosphate on arsenate uptake and translocation in the plants growing in 1.2 L 20% modified Hoagland's nutrient solution containing either 100 μM phosphate or no phosphate and 10 μM arsenate for 1, 2, 6, 12, 24 h, respectively. The results showed that the nonmetallicolous populations accumulated apparently more As in their fronds and roots than the metallicolous populations at both P supply levels. Phosphate significantly (P < 0.01) decreased frond and root concentrations of As during short time solution culture. In addition, the effects of phosphate on As translocation in P. vittata varied among different time-points during time-course hydroponics (1–24 h). The present results indicated that the inhibitory effect of phosphate on arsenate uptake was larger in the three nonmetallicolous populations than those in the two metallicolous populations of P. vittata.     

Effects of arsenate on the growth and microcystin production of Microcystis aeruginosa isolated from Taiwan as influenced by extracellular phosphate

Arsenic pollution and eutrophication are both prominent issues in the aquaculture ponds of Taiwan. It is important to study the effects of arsenic on algal growth and toxin production in order to assess the ecological risk of arsenic pollution, or at least to understand naturally occurring ponds. The sensitivity of algae to arsenate has often been linked to the structural similarities between arsenate and phosphate. Thus, in this study we examined the effects of arsenate (10⁻⁸ to 10⁻⁴ M) on Microcystis aeruginosa TY-1 isolated from Taiwan, under two phosphate regimes. The present study showed that M. aeruginosa TY-1 was arsenate tolerant up to 10⁻⁴ M, and that this tolerance was not affected by extracellular phosphate. However, it seems that extracellular phosphate contributed to microcystin production and leakage by M. aeruginosa in response to arsenate. Under normal phosphate conditions, total toxin yields after arsenate treatment followed a typical inverted U-shape hormesis, with a peak value of 2.25 ± 0.06 mg L⁻¹ in the presence of 10⁻⁷ M arsenate, whereas 10⁻⁸ to 10⁻⁶ M arsenate increased leakage of ∼75% microcystin. Under phosphate starvation, total toxin yields were not affected by arsenate, while 10⁻⁶ and 10⁻⁵ M arsenate stimulated microcystin leakage. It is suggested that arsenate may play a role in the process of microcystin biosynthesis and excretion. Given the arsenic concentrations in aquaculture ponds in Taiwan, arsenate favors survival of toxic M. aeruginosa in such ponds, and arsenate-stimulated microcystin production and leakage may have an impact on the food chain.     

A brief review of microbial arsenate respiration

In this review, we summarize the important recent findings relating to arsenate respi‐ration by bacteria. A brief discussion of freshwater arsenic cycling is provided, with attention placed on the microbial contributions to this cycle. The basic evidence for microbial growth on arsenate is presented for studies with both consortia and isolates, followed by a summary of the physiology and phytogeny of four arsenate‐respiring bac‐teria: Chrysiogenes arsenatis strain BAL‐1^T, Desulfotomaculum auripigmentum strain OREX‐4, Sulfurospirillum arsenophilus strain MIT‐13, and S. barnesii strain SES‐3. Drawing on biochemical studies of the arsenate reductasefrom S. barnesii strain SES‐3, a preliminary model for growth on arsenate is proposed. We conclude with a discussion of the importance of microbial arsenate reduction in the environment.     

Phosphorus Improves Arsenic Phytoremediation by Anadenanthera Peregrina by Alleviating Induced Oxidative Stress

Due to similarities in their chemical behaviors, studies examining interactions between arsenic (As)—in special arsenate—and phosphorus (P) are important for better understanding arsenate uptake, toxicity, and accumulation in plants. We evaluated the effects of phosphate addition on plant biomass and on arsenate and phosphate uptake by Anadenanthera peregrina, an important Brazilian savanna legume. Plants were grown for 35 days in substrates that received combinations of 0, 10, 50, and 100 mg kg^?1 arsenate and 0, 200, and 400 mg kg^?1 phosphate. The addition of P increased the arsenic-phytoremediation capacity of A. peregrina by increasing As accumulation, while also alleviating As-induced oxidative stress. Arsenate phytotoxicity in A. peregrina is due to lipid peroxidation, but not hydrogen peroxide accumulation. Added P also increased the activity of important reactive oxygen species-scavenging enzymes (catalase and ascorbate peroxidase) that help prevent lipid peroxidation in leaves. Our findings suggest that applying P represents a feasible strategy for more efficient As phytoremediation using A. peregrina.     

Arsenic resistance in Pteris vittata L.: identification of a cytosolic triosephosphate isomerase based on cDNA expression cloning in Escherichia coli

