Arsenic Hyperaccumulator Research Articles

ヒ素高蓄積植物による土壌ヒ素の吸収・除去と土壌微生物の亜ヒ酸酸化に関する研究

モエジマシダ(Pteris vittata)等のヒ素高蓄積植物はヒ素土壌汚染の浄化に適用可能と考えられている．本研究では, モエジマシダを用いたヒ素のファイトエクストラクション法による浄化技術に着目し, それによるヒ素吸収における微生物の関与の度合いを明らかにすることを目的として行った. 微生物植種の添加に伴うシダの栽培実験と土壌ヒ素の酸化試験を行い, ヒ素の土壌からシダへの移行特徴および形態変化を解析した. その結果, 土壌中の亜ヒ酸はヒ酸に酸化されたのち, その多くはシダにより吸収されることと, このヒ素酸化は微生物による作用であることが強く示された. さらに, シダ根圏試料に対して亜ヒ酸酸化酵素遺伝子aroAをターゲットとしたT-RFLP解析より, シダによるヒ素吸収に先立つヒ素形態の変化には, 土壌中のaroAを有する微生物が関与していると示唆された.

Compartmentation and complexation of metals in hyperaccumulator plants

Hyperaccumulators are being intensely investigated. They are not only interesting in scientific context due to their “strange” behavior in terms of dealing with high concentrations of metals, but also because of their use in phytoremediation and phytomining, for which understanding the mechanisms of hyperaccumulation is crucial. Hyperaccumulators naturally use metal accumulation as a defense against herbivores and pathogens, and therefore deal with accumulated metals in very specific ways of complexation and compartmentation, different from non-hyperaccumulator plants and also non-hyperaccumulated metals. For example, in contrast to non-hyperaccumulators, in hyperaccumulators even the classical phytochelatin-inducing metal, cadmium, is predominantly not bound by such sulfur ligands, but only by weak oxygen ligands. This applies to all hyperaccumulated metals investigated so far, as well as hyperaccumulation of the metalloid arsenic. Stronger ligands, as they have been shown to complex metals in non-hyperaccumulators, are in hyperaccumulators used for transient binding during transport to the storage sites (e.g., nicotianamine) and possibly for export of Cu in Cd/Zn hyperaccumulators [metallothioneins (MTs)]. This confirmed that enhanced active metal transport, and not metal complexation, is the key mechanism of hyperaccumulation. Hyperaccumulators tolerate the high amount of accumulated heavy metals by sequestering them into vacuoles, usually in large storage cells of the epidermis. This is mediated by strongly elevated expression of specific transport proteins in various tissues from metal uptake in the shoots up to the storage sites in the leaf epidermis. However, this mechanism seems to be very metal specific. Non-hyperaccumulated metals in hyperaccumulators seem to be dealt with like in non-hyperaccumulator plants, i.e., detoxified by binding to strong ligands such as MTs.

Impact of arsenic on uptake and bio-accumulation of antimony by arsenic hyperaccumulator Pteris vittata

Individual uptake of As and Sb species in Pteris vittata have been investigated, but little information is available how uptake is affected if both metalloids are simultaneously present in different amounts. We investigated the uptake of antimony and its speciation in Pteris vittata cultivated in quartz substrate with, versus without, co-contamination with arsenic and a contaminated soil for 7 weeks. Applying HPLC-ICP-MS technique Sb(V), Sb(III), As(III), and As(V) could be identified as main species in aqueous extracts of roots and fronds with up to 230 mg kg−1 of total Sb in the roots. Adding increasing amounts of As to the quartz substrate resulted in increasing uptake of Sb. In contrast to As, which is readily transferred to the fronds, Sb is primarily accumulated in the roots with Sb(V) being the dominant species (>90% of Sb). The addition of As doesn't result in enhanced translocation of Sb into the fronds.

