Low Arsenic Content Research Articles

Unusual holopelagic Sargassum mass beaching in Northwest Africa: Morphotypes, chemical composition, and potential valorisation

The rapid proliferation of holopelagic Sargassum spp. in the tropical Atlantic Ocean presents environmental challenges and economic opportunities. In 2022, Senegal witnessed its first significant holopelagic Sargassum beaching event, triggering widespread concern and interest from civil society, industrial sectors, and government. This study represents the first analysis of stranded holopelagic Sargassum's morphotypes and chemical composition in Northwest Africa. We highlight the nature of Sargassum stranding, dominated by S. fluitans III, and describe a putative new morphotype. Compared to most of the studies in the tropical Atlantic, Senegalese Sargassum displayed lower arsenic concentrations (9–29 ppm), higher cadmium levels (9–15 ppm), and increased mercury content (0.47–0.57 ppm). In addition, Senegalese Sargassum showed higher levels of iron (237–1017 ppm) and phosphorus (1300–1772 ppm). The biochemical analysis revealed high total protein levels (15–40 % DW) in Senegalese samples, though further analysis is required to confirm this. Furthermore, variations in biochemical composition within various parts of the Sargassum thallus were observed. The low arsenic content makes the beached Senegalese Sargassum attractive for valorisation and sets it apart from holopelagic Sargassum from all other regions where it occurs. However, caution should be taken regarding the high concentrations of cadmium. Our study highlights promising applications in Senegal and neighbouring countries, particularly in animal feed and agriculture. Noteworthy is the notable palladium content (2 ppm), valuable phenolic compounds, and mannitol, which present additional opportunities for the chemical industry. Our interdisciplinary approach enhances the global scientific understanding of the Sargassum issue. With the anticipation of more frequent Sargassum beaching events and, more generally, for seaweed exploitation, we advocate for inter-governmental African organisations to establish standardised norms for their exploitation. We recommend that the Food and Agriculture Organization/World Health Organization consider incorporating more seaweed in the Codex Alimentarius to facilitate their uses particularly when states deal with algal blooms.

Scorodite synthesis process using solid iron oxides

The immobilization of arsenic in the form of scorodite (FeAsO4·2H2O), which has excellent chemical stability, is attracting attention as a method for treating wastewater containing high concentrations of arsenic generated at nonferrous metal smelting plants. Scorodite, with its low solubility and high arsenic content per volume, is expected to be an environmentally friendly arsenic fixation method suitable for final disposal. Various scorodite synthesis methods have been studied. The first synthesis method proposed is the hydrothermal method using an autoclave to synthesize highly crystalline scorodite. Although the hydrothermal method is capable of synthesizing scorodite with good crystallinity, from an economic point of view, scorodite synthesis under ambient pressure and at low temperatures is more attractive. As a low-temperature scorodite synthesis method under atmospheric pressure, the oxidation of Fe(II) process by O2 bubbling was proposed. In this method, ferrous sulfate is added to an arsenic-containing solution as a Fe ion source, and in situ oxidation of Fe(II) by O2 or air bubbling to form scorodite with good crystallinity. In addition to temperature, other conditions, pH, Fe/As ratio, and reaction time have been reported to affect scorodite crystallization. Recently, scorodite synthesis using solid iron oxide as the Fe source for scorodite synthesis, instead of aqueous Fe salt solutions, has attracted much attention. In this presentation, we will report on the investigation of reaction parameters, such as the type of iron oxide and reaction temperature, for the scorodite synthesis using solid iron oxide.

Gold source and deposition in the Sanakham gold deposit, SW Laos: Constrains from textures, trace element geochemistry and in-situ sulfur isotopes of pyrite

