Spectroscopic study of the impact of arsenic speciation on arsenic/phosphorus uptake and plant growth in tumbleweed (Salsola kali)

Chemical transformations of arsenic in the rhizosphere-root interface of Pityrogramma calomelanos and Pteris vittata

Pityrogramma calomelanos and Pteris vittata are cosmopolitan fern species that are the strongest known arsenic (As) hyperaccumulators, with potential to be used in the remediation of arsenic-contaminated mine tailings. However, it is currently unknown what chemical processes lead to uptake of As in the roots. This information is critical to identify As-contaminated soils that can be phytoremediated, or to improve the phytoremediation process. Therefore, this study identified the in situ distribution of As in the root interface leading to uptake in P. calomelanos and P. vittata, using a combination of synchrotron micro-X-ray fluorescence spectroscopy and X-ray absorption near-edge structure imaging to reveal chemical transformations of arsenic in the rhizosphere-root interface of these ferns. The dominant form of As in soils was As(V), even in As(III)-dosed soils, and the major form in P. calomelanos roots was As(III), while it was As(V) in P. vittata roots. Arsenic was cycled from roots growing in As-rich soil to roots growing in control soil. This study combined novel analytical approaches to elucidate the As cycling in the rhizosphere and roots enabling insights for further application in phytotechnologies to remediated As-polluted soils.

Arsenate-reducing bacteria affect As accumulation and tolerance in Salix atrocinerea

Arsenic (As)-reducing bacteria are able to influence As-speciation and, in this way, change As bio-availability. In consequence, this has an impact on As uptake by plants growing on polluted soil and on the effectiveness of the phytoremediation process. To be able to efficiently utilize these bacteria for As-phytoremediation in the field, a better understanding of the plant-bacterial interactions involved in As-tolerance or toxicity is needed. In this work, seedlings of a clone of Salix atrocinerea derived from a specimen naturally growing on an As-polluted brownfield were grown under gnotobiotic conditions exposed to As, and in the presence or absence of two of its field-associated and in vitro characterized plant growth-promoting (PGP) bacteria. The inoculation with Pantoea sp., induced a moderate reduction of AsV to AsIII in the exposure medium that, together with a coordinated plant response of As uptake, chelation and sequestration, increased As accumulation in roots; which is reflected into a higher phytostabilization. However, inoculation with Rhodococcus erythropolis due to a higher disproportionate reduction of AsV to AsIII in the medium caused less As accumulation in roots that non-bioaugmented plants and despite the lower As content, the concentrations of AsIII present in the medium and the damage suffered in roots and leaves, indicated that As tolerance mechanisms (such as prevention of AsIII uptake and efflux) did not occur in time to avoid physical disturbance and plants growth reduction. Interestingly, by two different metabolic pathways -coordinated by different key transporters mediating As uptake, tolerance, distribution and vacuolar accumulation at the roots- both bacteria limited As accumulation in Salix shoots. Our results provide for the first time a detailed insight in the plant-bacterial responses and physiological changes contributing to As tolerance in S. atrocinerea, that will facilitate the design of effective strategies for exploitation of plant-associated microorganisms for phytoremediation.

Biochars mitigate greenhouse gas emissions and bioaccumulation of potentially toxic elements and arsenic speciation in Phaseolus vulgaris L