ARSENIC IN THE RHIZOSPHERE SOIL SOLUTION OF FERNS

The aim of this study was to explore the evidence of arsenic hyperaccumulation in plant rhizosphere solutions. Six common fern plants were selected and grown in three types of substrate: arsenic (As) -tailings, As-spiked soil, and soil-As-tailing composites. A rhizobox was designed with an in-situ collection of soil solutions to analyze changes in the As concentration and valence as well as the pH, dissolved organic carbon (DOC) and total nitrogen (TN). Arsenite composed less than 20% of the total As, and As depletion was consistent with N depletion in the rhizosphere solutions of the various treatments. The As concentrations in the rhizosphere and non-rhizosphere solutions in the presence of plants were lower than in the respective controls without plants, except for in the As-spiked soils. The DOC concentrations were invariably higher in the rhizosphere versus non-rhizosphere solutions from the various plants; however, no significant increase in the DOC content was observed in Pteris vittata, in which only a slight decrease in pH appeared in the rhizosphere compared to non-rhizosphere solutions. The results showed that As reduction by plant roots was limited, acidification-induced solubilization was not the mechanism for As hyperaccumulation.

Hijacking membrane transporters for arsenic phytoextraction

Arsenic is a toxic metalloid and recognized carcinogen. Arsenate and arsenite are the most common arsenic species available for uptake by plants. As an inorganic phosphate (Pi) analog, arsenate is acquired by plant roots through endogenous Pi transport systems. Inside the cell, arsenate is reduced to the thiol-reactive form arsenite. Glutathione (GSH)-conjugates of arsenite may be extruded from the cell or sequestered in vacuoles by members of the ATP-binding cassette (ABC) family of transporters. In the present study we sought to enhance both plant arsenic uptake through Pi transporter overexpression, and plant arsenic tolerance through ABC transporter overexpression. We demonstrate that Arabidopsis thaliana plants overexpressing the high-affinity Pi transporter family members, AtPht1;1 or AtPht1;7, are hypersensitive to arsenate due to increased arsenate uptake. These plants do not exhibit increased sensitivity to arsenite. Co-overexpression of the yeast ABC transporter YCF1 in combination with AtPht1;1 or AtPht1;7 suppresses the arsenate-sensitive phenotype while further enhancing arsenic uptake. Taken together, our results support an arsenic transport mechanism in which arsenate uptake is increased through Pi transporter overexpression, and arsenic tolerance is enhanced through YCF1-mediated vacuolar sequestration. This work substantiates the viability of coupling enhanced uptake and vacuolar sequestration as a means for developing a prototypical engineered arsenic hyperaccumulator.

Bacteria-Mediated Arsenic Oxidation and Reduction in the Growth Media of Arsenic Hyperaccumulator Pteris vittata

Microbes play an important role in arsenic transformation and cycling in the environment. Microbial arsenic oxidation and reduction were demonstrated in the growth media of arsenic hyperaccumulator Pteris vittata L. All arsenite (AsIII) at 0.1 mM in the media was oxidized after 48 h incubation. Oxidation was largely inhibited by antibiotics, indicating that bacteria played a dominant role. To identify AsIII oxidizing bacteria, degenerate primers were used to amplify ∼500 bp of the AsIII oxidase gene aioA (aroA) using DNA extracted from the media. One aioA (aroA)-like sequence (MG-1, tentatively identified as Acinetobacter sp.) was amplified, exhibiting 82% and 91% identity in terms of gene and deduced protein sequence to those from Acinetobacter sp. 33. In addition, four bacterial strains with different arsenic tolerance were isolated and identified as Comamonas sp.C-1, Flavobacterium sp. C-2, Staphylococcus sp. C-3, and Pseudomonas sp. C-4 using carbon utilization, fatty acid profiles, and/or sequencing 16s rRNA gene. These isolates exhibited dual capacity for both AsV reduction and AsIII oxidation under ambient conditions. Arsenic-resistant bacteria with strong AsIII oxidizing ability may have potential to improve bioremediation of AsIII-contaminated water using P. vittata and/or other biochemical strategies.