The Luang Prabang (Laos) – Loei (Thailand) metallogenetic belt is endowed with major porphyry-skarn, and epithermal gold deposits. The Sanakham gold deposit is a newly discovered hydrothermal vein-type orogenic gold deposits (10.6 t Au @ 3.05 g/t) in the belt, and consist of a series of quartz-sulfide veins that are controlled by secondary structures of the regional NNE-trending brittle-ductile shear zone. Its mineralization can be divided into four stages: (I) quartz–pyrite–arsenopyrite, (II) quartz–pyrite–pyrrhotite–chalcopyrite, (III) quartz–pyrite–base-metal sulfide, and (IV) quartz–carbonate. Three pyrite generations were identified: py1 (stage I) as disseminated euhedral to subhedral coarse grains; py2 (stage II) as subhedral to anhedral fine grains occur around the cataclastic and corrosion textures in py1; py3 (stage III) as subhedral to anhedral fine grains occurred in sulfide veins or veinlets that overprint the existing quartz-sulfides. All three generations of pyrite have low trace-element contents including very low lattice-bound gold (py1: 0.040 ppm; py2: 0.098 ppm; py3: 0.028 ppm) and arsenic (py1: 227 ppm; py2: 133 ppm; py3: 13.3 ppm) contents. Visible gold grains are sited in textures of pyrite grains that relate to brittle cataclastic deformation. Considering the relatively low contents of pyrite in the ores of Sanakham, it is impossible to derive the high-gold grade orebodies through local remobilization of originally lattice-bound gold in pyrite. The py2 might be formed during hydrothermal overprinting and dissolution of py1 led by continuous supply of ore fluids with relatively higher contents of Au, Ag, Cu, Sb, Te, Pb, and Bi. A reduction reaction related to reduced carbon species in wallrock is an assignable cause for gold deposition, even though the fluid-rock interaction and fluid immiscibility processes might be important. The Au- and As-poor pyrites have similar δ34S values (py1: –2.58 to –0.83 ‰; py2: –2.17 ‰ to + 0.25 ‰; py3: –1.24 to –0.60 ‰ ppm) that close to 0 ‰, indicating the sulfur may have been originated from magmatic or mantle sources. Therefore, we propose that the Sankham gold deposit is a member of the orogenic deposit class, but notably characterized by the low concentrations of invisible gold in the relatively As-poor pyrite lattice.

Arsenic effects and behavior during the transformation of struvite to newberyite: Implications for applications of green fertilizers

Struvite recovered from wastewaters is an increasingly popular green fertilizer but can sequester hazardous heavy metal(loid)s, including arsenic. The recycled struvite applied as fertilizers in soils often undergoes dissolution and subsequent transformation to newberyite over time, potentially resulting in arsenic contamination. Therefore, understanding the fate of arsenic during struvite transformation to newberyite is critical to its safe application. Herein, we have investigated the effects of arsenic on the transformation of struvite to newberyite and examined the associated consequences of this transformation for arsenic mobilization. Our results demonstrate that low arsenic contents have minor but detectable retardation on the struvite-newberyite transformation, and high arsenic loading can severely inhibit this transformation. However, the mechanisms of the struvite-newberyite transformation are independent of the arsenic concentrations. Most arsenic is retained in the solids during the struvite-newberyite transformation. Also, the arsenic contents in both struvite and newberyite increase with increasing the initial pH values. Moreover, spectroscopic analyses and first-principles calculations show that the dominant arsenic species incorporated is AsO43- in struvite but HAsO42- in newberyite. These results have important implications for understanding the roles of arsenic in struvite-based green fertilizers and developing optimal applications of these materials for the remediation of arsenic contamination in aqueous environments.

Distribution characteristics of selenium, cadmium and arsenic in rice grains and their genetic dissection by genome-wide association study.

High selenium (Se) and low cadmium (Cd) and arsenic (As) contents in rice grains were good for human health. The genetic basis and relationship of Se, Cd and As concentrations in rice grains are still largely unknown. In the present study, large variations were observed in Se, Cd and As concentrations in brown and milled rice in normal and Se treatment conditions in 307 rice accessions from 3K Rice Genomes Project. Se fertilizer treatment greatly increased Se concentrations but had no obvious changes in concentrations of Cd and As both in brown and milled rice. Total of 237 QTL were identified for Se, Cd and As concentrations in brown and milled rice in normal and Se treatment conditions as well as ratio of concentrations under Se treatment to normal conditions. Only 19 QTL (13.4%) were mapped for concentrations of Se and Cd, Se and As, and Se, Cd and As in the same or adjacent regions, indicating that most Se concentration QTL are independent of Cd and As concentration QTL. Forty-three favorable alleles were identified for 40 candidate genes by gene-based association study and haplotype analysis in 14 important QTL regions. Se-enriched rice variety will be developed by pyramiding favorable alleles at different Se QTL and excluding undesirable alleles at Cd and As QTL, or combining favorable alleles at Se QTL with the alleles at Se-sensitive QTL by marker-assisted selection.

Genome-Scale Profiling and High-Throughput Analyses Unravel the Genetic Basis of Arsenic Content Variation in Rice.