Arsenic hyperaccumulator Pteris vittata L. (Chinese brake fern) grows well in arsenic-contaminated media, with an extraordinary ability to tolerate high levels of arsenic. An expression cloning strategy was employed to identify cDNAs for the genes involved in arsenic resistance in P. vittata. Excised plasmids from the cDNA library of P. vittata fronds were introduced into Escherichia coli XL-1 Blue and plated on medium containing 4 mM of arsenate, a common form of arsenic in the environment. The deduced amino acid sequence of an arsenate-resistant clone, PV4-8, had cDNA highly homologous to plant cytosolic triosephosphate isomerases (cTPI). Cell-free extracts of PV4-8 had 3-fold higher level of triosephosphate isomerase (TPI) specific activities than that found in E. coli XL-1 Blue and had a 42 kD fusion protein immunoreactive to polyclonal antibodies raised against recombinant Solanum chacoense cTPI. The PV4-8 cDNA complemented a TPI-deficient E. coli mutant. PV4-8 expression improved arsenate resistance in E. coli WC3110, a strain deficient in arsenate reductase but not in AW3110 deficient for the whole ars operon. This is consistent with the hypothesis that PV4-8 TPI increased arsenate resistance in E. coli by directly or indirectly functioning as an arsenate reductase. When E. coli tpi gene was expressed in the same vector, bacterial arsenate resistance was not altered, indicating that arsenate tolerance was specific to P. vittata TPI. Paradoxically, P. vittata TPI activity was not more resistant to inhibition by arsenate in vitro than its bacterial counterpart suggesting that arsenate resistance of conventional TPI reaction was not the basis for the cellular arsenate resistance. P. vittata TPI activity was inhibited by incubation with reduced glutathione while bacterial TPI was unaffected. Consistent with cTPI’s role in arsenate reduction, bacterial cells expressing fern TPI had significantly greater per cent of cellular arsenic as arsenite compared to cells expressing E. coli TPI. Excised frond tissue infiltrated with arsenate reduced arsenate significantly more under light than dark. This research highlights a novel role for P. vittata cTPI in arsenate reduction.     

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.     

Mutational and gene expression analysis of mtrDEF,omcA and mtrCAB during arsenate and iron reduction in Shewanella sp. ANA‐3

Arsenate respiration and Fe(III) reduction are important processes that influence the fate and transport of arsenic in the environment. The goal of this study was to investigate the impact of arsenate on Fe(III) reduction using arsenate and Fe(III) reduction deficient mutants of Shewanella sp. strain ANA‐3. Ferrihydrite reduction in the absence of arsenate was similar for an arsenate reduction mutant (arrA and arsC deletion strain of ANA‐3) compared with wild‐type ANA‐3. However, the presence of arsenate adsorbed onto ferrihydrite impeded Fe(III) reduction for the arsenate reduction mutant but not in the wild‐type. In an Fe(III) reduction mutant (mtrDEF, omcA, mtrCAB null mutant of ANA‐3), arsenate was reduced similarly to wild‐type ANA‐3 indicating the Fe(III) reduction pathway is not required for ferrihydrite‐associated arsenate reduction. Expression analysis of the mtr/omc gene cluster of ANA‐3 showed that omcA and mtrCAB were expressed under soluble Fe(III), ferrihydrite and arsenate growth conditions and not in aerobically grown cells. Expression of arrA was greater with ferrihydrite pre‐adsorbed with arsenate relative to ferrihydrite only. Lastly, arrA and mtrA were simultaneously induced in cells shifted to anaerobic conditions and exposed to soluble Fe(III) and arsenate. These observations suggest that, unlike Fe(III), arsenate can co‐induce operons (arr and mtr) implicated in arsenic mobilization.     

Interaction of arsenate with phosphate-transport systems in wild- type and mutant Streptococcus faecalis

Harold, F. M. (National Jewish Hospital, Denver, Colo.), and J. R. Baarda. Interaction of arsenate with phosphate-transport systems in wild-type and mutant Streptococcus faecalis. J. Bacteriol. 91:2257-2262. 1966.-Arsenate competitively inhibits the growth of Streptococcus faecalis, primarily by competition with phosphate for a common transport system. Arsenate is itself accumulated by the cells; the uptake requires metabolic energy, and the intracellular arsenate level may reach 0.01 m. Cells loaded with arsenate have lost the capacity to take up radioactive glutamate, rubidium, phosphate, or arsenate itself, apparently by the uncoupling of adenosine triphosphate generation. The pH dependence of arsenate uptake is complex. At low concentrations of extracellular arsenate, uptake by the wild-type strain 9790 exhibits a single maximum about pH 8; mutant PT-1, previously shown to be defective in phosphate uptake, takes up essentially no arsenate. At high concentrations of arsenate, uptake by the wild type is bimodal with maxima at pH 5.5 and 9; the uptake curve for mutant PT-1 corresponds to the shoulder in the curve for the wild type. The apparent dissociation constant for arsenate uptake by the wild type is approximately 10(-5)m from pH 5 to 9, whereas that for mutant PT-1 is about 5 x 10(-5) M at pH 5 and rises rapidly with increasing pH. The results confirm the earlier conclusion that the lesion in mutant PT-1 resides in the transport of phosphate and arsenate. It is proposed that the wild type has two distinct transport systems, whereas the mutant has lost the one with alkaline pH optimum.     

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.     

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.     

Hydroponic Screening of Shrub Willow (Salix Spp.) for Arsenic Tolerance and Uptake

Shrub willows have demonstrated potential in many types of phytoremediation applications. Hydroponic culture was used to assess arsenic (As) tolerance and uptake by four shrub willow clones and to determine the effects of phosphate on As accumulation. After 4 weeks of growth in the absence of As, plants received one of four treatments: 0.25X Hoagland's minus P (?P), 0.25X Hoagland's minus P plus 100 μM arsenate (As100(?P)), 0.25X Hoagland's minus P plus 250 μM arsenate (As250(?P)), and 0.25X Hoagland's plus 250 μM arsenate (As250(+P)). Except for treatment As250(+P), phosphate was excluded due to its tendency to interfere with As uptake. After 3 weeks of treatment, plants were separated into root, leaf, and stem tissues. Biomass production and transpiration were used to quantify As tolerance. There was wide variation among clones in As tolerance and uptake. The presence of phosphate in solution alleviated the negative impacts of As on biomass and transpiration and also increased aboveground As accumulation, suggesting that phosphate may play a role in reducing toxicity and enhancing As uptake by willow shrubs. These findings offer insight into As tolerance and uptake in Salix spp. and add to the growing body of evidence supporting the use of shrub willow for phytoremediation.