Arsenic-induced morphogenic response in roots of arsenic hyperaccumulator fern Pteris vittata

On the assumption that arsenic induces stress morphogenetic responses involved in As tolerance and hyperaccumulation in the Pteris vittata fern, we analyzed the root system of young sporophytes grown in 250, 334, and 500μM As for five days and for 14 days. Anatomical and histological analyses were performed in plants grown for five days to evaluate the number, position, length and differentiation pattern of root hairs. AgNOR staining, employed to study nucleolus behavior in root apices, showed that arsenic influences nucleolar activity (evaluated by nucleolus size, number and absorbance) in the root meristem. In plants treated with 250 and 334μM As an acropetal shift of root hair development and an increase in hair length and density were observed, linked to an ectopic pattern of differentiation. The opposite trend was recorded in plants treated with 500μM As. It is worth noting the presence of living border-like cells, not yet observed in ferns, and their increase following As treatments. Analysis and vitality of border-like cells were surveyed after 14 days of treatments. In conclusion As treatments elicited a stress-induced morphogenic response which, by modifying the differentiation pattern, number and length of root hairs, modulating nucleolar activity and interacting with the rhizosphere by inducing border-like cell production, may adjust the rate of root uptake and its metabolic activity.

Antioxidative mechanisms on selenium accumulation in Pteris vittata L., a potential selenium phytoremediation plant

Selenium (Se) contamination due to industrial activities has received increasing concerns. Phytoremediation has been suggested to be an efficient and feasible way to remove Se from Se-contaminated environment. Recently, an arsenic (As) hyperaccumulator Pteris vittata L. (Chinese Brake fern) was found to be a Se accumulator. This study was carried out to investigate Se accumulation mechanisms concentrating on antioxidant responses of this plant to six levels of selenite (0, 1, 2, 5, 10, and 20 mg/L). The results showed that Chinese Brake fern can accumulate a large amount of Se without any visible toxic symptoms and significant decreases in its biomass. However, the root took up more Se than the fronds. The highest concentration of Se in the roots and fronds was 1.536 mg/kg and 242 mg/kg, respectively, demonstrating a typical accumulation character to Se. Addition of 2 mg/L Se decreased, but ≥ 5 mg/L Se enhanced the production of malondialdehyde (MDA), suggesting an antioxidant role of low dosages of Se. The enzymes of catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POD) contributed their anti-oxidative functions only under low dosages of Se, as shown by their increased activities at Se levels ≤ 5 mg/L and lowered activities at Se levels > 5 mg/L. The concentration of glutathione (GSH) and enzyme activity of glutathione reductase (GR) were stimulated by ≥ 5 mg/L Se. Superoxide dismutase (SOD) activity was also enhanced by 20 mg/L Se. Our results suggest that SOD, GSH and GR were likely responsible for, but enzymes of POD, APX, and CAT have limited roles in Se accumulation in Chinese Brake fern.  

Arsenic and Antimony Transporters in Eukaryotes

Arsenic and antimony are toxic metalloids, naturally present in the environment and all organisms have developed pathways for their detoxification. The most effective metalloid tolerance systems in eukaryotes include downregulation of metalloid uptake, efflux out of the cell, and complexation with phytochelatin or glutathione followed by sequestration into the vacuole. Understanding of arsenic and antimony transport system is of high importance due to the increasing usage of arsenic-based drugs in the treatment of certain types of cancer and diseases caused by protozoan parasites as well as for the development of bio- and phytoremediation strategies for metalloid polluted areas. However, in contrast to prokaryotes, the knowledge about specific transporters of arsenic and antimony and the mechanisms of metalloid transport in eukaryotes has been very limited for a long time. Here, we review the recent advances in understanding of arsenic and antimony transport pathways in eukaryotes, including a dual role of aquaglyceroporins in uptake and efflux of metalloids, elucidation of arsenic transport mechanism by the yeast Acr3 transporter and its role in arsenic hyperaccumulation in ferns, identification of vacuolar transporters of arsenic-phytochelatin complexes in plants and forms of arsenic substrates recognized by mammalian ABC transporters.