Ionomics, the study of the composition of mineral nutrients and trace elements in organisms that represent the inorganic component of cells and tissues, has been widely studied to explore to unravel the molecular mechanism regulating the elemental composition of plants. However, the genetic factors of rice subspecies in the interaction between arsenic and functional ions have not yet been explained. Here, the correlation between As and eight essential ions in a rice core collection was analyzed, taking into account growing condition and genetic factors. The results demonstrated that the correlation between As and essential ions was affected by genetic factors and growing condition, but it was confirmed that the genetic factor was slightly larger with the heritability for arsenic content at 53%. In particular, the cluster coefficient of japonica (0.428) was larger than that of indica (0.414) in the co-expression network analysis for 23 arsenic genes, and it was confirmed that the distance between genes involved in As induction and detoxification of japonica was far than that of indica. These findings provide evidence that japonica populations could accumulate more As than indica populations. In addition, the cis-eQTLs of AIR2 (arsenic-induced RING finger protein) were isolated through transcriptome-wide association studies, and it was confirmed that AIR2 expression levels of indica were lower than those of japonica. This was consistent with the functional haplotype results for the genome sequence of AIR2, and finally, eight rice varieties with low AIR2 expression and arsenic content were selected. In addition, As-related QTLs were identified on chromosomes 5 and 6 under flooded and intermittently flooded conditions through genome-scale profiling. Taken together, these results might assist in developing markers and breeding plans to reduce toxic element content and breeding high-quality rice varieties in future.

Responses of potential ammonia oxidation and ammonia oxidizers community to arsenic stress in seven types of soil

Soil arsenic contamination is of great concern because of its toxicity to human, crops, and soil microorganisms. However, the impacts of arsenic on soil ammonia oxidizers communities remain unclear. Seven types of soil spiked with 0 or 100 mg arsenic per kg soil were incubated for 180 days and sampled at days 1, 15, 30, 90 and 180. The changes in the community composition and abundance of ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) were analyzed by terminal restriction fragment length polymorphism (T-RFLP) analysis, clone library sequencing, and quantitative PCR (qPCR) targeting amoA gene. Results revealed considerable variations in the potential ammonia oxidation (PAO) rates in different soils, but soil PAO was not consistently significantly inhibited by arsenic, probably due to the low bioavailable arsenic contents or the existence of functional redundancy between AOB and AOA. The variations in AOB and AOA communities were closely associated with the changes in arsenic fractionations. The amoA gene abundances of AOA increased after arsenic addition, whereas AOB decreased, which corroborated the notion that AOA and AOB might occupy different niches in arsenic-contaminated soils. Phylogenetic analysis of amoA gene-encoded proteins revealed that all AOB clone sequences belonged to the genus Nitrosospira, among which those belonging to Nitrosospira cluster 3a were dominant. The main AOA sequence detected belonged to Thaumarchaeal Group 1.1b, which was considered to have a high ability to adapt to environmental changes. Our results provide new insights into the impacts of arsenic on the soil nitrogen cycling.

Strategies for arsenic pollution control from copper pyrometallurgy based on the study of arsenic sources, emission pathways and speciation characterization in copper flash smelting systems.

Arsenic in copper flash smelting (FS) systems not only affects the quality of products but also poses significant technological and environmental problems. Based on the assessment of arsenic mass partitioning in the FS system, arsenic elimination in off-gassing and tailings is 22%, and most of the arsenic output (69%) is recycled in the FS system. Circulating arsenic, especially arsenic in recycled dust and slag concentrate, is the key reason for high-arsenic-content feed. Dust-type recycled materials (RMs) contribute much more arsenic to the feed than slag-type RMs. Flash smelting furnace electrostatic precipitator (FSF ESP) dust contributes makese the largest contribution to arsenic among the dust-type RMs of mixed dust, especially trivalent arsenic, followed by FSF and flash converting furnace waste heat boiler (FCF WHB) dust, which contributes pentavalent arsenic. FCF WHB dust exhibits a relatively low arsenic content, consisting mainly of As(V)-O. Slag-type recycled materials contribute As(V)-O to the total feed, and As(III) originates from copper concentrates. Considering the arsenic contribution and environmental risk, reducing the recovery of FSF ESP dust can greatly decrease the arsenic grade of FSF feed and volatile As2O3. As one of the main arsenic sources in feed, FSF slag concentrate should be carefully disposed of if separated from feed materials because of its high arsenic-related environmental risk. In contrast, WHB dust and FCF slag are more suitable as RM due to their high copper content and low arsenic risk.

