Arsenic biotransformation and volatilization in transgenic rice

Microbial-driven mechanisms of arsenic methylation during Chinese rice wine fermentation

Arsenic (As) may be a potential contaminant in Chinese rice wine. The methylation of As, which was evaluated as a possible detoxification mechanism to solve this issue. In this study, we simulated Chinese rice wine fermentation under laboratory conditions to explore As methylation behavior. The total As content of Chinese rice wine decreased by 68.6 % after fermentation. As species were transformed during fermentation; inorganic arsenic (iAs) was converted to methylated As; iAs decreased by 73.29 %, and the dimethylarsenic acid (DMA) percentage increased from 4.4 % to 19.9 %. We found that the As methylation gene (arsM) was ubiquitous in Chinese rice wine. Correlation analysis showed that physicochemical properties determined arsM abundance and DMA content during fermentation. The microbial As methylation pathway was summarized based on the annotated As functional genes from the Kyoto Encyclopedia of Genes and Genomes database. Higher relative abundances of glutathione S-transferase and arsC promoted the formation of more trivalent As substrates and further promoted methylation behavior for As detoxification during fermentation. According to the microbial arsM contribution analysis, Mycobacteroides, Rhizopus, and Jimgerdemannia were the primary As methylation microorganisms in Chinese rice wine. These results highlighted the specific As methylation process during fermentation, which could improve the control of As contamination in Chinese rice wines.

New concerns about arsenic contamination in agricultural fields: an in-depth understanding of the occurrence and regulatory strategies for rice straighthead disease

Rice, a pivotal staple crop globally, faces a significant threat from straighthead disease, a pervasive physiological disorder that can diminish or obliterate yields, thereby jeopardizing food security. Currently, it is established that dimethylarsinic acid (DMA), a form of organic arsenic (As) commonly found in paddy fields, is the causal agent inducing rice straighthead disease. However, a systematic understanding of the mechanisms underlying DMA absorption, transport, toxicity, and the induction of straighthead disease remains lacking, as do effective methods for disease prevention and control. This review introduces the incidence of straighthead disease caused by DMA and delves into the potential physiological mechanisms. It synthesizes the factors influencing the uptake of methylated As and their association with straighthead disease in rice. The efficient translocation of DMA from roots to shoots, coupled with its high oxidative toxicity, leads to the manifestation of straighthead disease. Hydrogen peroxide appears to serve as a critical signaling molecule under DMA stress, elevating cell wall pectin levels and inhibiting cell expansion, leading to deformed panicles in diseased rice. Agricultural strategies aimed at diminishing the accumulation of methylated As in rice, such as intermittent flooding, minimizing organic matter input, applying chemical amendments, selecting varieties with low DMA accumulation, and employing bioremediation techniques, could mitigate straighthead disease. This review aims to heighten awareness of methylated As toxicity and straighthead disease, providing a foundational reference to guide future efforts in disease management within rice cultivation.

Control of arsenic methylation in paddy soils by iron nanoparticles

Rice, as the most essential food grain, is frequently exposed to high concentrations of arsenic. Among the arsenic species, dimethylarsenate (DMAs(V)) is preferentially translocated from paddy soils to rice grains, posing serious threats to food safety and yield. Herein, we report an efficient strategy for DMAs(V) mitigation in paddy soils with nanoscale Zero-Valent Iron (nZVI). Species and concentrations of arsenic in paddy porewater were monitored during a 28-d soil-water incubation. Effects of nZVI dose towards microbial sulfate reduction and methane generation potential in paddy soils, which are crucial for arsenic methylation and demethylation, were analyzed via metagenomic sequencing. Results demonstrated that the maximal DMAs(V) concentration in paddy porewater decreased from 0.37 to 0.04 μM in arsenic-contaminated paddy soils with nZVI dose increasing from 0 to 5.0 g/kg. Accordingly, the maximal concentration of inorganic arsenite (iAs(III)), which is the precursor of DMAs(V), decreased from 1.39 to 0.23 μM. Furthermore, the application of nZVI reshaped the structure of microbial community in paddy soils. Specifically, the relative abundance of δ-proteobacteria involved in sulfate reduction, which is crucial for iAs(III) methylation, waned from 7.62 % to 3.17 %, while that of Methanomicrobia for DMAs(V) demethylation and methanogenesis proliferated from 7.03 % to 13.62 %, with nZVI dose increasing from 0 to 5.0 g/kg. Via simultaneous inhibition of DMAs(V) formation and acceleration of DMAs(V) transformation, nZVI efficiently controls the accumulation of DMAs(V) in paddy porewater. In conclusion, these findings prove the efficient performance for DMAs(V) mitigation with nZVI and uncover its underlying mechanisms.