Anthropogenic and natural activities can lead to increased greenhouse gas emissions and discharge of potentially toxic elements (PTEs) into soil environment. Biochar amendment to soils is a cost-effective technology and sustainable approach used to mitigate greenhouse gas emissions, improve phytoremediation, and minimize the health risks associated with consumption of PTE-contaminated vegetables. Greenhouse pot experiments were conducted to investigate the effects of peanut shell biochar (PNB) and sewage sludge biochar (SSB) on greenhouse gas (GHG) emissions, plant growth, PTE bioaccumulation, and arsenic (As) speciation in bean plants. Results indicated that amendments of PNB and SSB increased plant biomass production by increasing soil fertility and reducing bioavailability of PTEs. Addition of biochars also increased soil pH, total nitrogen (TN), total carbon (TC), dissolved organic carbon (DOC), and ammonium-nitrogen (NH₄-N) but decreased available concentrations of PTEs such as cadmium (Cd), lead (Pb), and As. The concentration of nitrate-nitrogen (NO₃^--N) was also decreased in biochar-amended soils. In addition, PNB and SSB amendments significantly (P < 0.01) reduced the bioaccumulation of chromium (Cr), As, Cd, Pb, and nickel (Ni) in stalks, leaves, and fruits of Phaseolus vulgaris L. Similarly, PNB and SSB amendments significantly (P ≤ 0.05) reduced inorganic As species like arsenite (As (III)) and arsenate (As (V)). Greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) emissions were significantly (P < 0.01) reduced but nitrous oxide (N₂O) emissions first increased and then decreased amended with both biochars. Current findings demonstrate that SSB and PNB are two beneficial soil amendments simultaneous mitigating greenhouse gas emissions and PTE bioaccumulation as well as arsenic speciation in P. vulgaris L.

OsPT4 Contributes to Arsenate Uptake and Transport in Rice

Arsenic (As) is toxic to organisms, and elevated As accumulation in rice (Oryza sativa) grain may pose a significant health risk to humans. The predominant form of As in soil under aerobic conditions is As(V), which has a chemical structure similar to that of PO₄^3-. Rice roots take up As(V) by phosphate (Pi) transporters, such as OsPT1 and OsPT8. In the present study, we investigated the contribution of OsPT4, belonging to the Pht1 family, on rice As(V) uptake and transport. We determined the mRNA amounts of OsPTs in rice seedlings, and expressions of OsPT1, OsPT4, and OsPT8 were up-regulated under As(V) conditions. OsPT4-overexpressing plants were obtained to examine the As (V) transport activity of OsPT4 in rice. When transgenic rice grew in hydroponic culture with 25 and 50 μM As(V), the plants showed sensitivity to As(V) stress with aboveground parts showing delayed growth and the roots stunted. The OsPT4 CRISPR lines showed the opposite phenotype. When plants were grown in 5 μM As(V) solution for 7 days, the As accumulation of OsPT4-overexpressing plants increased up to twice in roots and shoots. Furthermore, the arsenate uptake rates of OsPT4-overexpressing lines were higher compared with wild type. The V_max of As(V) uptake in OsPT4-overexpressing plants increased 23-45% compared with Nipponbare. In the flooded soil, the As accumulation of OsPT4-overexpressing plants increased 40-66% and 22-30% in straw and grain, respectively. While in OsPT4-cr plants As accumulation in roots decreased 17-30% compared with Nipponbare. Therefore, the present study indicates that OsPT4 is involved in As(V) uptake and transport and could be a good candidate gene to generate low As-accumulating rice.

Arsenic uptake and speciation in Arabidopsis thaliana under hydroponic conditions

Arsenic (As) uptake and species in Arabidopsis thaliana were evaluated under hydroponic conditions. Plant nutrient solutions were treated with arsenite [As(III)] or arsenate [As(V)], and aqueous As speciation was conducted using a solid phase extraction (SPE) cartridge. Arabidopsis reduced As(V) to As(III) in the nutrient solution, possibly due to root exudates such as organic acids or the efflux of As(III) from plant roots after in vivo reduction of As(V) to As(III). Arsenic uptake by Arabidopsis was associated with increased levels of Ca and Fe, and decreased levels of K in plant tissues. Arsenic in Arabidopsis mainly occurred as As(III), which was coordinated with oxygen and sulfur based on XANES and EXAFS results. The existence of As(III)O and As(III)S in EXAFS indicates partial biotransformation of As(III)O to a sulfur-coordinated form because of limited amount of glutathione in plants. Further understanding the mechanism of As biotransformation in Arabidopsis may help to develop measures that can mitigate As toxicity via genetic engineering.