Assessment of heavy metals contamination of the water resources in the vicinity of Barapukuria Coal Mine, Phulbari, Dinajpur, NW Bangladesh

The present research work shows that the concentrations of heavy metals can be expressed as Fe>Mn>Zn>Ni>Cr>Pb>Co>As in the groundwater of Phulbari area, Dinajpur, Bangladesh on the basis of their abundance. The Plio-Pleistocene Dupi Tila aquifer sediment is principally composed of quartz, feldspar and lithic grains and is characterized by low ZTR index. Seventeen groundwater samples have higher iron concentrations than the permissible limit of WHO, 2004. The chemical weathering of pyrite release iron in the natural water of the study area. The manganese content of two groundwater samples is higher than the acceptable limit. The high abundance of potassium in 11.11% water samples is indicative of the influence of agricultural activity in the investigated area. The factor analysis reveals the dominance of four factors that comprise 77.276% of the groundwater sample. The contamination index (Cd) for the heavy metals were performed and 88.89% of the groundwater samples have low contamination index. The calcium of the groundwater is resulted from the dissolution of carbonate rocks. The low arsenic concentrations indicate the fast flowing nature of the groundwater and/or low arsenic content of the aquifer sediments. The low cobalt content of the groundwater reflects the low cobalt concentration of the aquifer sediments. The heavy metal chemistry of the groundwater of the study area reveals that the quality of the water is influenced by the geogenic sources as well as the anthropogenic activities like coal mining and agricultural activities. Â

Multistage mineralization in the Haoyaoerhudong gold deposit, Central Asian Orogenic Belt: Constraints from the sedimentary-diagenetic and hydrothermal sulfides and gold

The Central Asian Orogenic Belt, as one of world-class gold economic belts, preserves a number of giant, large black shale-hosted gold deposits, while it is still debated for origin of sulfides and gold mainly due to lack of identification for multiple stages of sulfides. The Haoyaoerhudong gold deposit is hosted in a sequence of Mesoproterozoic carbonaceous and pyritic slate, phyllite, and schist that form a tight syncline along the north margin of the North China Craton. Detailed petrography of the host rocks and mineralization have defined five stages of pyrites. The earliest form of pyrite (Py1) occurs as fine-grained dispersed pyrite in black carbonaceous slate and medium- to coarse-grained disseminated pyrite in pyrite-rich layers, contains relative low gold and high arsenic content, indicating a syn-sedimentary or diagenetic in origin. Stage II pyrite (Py2) occurs with garnet and quartz inclusions and Py3 occurs as pyrite veins, contains higher gold and lower As content, and are interpreted to have formed from the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism. Stage IV pyrite (Py4) from the pyrite-quartz veins crosscut the metamorphic garnet, contains the highest gold concentrations and other trace elements, and is considered to have formed post-peak metamorphism. Abundant native gold, electrum, and maldonite occur as inclusions within Py4 and monazite and in fractures that crosscut garnet. While, Py5 with typical remobilized feature is thought to be a product of melting of former pyrites (Py1 to Py4) triggered by the large-scale Hercynian magmatism. The sedimentary/diagenetic Py1 have δ34S values that range from +12.4‰ to +16.2‰. Later generations of sulfides, including Py2 to Py5, and Ccp2 to Ccp3, have δ34S values from +9.5‰ to +12.7‰. Monazite with maldonite inclusions from quartz-pyrite veins yielded an intercept age of 341.3 ​± ​6.6 ​Ma, while coarse grained monazite associated biotite along fractures in the reefs yielded an intercept age of 254.6 ​± ​8.2 ​Ma.The paragenetic, textural, chemical, and isotopic data suggest three distinct gold producing episodes at Haoyaoerhudong gold deposit. Gold and arsenic were clearly initially concentrated in organic muds, and enriched along the structures of diagenetic arsenic-rich pyrite. Subsequently, accompanying metamorphism and deformation, gold was liberated from the dissolution of diagenetic pyrites to form the pyrite veins. Finally, accompanying transformation of pyrite into pyrrhotite, gold was released into the metamorphic fluids to become concentrated as native gold, electrum, and maldonite in pyrite-quart veins. Monazite with age of 341 ​Ma from quartz-pyrite veins suggests that the third major gold mineralizing event in Haoyaoerhudong occurred before the Hercynian magmatism, suggesting that the Haoyaoerhudong deposit is a typical orogenic gold deposit rather than intrusion-related deposit.