Anoxygenic phototrophic purple non-sulfur bacteria: tool for bioremediation of hazardous environmental pollutants

The extraordinary metabolic flexibility of anoxygenic phototrophic purple non-sulfur bacteria (PNSB) has been exploited in the development of various biotechnological applications, such as wastewater treatment, biohydrogen production, improvement of soil fertility and plant growth, and recovery of high-value compounds. These versatile microorganisms can also be employed for the efficient bioremediation of hazardous inorganic and organic pollutants from contaminated environments. Certain members of PNSB, especially strains of Rhodobacter sphaeroides and Rhodopseudomonas palustris, exhibit efficient remediation of several toxic and carcinogenic heavy metals and metalloids, such as arsenic, cadmium, chromium, and lead. PNSB are also known to utilize diverse biomass-derived lignocellulosic organic compounds and xenobiotics. Although biodegradation of some substituted aromatic compounds by PNSB has been established, available information on the involvement of PNSB in the biodegradation of toxic organic pollutants is limited. In this review, we present advancements in the field of PNSB-based bioremediation of heavy metals and organic pollutants. Furthermore, we highlight that the potential role of PNSB as a promising bioremediation tool remains largely unexplored. Thus, this review emphasizes the necessity of investing extensive research efforts in the development of PNSB-based bioremediation technology.

Novel cost-effective design for bio-volatilization studies in photosynthetic microalgae exposed to arsenic with emphasis on growth and glutathione modulation

A novel laboratory model was designed to study the arsenic (As) biotransformation potential of the microalgae Chlorella vulgaris and Nannochloropsis sp. and the cyanobacterium Anabaena doliolum. The Algae were treated under different concentrations of As(III) to check their growth, toxicity optimization, and volatilization potential. The results revealed that the alga Nannochloropsis sp. was better adopted in term of growth rate and biomass than C. vulgaris and A. doliolum. Algae grown under an As(III) environment can tolerate up to 200 μM As(III) with moderate toxicity impact. Further, the present study revealed the biotransformation capacity of the algae A. doliolum, Nannochloropsis sp., and Chlorella vulgaris. The microalga Nannochloropsis sp. volatilized a large maximum amount of As (4,393 ng), followed by C. vulgaris (4382.75 ng) and A. doliolum (2687.21 ng) after 21 days. The present study showed that As(III) stressed algae-conferred resistance and provided tolerance through high production of glutathione content and As-GSH chemistry inside cells. Thus, the biotransformation potential of algae may contribute to As reduction, biogeochemistry, and detoxification at a large scale.

Heavy metals and arsenic stress in food crops: Elucidating antioxidative defense mechanisms in hyperaccumulators for food security, agricultural sustainability, and human health

The spread of heavy metal(loid)s at soil-food crop interfaces has become a threat to sustainable agricultural productivity, food security, and human health. The eco-toxic effects of heavy metals on food crops can be manifested through reactive oxygen species that have the potential to disturb seed germination, normal growth, photosynthesis, cellular metabolism, and homeostasis. This review provides a critical overview of stress tolerance mechanisms in food crops/hyperaccumulator plants against heavy metals and arsenic (HM-As). The HM-As antioxidative stress tolerance in food crops is associated with changes in metabolomics (physico-biochemical/lipidomics) and genomics (molecular level). Furthermore, HM-As stress tolerance can occur through plant-microbe, phytohormone, antioxidant, and signal molecule interactions. Information regarding the avoidance, tolerance, and stress resilience of HM-As should help pave the way to minimize food chain contamination, eco-toxicity, and health risks. Advanced biotechnological approaches (e.g., genome modification with CRISPR-Cas9 gene editing) in concert with traditional sustainable biological methods are useful options to develop 'pollution safe designer cultivars' with increased climate change resilience and public health risks mitigation. Further, the usage of HM-As tolerant hyperaccumulator biomass in biorefineries (e.g., environmental remediation, value added chemicals, and bioenergy) is advocated to realize the synergy between biotechnological research and socio-economic policy frameworks, which are inextricably linked with environmental sustainability. The biotechnological innovations, if directed toward 'cleaner climate smart phytotechnologies' and 'HM-As stress resilient food crops', should help open the new path to achieve sustainable development goals (SDGs) and a circular bioeconomy.

Recent advances in arsenic mitigation in rice through biotechnological approaches

Arsenic (As) is a major threat to the environment and human health due to its toxicity and carcinogenicity. Occurrence of alarming concentrations of As in water and soil leads to its bioaccumulation in crops which is a major health concern globally. Rice (Oryza sativa) is a staple food for a large population staying in As contaminated areas so, it is of utmost importance to reduce As levels in rice, especially grains. Amongst several strategies in practice, biotechnology may provide an effective option to reduce As accumulation in rice grains. Genetic engineering can be a viable approach to exploit potential genes playing roles in As metabolism pathway in plants. Besides, developing low As accumulating rice varieties through breeding is also an important area. Identifying genotypic variation in rice is a crucial step toward the development of a safe rice cultivar for growing in As-affected areas. Significant genotypic variation has been found in rice varieties for As accumulation in grains and that is attributable to differential expression of transporters, radial oxygen loss, and other regulators of As stress. This review provides recent updates on the research advances leading to transgenic and breeding approaches adopted to reduce As levels in rice, especially grains.