Bioaccumulation of Cr, Hg, As, Pb, Cu and Ni with the ability for hyperaccumulation by Amaranthus dubius

Amaranthus dubius is a nutritious leafy vegetable that is widely distributed in Africa, Asia and South America. The rapid growth and biomass makes them one of the highest yielding leafy crops, which may be valuable for phytoremediation. Thus, this study investigates the potential of A. dubius for the hyperaccumulation and distribution of chromium (Cr), mercury (Hg), arsenic (As), lead (Pb), copper (Cu) and nickel (Ni) in the different plant organs. The phytoremediation was calculated by the bioconcentration factor and translocation factor of plants grown under controlled conditions in a tunnel house. The metal accumulated was investigated using inductively coupled plasma mass spectroscopy (ICPMS). Following exposure to 25, 75 and 100 ppm of Cr, Hg, As, Pb, Cu and Ni respectively for four days, A. dubius accumulates Cr, Hg and Pb at the low concentration (25 ppm) and translocates it to the shoots, but at higher concentrations (75 and 100 ppm) the metal is only stored in the roots. In the case of As accumulation at all concentrations, the metal is translocated to the shoots; Cu and Ni indicate that the metal is stored in the roots mainly. This study indicates that A. dubius has limited potential for the bioaccumulation of Cr, Hg, Pb, Cu and Ni, but is capable of hyperaccumulating of As. Key words: Amaranthus dubius, bioconcentration factor, translocation factor, inductively coupled plasma mass spectroscopy.

砷形态对黑藻和竹叶眼子菜有机酸含量的影响

Organic acids widely exist in plants and environment.They can chelate metals and participate in the metabolitic process of metal uptake,translocation and accumulation,and thus play an important role in plant metal tolerance and detoxification.The changes of organic acid contents in terrestrial plants under arsenic stress have been well documented,but little information is available regarding the changes of organic acids in aquatic plants under different arsenic speciation treatments.The arsenic uptake abilities varied among the aquatic plants.The comparsion between plant species regarding their arsenic-accumulating abilities could provide valuable information in understanding arsenic uptake mechanism.The results of a previous study showed that Hydrilla verticillata can hyperaccumulate arsenic with an average concentration of 282 mg/kg in plants from water containing 0.046 mg/L As.However,the concentrations of arsenic in Potamogeton malaianus were only 129 and 121 mg/kg when exposed to 5.0 mg/L As(Ⅲ) or As(Ⅴ),respectively. In the present research,a hydroponic culture method,together with gas chromatography-mass spectrometry(GC-MS),was used to evaluate the organic acid contents in plants and root exudation of Hydrilla verticillata(arsenic hyperaccumulator) and Potamogeton malaianus(non-hyperaccumulator) as affected by arsenic of different species [As(Ⅲ),As(Ⅴ),dimethylarsinate(DMA)].Arsenic concentrations were applied in the range of 0—5.0 mg/L.The results showed that the natural contents of oxalic,malonic,citric,malic and palmitic acids in H.verticillata were significantly higher than those of P.malaianus(P0.05).In As(Ⅲ) treatments,the contents of citric,malic,palmitic,linolenic and total organic acids significantly decreased in H.verticillata,and the similar decreasing trend was also observed for palmitic acid content in P.malaianus.In the As(Ⅴ) treatments,a significant decrease was observed for the contents of palmitic acid and total organic acids in H.verticillata and palmitic acid in P.malaianus.In the DMA treantments,the contents of oxalic,citric and malic acids significantly decreased in H.verticillata,and the decreasing trend was also observed for the contents of palmitic,linolenic and total organic acids in P.malaianus.The contents of palmitic acid,linolenic acid,or total organic acid was negatively correlated to arsenic concentrations in H.verticillata in As(Ⅲ) treantments,and the negative correlation was also found between oxalic acid and arsenic concentration in As(Ⅴ) treantments(P0.05).A significantly negative correlation was also noted between the contents of palmitic acid,linolenic acid,or total organic acid and concentrations of arsenic in P.malaianus in As(Ⅲ) or DMA treantments.Citric acid content also negatively correlated to arsenic uptake in DMA treantment(P0.05).Oxalic,malonic,succinic and palmitic acids were determined in the root exudates of two tested plants,and a significantly positive correlation was observed between the concentration of oxalic exudation and that of arsenic in H.verticillata in As(Ⅲ) or As(Ⅴ) treantments.The present study concludes that root exudation of oxalic is one of impotrant mechanisms in arsenic hyperaccumulation by H.verticillata in As(Ⅲ) or As(Ⅴ) treantment.