STUDY ON ARSENIC MITIGATION BY LIMING METHOD COMBINING ORGANIC MATERIAL ON SOYBEAN AND CORN IN AN GIANG PROVINCE

The study on arsenic mitigation by liming method, combining organic material on soybean and corn crops on the dyke in An Giang, was conducted to determine the effect of lime combination rice husk ash (03 tons/ha) on the uptake of arsenic in soybean, and the effect of lime combination sawdust (02 tons/ha) on the uptake of arsenic in corn crops. The experiment was arranged in a completely randomized block field format, where the experiments were arranged with 2 treatments with 4 replicates: Treatment 1 (NT1): control (No liming combined with organic materials); Treatment 2 (NT2): liming combined with organic materials in a ratioof 1: 1 (Dosage: 03 tons / ha of lime mixture with rice husk biochar for soybean crop land, 02 tons / ha of mixed lime combined with sawdust for corn crop land). The results revealed that liming treatments combined withorganic materials increased pH H2O and arsenic in soil - the lime combination rice husk ash arsenic content in roots (0.836 mg/kg), leaf stems (0.83 mg/kg) and seeds (0.06 mg/kg) had results lower than the control treatments, 33.1%; 32.5% and 45.5% respectively. Applying and combining lime with sawdust resulted in lower arsenic content in leaf stems (95.3 mg/kg) and seeds (6.33 mg/kg) compared to the control treatmentsby 31.9% and 49.4%. Therefore, added lime combined with organic matter can decrease arsenic content in plants, and it is recommended that growers apply this technique to reduce the absorption of arsenic into crops.

Iron (III) oxide nanoparticles alleviate arsenic induced stunting in Vigna radiata

Iron nanoparticles (NPs) are widely used for the removal of arsenic from water. In this study, we evaluated the interaction between arsenate (AsO43−) and Fe2O3-NPs on early seedling growth of Vigna radiata. Seedlings were raised in AsO43− and Fe2O3-NPs, alone and in combination. While Fe2O3-NPs slightly promoted seedling growth, AsO43− reduced seedling growth drastically. AsO43--induced decline in the seedling growth was recovered by Fe2O3-NPs. In contrast, equivalent concentrations of FeCl3, alone and together with AsO43−, inhibited seed germination completely. Lower arsenic content in seedlings raised in the presence of Fe2O3-NPs indicated that Fe2O3-NPs restricted arsenic uptake. Ability of Fe2O3-NPs to restrict the arsenic uptake of the seedlings was due to adsorption of AsO43−, as revealed by transmission and scanning electron microscopy. Non-toxic levels of iron in seedlings were due to restriction of Fe2O3-NPs to root-surface. AsO43− enhanced the ferric chelate reductase activity of root which was recovered by Fe2O3-NPs. The AsO43--induced oxidative stress, evident from high levels of proline, H2O2 and malondialdehyde, and lowered root oxidisability was ameliorated by Fe2O3-NPs. AsO43-induced enhancement in total antioxidant capacity, superoxide dismutase and catalase activity, and decline in guaiacol peroxidase activity were antagonized by Fe2O3-NPs. Our findings reveal that Fe2O3-NPs provide effective resistance/amelioration to arsenic toxicity by reducing arsenic availability to plants.

Producing Metallic Antimony with Low Arsenic Content from Antimony Concentrate

The basis of the concentrate is sodium hexahydroxoantimonate or mineral mopungite. Upon reduction of the concentrate with coke, ground antimony containing 0.34% arsenic was obtained. To reduce the arsenic content in the rough metal to 0.1% and exclude the stages of antimony refining from arsenic, reductive melting is proposed in the presence of lead compounds. Because of the smelting reduction of the antimonate concentrate in the presence of sodium plumbite or lead oxide, a rough antimony with an arsenic content of 0.07–0.1% was obtained. The process of reductive smelting of the antimonate concentrate on black antimony was carried out in an oven with silicate heaters in alundum crucibles with batches of charge of 100–150 grams. The content of impurities and the base metal in antimony was determined by chemical and atomic absorption methods. The form of arsenic in the concentrate was determined by X-ray phase analysis. The analysis was carried out on an automated diffractometer DRON-3 with CuKα radiation, s-filter. The concentration of arsenic in the slag phase in the form of lead diarsenate Pb2As2O7 is shown. Thermal gravimetric analysis of the smelting reduction process of the antimonate concentrate was studied on the Q-1000/D derivatograph of the F. Paulik, J. Paulik and L. Erdey systems of the “MOM” company. Thermogravimetric researches of process of recovery melting of the furnace charge consisting of an antimony concentrate, lead oxide and coke as a result of which it is established that process of formation of metal antimony proceeds in the range of temperatures 445–950°C are conducted.