Interaction of arsenic and phosphate on their uptake and accumulation in Chinese brake fern

Interactive effects of arsenate (As (V)) and phosphate (Pi) were investigated under hydroponic culture. Arsenic concentrations in fronds and roots of Chinese brake fern (Pteris vittata L.) significantly (p < 0.05) increased with increasing As (V), but decreased (p < 0.05) with increasing Pi in nutrient solution. Phosphate uptake was significantly (p < 0.05) inhibited by 1000 micromol L(-1) As (V). Under 100 micromol L(-1) As (V), frond phosphorus (P) increased at 100 and 1000 micromol L(-1) Pi, and root P increased at 250 micromol L(-1) Pi exposures. Arsenic and P concentrations in fronds and roots of Chinese brake fern were negatively correlated (p < 0.05). Arsenate treatments enhanced As and P transport to fronds, while increasing Pi inhibited their transportation, with highest frond P and As (%) obtained under 100 micromol L(-1) treatment. pH values in nutrient solution increased with increasing exposure time, but decreased with increasing Pi levels. Dissolved organic carbon (DOC) contents (dry weight) in nutrient solution decreased with increasing Pi levels, both for treatments with and without As (V). Arsenate at 1000 micromol L(-1) significantly (p < 0.05) increased DOC contents, especially for treatment without Pi. Six organic acids were detected in root exudates of Chinese brake fern, with oxalic and malic acids being most dominant.

Arsenic speciation in tissues of the hyperaccumulator P. calomelanos var. austroamericana using X-ray absorption spectroscopy

The fate and chemical speciation of arsenic (As) during uptake, translocation, and storage by the As hyperaccumulating fern Pityrogramma calomelanos var. austroamericana (Pteridaceae) were examined using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and synchrotron-based micro-X-ray absorption near edge structure (micro-XANES) and micro-X-ray fluorescence (micro-XRF) spectroscopies. Chemical analysis revealed total As concentration was ca. 6.5 times greater in young fronds (5845 mg kg(-1) dry weight (DW)) than in old fronds (903 mg kg(-1) DW). In pinnae, As concentration decreased from the base (6822 mg kg(-1) DW) to the apex (4301 mg kg(-1) DW) of the fronds. The results from micro-XANES and micro-XRF of living tissues suggested that more than 60% of arsenate (As(V)) absorbed was reduced to arsenite (As(III)) in roots, prior to transport through vascular tissues as As(V) and As(III). In pinnules, As(III) was the predominant redox species (72-90%), presumably as solvated, oxygen coordinated compounds. The presence of putative As(III)-sulphide (S(2-)) coordination throughout the fern tissues (4-25%) suggests that S(2-) functional groups may contribute in the biochemical reduction of As(V) to As(III) during uptake and transport at a whole-plant level. Organic arsenicals and thiol-rich compounds were not detected in the species and are unlikely to play a role in As hyperaccumulation in this fern. The study provides important insights into homeostatic regulation of As following As uptake in P. calomelanos var. austroamericana.

Accumulation, speciation, and coordination of arsenic in an inbred line and a wild type cultivar of the desert plant species Chilopsis linearis (Desert willow)

This study investigated the absorption of arsenic (As), sulfur (S), and phosphorus (P) in the desert plant Chilopsis linearis (Desert willow). A comparison between an inbred line (red flowered) and wild type (white flowered) plants was performed to look for differential responses to As treatment. One month old seedlings were treated for 7 days with arsenate (As(2)O(5), As(V)) at 0, 20, and 40 mg As(V)L(-1). Results from the ICP-OES analysis showed that at 20mg As(V)L(-1), red flowered plants had 280+/-11 and 98+/-7 mg As kg(-1) dry wt in roots and stems, respectively, while white flowered plants had 196+/-30 and 103+/-13 mg As kg(-1) dry wt for roots and stems. At this treatment level, the concentration of As in leaves was below detection limits for both plants. In red flowered plants treated with 40 mg As(V)L(-1), As was at 290+/-77 and 151+/-60 mg As kg(-1) in roots and stems, respectively, and not detected in leaves, whereas white flowered plants had 406+/-36, 213+/-12, and 177+/-40 mg As kg(-1) in roots, stems, and leaves. The concentration of S increased in all As treated plants, while the concentration of P decreased in roots and stems of both types of plants and in leaves of red flowered plants. X-ray absorption spectroscopy analyses demonstrated partial reduction of arsenate to arsenite in the form of As-(SX)(3) species in both types of plants.