Mechanisms of Efficient Arsenite Uptake by Arsenic Hyperaccumulator Pteris vittata

Arsenate (AsV) and arsenite (AsIII) are two dominant arsenic species in the environment. While arsenate uptake is via phosphate transporter in plants, including arsenic hyperaccumulator Pteris vittata , AsIII uptake mechanisms by P. vittata are unclear. In this study, we investigated AsIII uptake by P. vittata involving root radial transport from external medium to cortical cells and xylem loading. In the root symplastic solution, AsIII was the predominant species (90-94%) and its concentrations were 1.6-21 times those in the medium. AsIII influx into root symplast followed Michaelis-Menten kinetics with K(m) of 77.7 μM at external AsIII concentrations of 2.6-650 μM. In the presence of metabolic inhibitor 2,4-dinitrophenol (DNP), arsenic concentrations in the root symplast were reduced to the levels lower than in the medium, indicating that a transporter-mediated active process was mainly responsible for AsIII influx into P. vittata roots. Unlike radial transport, AsIII loading into xylem involved both high- and low-affinity systems with K(m) of 8.8 μM and 70.4 μM, respectively. As indicated by the effect of 2,4-DNP, passive diffusion became more important in arsenic loading into xylem at higher external AsIII. The unique AsIII uptake system in P. vittata makes it a valuable model to understand the mechanisms of arsenic hyperaccumulation in the plant kingdom.

First evidence on different transportation modes of arsenic and phosphorus in arsenic hyperaccumulator Pteris vittata

Arsenic (As) reduction and translocation are key processes for As hyperaccumulation by the hyperaccumulator Pteris vittata L. Micro-X-ray adsorption spectroscopy of P. vittata’s rhizoid tissues revealed that As reduction mainly occurred in endodermis during translocation from epidermis to vascular bundle. Prior to reduction, arsenate (As (V)) translocation was an active process requiring energy and employing a phosphate (P) transporter. Use of a synchrotron X-ray microprobe showed that As (V) and P were cotransported and that this process could be enhanced by As (V) exposure or P deficiency but restrained by energy release inhibition caused by 2,4-dinitrophenol or sodium orthovanadate. In contrast, after As reduction, As(III) translocation differed from P translocation and was more efficient, appearing free from the apparent endodermal blockage. The results here revealed the role of the P transporter on As translocation as well as the key role of As reduction in As hyperaccumulation by P. vittata.

Phytoremediation Potential of Pityrogramma Calomelanos Var. Austroamericana and Pteris Vittata L. Grown at a Highly Variable Arsenic Contaminated Site

This study examined the phytoextraction potential of two arsenic (As) hyperaccumulators, Pteris vittata L. and Pityrogramma calomelanos var. austroamericana at a historical As-contaminated cattle dip site in northern New South Wales (NSW), Australia. Total As concentration in the surface soil (0–20 cm) showed a better spatial structure than phosphate-extractable As in the surface and sub-surface soil at this site. P. calomelanos var. austroamericana produced greater frond dry biomass (mean = 130 g plant−1) than P. vittata (mean = 81 g plant−1) after 10 months of growth. Arsenic concentration and uptake in fronds were also significantly higher in P. calomelanos var. austroamericana (means = 887 mg kg−1 and 124 mg plant−1) than in P. vittata (means = 674 mg kg−1and 57 mg plant−1). Our results showed that under the field conditions and highly variable soil As at the site, P. calomelanos var. austroamericana performed better than P. vittata. We predict that P. calomelanos var. austroamericana would take approximately 100 years to reduce the total As to below 20 mg kg−1 at the site compared to ≥ 200 years estimated for P. vittata. However, long-term data are required to confirm these observations under field conditions.