Role of high-elevation groundwater flows in the hydrogeology of the Cimino volcano (central Italy) and possibilities to capture drinking water in a geogenically contaminated environment

Origin, yield and quality of the groundwater flows at high elevation in the Cimino volcano (central Italy) were examined. In this area, groundwater is geogenically contaminated by arsenic and fluoride, yet supplies drinking water for approximately 170,000 inhabitants. The origin of the high-elevation groundwater flows is strictly related to vertical and horizontal variability of the rock types (lava flows, lava domes and ignimbrite) in an area of limited size. In some cases, groundwater circuits are related to perched aquifers above noncontinuous aquitards; in other cases, they are due to flows in the highly fractured dome carapace, limited at the bottom by a low-permeability dome core. The high-elevation groundwater outflow represents about 30% of the total recharge of Cimino’s hydrogeological system, which has been estimated at 9.8 L/s/km2. Bicarbonate alkaline-earth, cold, neutral waters with low salinity, and notably with low arsenic and fluoride content, distinguish the high-elevation groundwaters from those of the basal aquifer. Given the quantity and quality of these resources, approaches in the capture and management of groundwater in this hydrogeological environment should be reconsidered. Appropriate tapping methods such as horizontal drains, could more efficiently capture the high-elevation groundwater resources, as opposed to the waters currently pumped from the basal aquifer which often require dearsenification treatments.

Computational analysis of genes encoding for molecular determinants of arsenic tolerance in rice (Oryza sativa L.) to engineer low arsenic content varieties

Computational analysis of genes encoding for molecular determinants of arsenic tolerance in rice (<i>Oryza sativa</i> L.) to engineer low arsenic content varieties

Arsenic Transformation in Swine Wastewater with Low-Arsenic Content during Anaerobic Digestion

In this study, the raw wastewater (RW), and effluents from the acidogenic phase (AP) and methanogenic phase (MP) in a swine wastewater treatment plant were collected to investigate the occurrence and transformation of arsenic (As), as well as the abundance of As metabolism genes during the anaerobic digestion (AD) process. The results showed that total concentrations of As generally decreased by 33–71% after AD. Further analysis showed that the As species of the dissolved fractions were present mainly as dimethylarsinic acid (DMA), with arsenite (As(III)) and arsenate (As(V)) as the minor species. Moreover, real-time PCR (qPCR) results showed that As metabolism genes (arsC, arsenate reduction gene; aioA, arsenite oxidation gene and arsM, arsenite methylation gene) were highly abundant, with arsM being predominant among the metabolism genes. This study provides reliable evidence on As biotransformation in swine wastewater treatment process, suggesting that AD could be a valuable treatment to mitigate the risk of As in wastewater.

Molecular Study of Indigenous Bacterial Community Composition on Exposure to Soil Arsenic Concentration Gradient.

Community structure of bacteria present in arsenic contaminated agricultural soil was studied with qPCR (quantitative PCR) and DGGE (Denaturing Gradient Gel Electrophoresis) as an indicator of extreme stresses. Copy number of six common bacterial taxa (Acidobacteria, Actinobacteria, α-, β- and γ-Proteobacteria, Firmicutes) was calculated using group specific primers of 16S rDNA. It revealed that soil contaminated with low concentration of arsenic was dominated by both Actinobacteria and Proteobacteria but a shift towards Proteobacteria was observed with increasing arsenic concentration, and number of Actinobacteria eventually decreases. PCA (Principle Component Analysis) plot of bacterial community composition indicated a distinct resemblance among high arsenic content samples, while low arsenic content samples remained separated from others. Cluster analysis of soil parameters identifies three clusters, each of them was related to the arsenic content. Further, cluster analysis of 16S rDNA based DGGE fingerprint markedly distributed the soil bacterial populations into low (< 10 ppm) and high (> 10 ppm) arsenic content subgroups. Following analysis of diversity indices shows significant variation in bacterial community structure. MDS (Multi Dimensional Scaling) plot revealed distinction in the distribution of each sample denoting variation in bacterial diversity. Phylogenetic sequence analysis of fragments excised from DGGE gel revealed the presence of γ-Proteobacteria group across the study sites. Collectively, our experiments indicated that gradient of arsenic contamination affected the shape of the soil bacterial population by significant structural shift.