Effect of external and internal phosphate status on arsenic toxicity and accumulation in rice seedlings

Phosphorus (P) deficiency is thought to exacerbate the arsenic (As) phytotoxicity in paddy rice. The experiments were conducted to investigate the effects of external phosphate supply on As accumulation in rice and its toxicity under phosphate deficiency conditions. Rice seedlings pretreated with a phosphorus deficient nutrient solution (-P) for 14 d accumulated more As than those pretreated with a normal phosphorus supply nutrient solution (+P). Rice protreated with -P showed As toxicity symptoms after being exposed to 50 micromol/L arsenate for 4 h, while +P rice did not show any toxicity symptoms. Arsenic toxicity symptoms can be alleviated by increasing external P concentrations. The arsenate uptake rate and accumulation corresponded with the As toxicity in rice plants. Arsenic concentrations in rice roots decreased with increasing external phosphate concentrations. The lowest As accumulation and the highest P accumulation were found when the external P concentration reached 100 micromol/L. In short, P deficiency increased the sensitivity of rice to arsenate and increasing of external phosphate supply could alleviate As toxicity.

Arsenic uptake by aquatic macrophyte Spirodela polyrhiza L.: interactions with phosphate and iron

The uptake of arsenate (As(V)) and dimethylarsinic acid (DMAA) by aquatic macrophyte Spirodela polyrhiza L. was investigated to determine the influence of arsenic interaction with PO4(3-) and Fe ions. Plants were grown hydroponically on standard Murashige and Skoog (MS) culture solutions. Arsenic concentrations in Fe-oxide (Fe-plaque) on plant surfaces were determined by citrate-bicarbonate-ethylenediaminetetraacetic acid (CBE) technique. S. polyrhiza L. accumulated 51-fold arsenic from arsenate solution compared to that from DMAA solution with initial concentrations of 4.0 and 0.02microM of arsenic and phosphate, respectively. The arsenate uptake was negatively (p<0.001) correlated with phosphate uptake and positively (p<0.05) correlated with iron uptake. About 56% of the total arsenic was accumulated into the plant tissues while 44% was adsorbed on Fe-plaque (CBE-extract), when the plants were grown on arsenate solution. The DMAA uptake into the plant was neither affected by the phosphate concentrations nor correlated (p>0.05) with iron accumulation. The results suggest that adsorption of arsenate on Fe-plaque of the surface of S. polyrhiza L. contributes to the arsenic uptake significantly. Thus, arsenate uptake in S. polyrhiza L. occurred through the phosphate uptake pathway and by physico-chemical adsorption on Fe-plaques of plant surfaces as well. The S. polyrhiza L. uses different mechanisms for DMAA uptake.

Arsenic accumulation and toxicity in the earthworm Eisenia fetida affected by chloride and phosphate

Chloride and phosphate (PO4) were added to field soils contaminated with arsenic and heavy metals, and Eisenia fetida were inoculated in these treated soils for 70 d. After 28 d, earthworm survival, body weight, and cocoon production were measured. During the exposure period, the accumulation of As and four metals (Cu, Zn, Cd, and Pb) in E. fetida was analyzed. Bulk soils and soil solutions were characterized for the contents of As, metals, organic matter, and major cations and anions; cation exchange capacity; and pH. Although addition of Cl to the soil did not impact As toxicity, it significantly reduced As uptake. The addition of PO4 induced changes in As toxicity and reduced As accumulation even though the As concentration in the soil pore water increased because of the substitution of As by P at soil sorption sites. These results imply that the addition of Cl induced the formation of an As-Cl complex, thereby resulting in decreased As bioavailability; they also imply that monovalent phosphate (H2PO4(-)) competes with As for uptake through transporter-mediated mechanisms in cells.