Assessment of Bioaccumulation of Heavy Metal by Pteris Vittata L. Growing in the Vicinity of Fly Ash

Pteris vittata L. subsp. vittata, a potential arsenic hyperaccumulator fern, growing naturally in the vicinity of fly ash was analyzed for the concentration of nine heavy metals (Fe, Cu, Zn Ni, Al, Cr, Pb, Si, and As) from five different sites around of Kanti Thermal Power Station at Muzaffarpur in Bihar State, India. Metal accumulation in P. vittata was correlated with the level of pollution at five selected sampling sites. The results revealed significantly more accumulation of these metals in the above ground parts of the plant than the parts below ground. Statistical parameters such as the coefficient of variation (CV%) showed a higher for As, Cu, Cr, and a lower one for Fe, Ni, Al. There was high spatial variability in the total metal concentration at different sites. The present study confirmed that P. vittata is a heavy metals accumulator and that it is a highly suitable candidate for phytoremediation of metal contaminated wastelands.

Metabolic adaptation of Pteris vittata L. gametophyte to arsenic induced oxidative stress

The sporophyte and gametophyte of Pteris vittata are arsenic hyperaccumulators, however, little is known about the mechanism by which the gametophyte deals with this toxic element. An in vitro system (spores grown in arsenic amended nutrient media) was used to investigate the impact of arsenic on growth of the gametophyte and the role of antioxidative systems in combating As-stress. When mature spores of P. vittata were grown in medium amended with 0–50mgkg−1 of arsenic (as arsenate), the arsenic concentration in the gametophyte increased, with increasing arsenate in the media, but did not inhibit the spore germination and biomass development. Increases in the level of antioxidant enzymes, superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, and glutathione-Stransferase) and of ascorbic acid and glutathione probably enabled the gametophyte to withstand the oxidative stress caused by arsenate.

Arsenic-resistant bacteria solubilized arsenic in the growth media and increased growth of arsenic hyperaccumulator Pteris vittata L.

The role of arsenic-resistant bacteria (ARB) in arsenic solubilization from growth media and growth enhancement of arsenic-hyperaccumulator Pteris vittata L. was examined. Seven ARB (tolerant to 10mM arsenate) were isolated from the P. vittata rhizosphere and identified by 16S rRNA sequencing as Pseudomonas sp., Comamonas sp. and Stenotrophomonas sp. During 7-d hydroponic experiments, these bacteria effectively solubilized arsenic from the growth media spiked with insoluble FeAsO4 and AlAsO4 minerals (from <5μgL−1 to 5.04–7.37mgL−1 As) and enhanced plant arsenic uptake (from 18.1–21.9 to 35.3–236mgkg−1 As in the fronds). Production of (1) pyochelin-type siderophores by ARB (fluorescent under ultraviolet illumination and characterized with thin layer chromatography) and (2) root exudate (dissolved organic C) by P. vittata may be responsible for As solubilization. Increase in P. vittata root biomass from 1.5–2.2 to 3.4–4.2g/plant dw by ARB and by arsenic was associated with arsenic-induced plant P uptake. Arsenic resistant bacteria may have potential to enhance phytoremediation of arsenic-contaminated soils by P. vittata.

Organic acids in two arsenic hyperaccumulators and a non-hyperaccumulator of Pteris exposed to elevated arsenic concentrations

Six organic acids (oxalic, malonic, malic, citric, palmitic and linolenic acid) in two arsenic (As) hyperaccumulators (Pteris multifida and Pteris vittata) and a non-hyperaccumulator fern (Pteris semipinnata) exposed to different As concentrations (0, 5, 20 and 40 mg As L−1) under hydroponic conditions were determined using gas chromatography-mass spectrometry (GC-MS). Three fern species were collected from an uncontaminated site, but two As-hyperaccumulators were also collected from an As-contaminated site. Furthermore, the time-course effect of organic acid production in P. multifida and P. semipinnata collected from the uncontaminated site were also studied under 0 and 20 mg As L–1 treatments with a sampling interval at 0, 6, 12, 24 and 36 h. After esterifying (H2SO4–CH3OH), extracting (CH2Cl2), washing (saturated NaCl) and drying (anhydrous Na2SO4), these organic acids were isolated (Column: HP Ultra 2, 50 m × 0.2 mM × 0.33 µM; Carrier gas: He). There was no significant increase in the concentrations of the six organic acids in the fronds and roots of the three fern species sampled from an uncontaminated site under 5 or 40 mg As L−1 treatments. In addition, there were also no significant differences in the concentrations of organic acids among the three fern species under the same As concentration treatments. Concentrations of malic, oxalic and linolenic acids in the fronds and roots of two As hyperaccumulators collected from an As-contaminated or uncontaminated site and the non-hyperaccumulator collected from an uncontaminated site followed the sequence of malic acid > oxalic acid > linolenic acid. As for time-course effect, compared to P. semipinnata, a significant increase was observed in concentrations of oxalic and linolenic acid in P. multifida over time under 20 mg As L−1 treatment. In general, the changes of organic acid concentrations in As hyperaccumulators did not directly contribute to As accumulation.