Rhizospheric iron and arsenic bacteria affected by water regime: Implications for metalloid uptake by rice

Rice is characterized by high levels of arsenic accumulation, even if cultivated in non-contaminated soils. Given the limits for arsenic concentration in rice grain recently established by the European Community, it is essential to understand the mechanisms and find solutions to this issue. Arsenic bioavailability is strictly related to water management of the rice paddy as well as to iron- and arsenic-cycling bacterial populations inhabiting the rice rhizosphere.To evaluate the effect of different agronomic conditions on the root-soil microbiota involved in arsenic mobilization, rice plants were grown in macrocosms containing non contaminated field soil under either continuous flooding, aerobic rice regime or continuous flooding with a 14 day-period of drainage before flowering. Specific groups of iron- and arsenic-cycling bacteria were assessed by real time quantitative PCR and fluorescence in situ hybridization.Continuous flooding led to the release of arsenate and iron in soil solution and produced rice grains with arsenite and organic arsenic above the recently established limits, contrary to the other agronomic conditions.Iron-reducing bacteria affiliated to the family Geobacteraceae significantly increased under continuous flooding in rhizosphere soil, in concomitance to arsenate dissolution from iron minerals. The 14 day-period of drainage before flowering allowed the recycling of iron, with the increase of Gallionella-like iron-oxidizing bacteria. This phenomenon likely influenced the decrease of arsenic translocation in rice grains.Regardless of the water regime, genes for arsenite oxidation (aioA) were the most abundant arsenic-processing genes, explaining the presence of arsenate in soil solution. The presence of arsenite and organic arsenic in rice grains produced under continuous flooding might be related to the retrieval of genes for arsenate reduction (arsC) and for arsenite methylation (arsM) in the proximity of the roots.These outcomes indicate a potential active role of rhizospheric iron- and arsenic-cycling bacteria in determining arsenic accumulation in rice grains from plants cultivated under continuous flooding, even in soil with a low arsenic content.

Contamination and mobilization of arsenic in the soil and groundwater and its influence on the irrigated crops, Manipur Valley, India

The arsenic concentration in groundwater used for irrigating the rice crops varies from 10 to 475 ppb. The Disang shales, deposited under marine environment, appear to be the main source of arsenic contaminating the groundwater. The arsenic content in the rice roots vary from 32 to 52 ppm while the arsenic in the grains is far above the limit prescribed by WHO. Sequential extraction process results indicate that a large part of arsenic is present in the residual phases. High concentration of arsenic at root depth is present in crystalline Fe-oxides and is available for the rice plants. Sequential extraction experiment on soils indicate low arsenic content in organic matter compared to the above phases.

Metagenomic analysis revealed highly diverse microbial arsenic metabolism genes in paddy soils with low-arsenic contents

Microbe-mediated arsenic (As) metabolism plays a critical role in global As cycle, and As metabolism involves different types of genes encoding proteins facilitating its biotransformation and transportation processes. Here, we used metagenomic analysis based on high-throughput sequencing and constructed As metabolism protein databases to analyze As metabolism genes in five paddy soils with low-As contents. The results showed that highly diverse As metabolism genes were present in these paddy soils, with varied abundances and distribution for different types and subtypes of these genes. Arsenate reduction genes (ars) dominated in all soil samples, and significant correlation existed between the abundance of arr (arsenate respiration), aio (arsenite oxidation), and arsM (arsenite methylation) genes, indicating the co-existence and close-relation of different As resistance systems of microbes in wetland environments similar to these paddy soils after long-term evolution. Among all soil parameters, pH was an important factor controlling the distribution of As metabolism gene in five paddy soils (p = 0.018). To the best of our knowledge, this is the first study using high-throughput sequencing and metagenomics approach in characterizing As metabolism genes in the five paddy soil, showing their great potential in As biotransformation, and therefore in mitigating arsenic risk to humans.