Influence of EDTA and chemical species on arsenic accumulation in Spirodela polyrhiza L. (duckweed)

The influence of ethylenediaminetetraacetic acid (EDTA) and chemical species on arsenic accumulation in aquatic floating macrophyte Spirodela polyrhiza L. (duckweed) was investigated. The uptake of inorganic arsenic species (arsenate; As(V) and arsenite; As(III)) into the plant tissue and their adsorption on iron plaque of plant surfaces were significantly (p<0.05) higher than those of organic species (monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA)). The addition of EDTA to the culture media increased the uptake of As(V) and As(III) into the plant tissue though the MMAA and DMAA uptake were not affected. About 4-6% of the inorganic arsenic species were desorbed or mobilized from iron plaque by EDTA. Desorption of organic arsenic species was not affected by EDTA addition because the co-precipitation occurs only with inorganic species. Phosphate uptake was not affected by EDTA though its concentration in citrate-bicarbonate-EDTA (CBE)-extract was much higher than that of plant tissue. Iron uptake into the plant increased significantly (p>0.05) by EDTA addition to the culture media while its concentration in CBE-extract decreased significantly (p<0.05). The As(inorganic)/Fe ratios in plant were higher than those of CBE-extract which indicate the increased uptake of these arsenic species into the plant relative to the iron. The lower As(organic)/Fe ratios in plant and on CBE-extract suggest the reduction of accumulation of these arsenic species relative to the iron.

Arsenic accumulation in duckweed (Spirodela polyrhiza L.): a good option for phytoremediation

Some unavoidable drawbacks of traditional technologies have made phytoremediation a promising alternative for removal of arsenic from contaminated soil and water. In the present study, the potential of an aquatic macrophyte Spirodela polyrhiza L. for phytofiltration of arsenic, and the mechanism of the arsenic uptake were investigated. The S. polyrhiza L. were grown in three test concentrations of arsenate and dimethylarsinic acid (DMAA) (i.e. 1.0, 2.0 and 4.0microM) with 0 (control), 100 or 500microM of phosphate. One control treatment was also set for each test concentrations of arsenic. The PO(4)(3-) concentration in control treatment was 0.02microM. When S. polyrhiza L. was cultivated hydroponically for 6d in culture solution containing 0.02microM phosphate and 4.0microM arsenate or DMAA, the arsenic uptake was 0.353+/-0.003micromolg(-1) and 7.65+/-0.27nmolg(-1), respectively. Arsenic uptake into S. polyrhiza L. was negatively (p<0.05) correlated with phosphate uptake when arsenate was applied to the culture solutions owing to similar in the sorption mechanism between AsO(4)(3-) and PO(4)(3-), and positively (p<0.05) correlated with iron uptake due to adsorption of AsO(4)(3-) onto iron oxides. Thus, the S. polyrhiza L. accumulates arsenic by physico-chemical adsorption and via the phosphate uptake pathway when arsenate was added to the solutions. These results indicate that S. polyrhiza L. would be a good arsenic phytofiltrator. In contrast, DMAA accumulation into S. polyrhiza L. was neither affected by the phosphate concentration in the culture nor correlated (p>0.05) with iron accumulation in plant tissues, which indicates that S. polyrhiza L. uses different mechanisms for DMAA uptake.

Examination of arsenic(III) and (V) uptake by the desert plant species mesquite (Prosopis spp.) using X-ray absorption spectroscopy

This study describes the effects of Arsenic(III) and (V) on the growth and their uptake by the desert plant mesquite (Prosopis spp.). Seedlings were sown in agar-based medium containing a modified Hoagland's nutrient solution. After 1 week, the seedlings were transplanted to arsenic (As) treated agar media that contained 5 mgL(-1) of As either As(III) (As(2)O(3)) or As(V) (As(2)O(5)). The plants were harvested after 14 days of growth and sectioned into roots, stems, and leaves. After digestion, As concentrations in the roots, stems, and leaves were determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Our results showed that the As concentrations from As(V) were significantly higher than the As concentrations from As(III) in all portions of the plant. Plants exposed to As(V) concentrated (mg As kg(-1) d wt) about 770+/-191, 326+/-94, and 119+/-18 in roots, stems, and leaves, respectively. X-ray absorption spectroscopy (XAS) showed that As(V) was reduced to As(III) inside the mesquite plant. In addition, greater than 90% of the As(III) found in the mesquite plants was bound to sulfur ligands in the roots, stems and leaves.