Effect of Phytoaccumulation of Arsenic and Chromium on Structural and Ultrastructural Changes of Brake Fern (Pteris vittata)

Structural and ultrastructural changes caused by bioaccumulation of As and Cr in brake fern (Pteris vittata), a known arsenic hyperaccumulator, were investigated. Potted plants of brake fern were exposed to metal treatments of As and Cr for three weeks. Leaf, stem and root samples were collected periodically and fixed for LM (Light Microscopy), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) to evaluate anatomical changes. The fresh weights, dry weights, RWC (Relative Water Content) and plant heights were obtained before the brake fern plants were harvested for metal accumulation analysis. The As accumulated mainly in the shoots while Cr accumulated mainly in the roots of the metal-treated plants. Significant changes in the ferns physical characters, including fresh weight, dry weight, RWC, and plant height were observed for only Cr-treated plants but not for As-treated plants. Microscopic studies reveal the Cr accumulation resulted in dehydration and collapse of internal structure of leaves and cellular breakdown of roots. The As-treated plants showed no significant structural changes in leaves, stems and roots compared to control plants. Clotted depositions were observed in roots and stems of plant groups treated with highest concentration of Cr and As when compared to control (T0) group. Our study indicates that Cr has a profound impact on physiology and structure of fern plants. The accumulation of Cr resulted in decrease in growth rate, total biomass and RWC. We believe that brake fern plants can uptake, translocate and sequester As because it caused no significant structural changes in leaves, stems and roots of the plants.

Arsenic is not stored as arsenite - phytochelatin complexes in the seaweeds Fucus spiralis and Hizikia fusiforme

Environmental context Seaweeds hyperaccumulate the toxic metalloid arsenic, but seemingly achieve detoxification by transformation to arsenosugars. The edible seaweed hijiki is a notable exception because it contains high levels of toxic arsenate and arsenite. Terrestrial plants detoxify arsenic by forming arsenite–phytochelatin complexes. The hypothesis that seaweeds also synthesise phytochelatins to bind arsenite as a means of detoxification before arsenosugar synthesis is tested in this investigation. Abstract Phytochelatins (PCs), generic structure [γ-Glu-Cys]n-Gly, are peptides synthesised by terrestrial plants to bind toxic metal(loid)s such as cadmium and arsenic. Seaweeds are arsenic hyperaccumulators, seemingly achieving detoxification via arsenosugar biosynthesis. Whether seaweeds synthesise PCs to aid detoxification during arsenic exposure is unknown. Hizikia fusiforme (hijiki) and Fucus spiralis were used as model seaweeds: the former is known for its large inorganic arsenic concentration, whereas the latter contains mainly arsenosugars. F. spiralis was exposed to 0, 1 and 10 mg L–1 arsenate solutions for 24 h, whereas hijiki was analysed fresh. All samples contained AsIII, glutathione and reduced PC2, identified using HPLC-ICP-MS/ES-MS. Although hijiki contained no AsIII–PC complexes, arsenate exposed F. spiralis generated traces of numerous arsenic compounds that might be AsIII–GS or AsIII–PC2 complexes. AsIII–PC complexes seem not to be a principal storage form for long-term arsenic storage within seaweeds. However, 40 times higher glutathione concentrations were found in hijiki than F. spiralis, which may explain how hijiki deals with its high inorganic arsenic burden.