Harnessing plant microbiome for mitigating arsenic toxicity in sustainable agriculture

Heavy metal toxicity has become an impediment to agricultural productivity, which presents major human health concerns in terms of food safety. Among them, arsenic (As) a non-essential heavy metal has gained worldwide attention because of its noxious effects on agriculture and public health. The increasing rate of global warming and anthropogenic activities have promptly exacerbated As levels in the agricultural soil, thereby causing adverse effects to crop genetic and phenotypic traits and rendering them vulnerable to other stresses. Conventional breeding and transgenic approaches have been widely adapted for producing heavy metal resilient crops; however, they are time-consuming and labor-intensive. Hence, finding new mitigation strategies for As toxicity would be a game-changer for sustainable agriculture. One such promising approach is harnessing plant microbiome in the era of 'omics' which is gaining prominence in recent years. The use of plant microbiome and their cocktails to combat As metal toxicity has gained widespread attention, because of their ability to metabolize toxic elements and offer an array of perquisites to host plants such as increased nutrient availability, stress resilience, soil fertility, and yield. A comprehensive understanding of below-ground plant-microbiome interactions and their underlying molecular mechanisms in exhibiting resilience towards As toxicity will help in identifying elite microbial communities for As mitigation. In this review, we have discussed the effect of As, their accumulation, transportation, signaling, and detoxification in plants. We have also discussed the role of the plant microbiome in mitigating As toxicity which has become an intriguing research frontier in phytoremediation. This review also provides insights on the advancements in constructing the beneficial synthetic microbial communities (SynComs) using microbiome engineering that will facilitate the development of the most advanced As remedial tool kit in sustainable agriculture.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

Safe, efficient, and economically beneficial remediation of arsenic-contaminated soil: possible strategies for increasing arsenic tolerance and accumulation in non-edible economically important native plants

Anthropogenic activities, geological processes, and biogenic sources have led to the enhanced concentration of arsenic (As), a toxic metalloid in water and soil. Non-edible, economically important plants can be employed for safe As phytoremediation in addition to generating extra income. However, these plants may get affected by stressful local environmental conditions. Native plant species are adapted to local environmental conditions and hence overcome this problem. Native non-edible economic plant species which show high As tolerance and accumulation are promising candidate for safe, efficient, and economically beneficial phytoremediation of As-contaminated sites. The current review discusses the potential of native economic plant species that can be used in As phytoremediation programme. However, since their phytoremediation potential is moderate, possible strategies for increasing As  olerance and accumulation, especially genetic modification, have been discussed in detail. Knowledge gained from the review can be used for the development of As tolerance and accumulation in non-edible economic native plants.

Arsenic transformation and volatilization by arbuscular mycorrhizal symbiosis under axenic conditions

It is well known that arbuscular mycorrhizal (AM) fungi can enhance plant arsenic (As) resistance by influencing As uptake, translocation, and speciation; however, As transformation and volatilization by an entire plant inoculated with AM fungus remains uninvestigated. In the present study, AM symbiosis of Rhizophagus irregularis with unbroken Medicago sativa was successfully established in vitro. Afterwards, five concentrations of arsenate were applied to the culture media. The results showed that AM inoculation could methylate inorganic As into dimethylarsinic acid (DMA), dimethylarsine (DMAsH), and trimethylarsine (TMAs), which were detected in the plants, media, or air. Volatile As, accounting for a small proportion of total organic As, appeared under high arsenate exposure, accompanied by remarkable upregulation of root RiMT-11, an arsenite methyltransferase gene in R. irregularis. In addition, AM colonization significantly increased arsenite percentages in plant tissues and external media. Regardless of As species, AM inoculation tended to release the transformed As into the environment rather than transfer them to plant tissues. Our present study, for the first time, comprehensively verified As methylation, volatilization, and reduction by AM fungus associated with the entire plant under absolute axenic conditions and gained a deeper insight into As metabolism in AM symbionts.

Arsenic Accumulation in Hydroponically Grown Schizachyrium scoparium (Little Bluestem) Amended with Root-Colonizing Endophytes

We integrated microscopy, spectroscopy, culturing and molecular biology, and aqueous chemistry techniques to evaluate arsenic (As) accumulation in hydroponically grown Schizachyrium scoparium inoculated with endophytic fungi. Schizachyrium scoparium grows in historically contaminated sediment in the Cheyenne River Watershed and was used for laboratory experiments with As(V) ranging from 0 to 2.5 mg L^-1 at circumneutral pH. Arsenic accumulation in regional plants has been a community concern for several decades, yet mechanisms affecting As accumulation in plants associated with endophytic fungi remain poorly understood. Colonization of roots by endophytic fungi supported better external and vascular cellular structure, increased biomass production, increased root lengths and increased P uptake, compared to noninoculated plants (p value <0.05). After exposure to As(V), an 80% decrease of As was detected in solution and accumulated mainly in the roots (0.82-13.44 mg kg^-1) of noninoculated plants. Endophytic fungi mediated intracellular uptake into root cells and translocation of As. Electron microprobe X-ray mapping analyses detected Ca-P and Mg-P minerals with As on the root surface of exposed plants, suggesting that these minerals could lead to As adsorption on the root surface through surface complexation or coprecipitation. Our findings provide new insights regarding biological and physical-chemical processes affecting As accumulation in plants for risk assessment applications and bioremediation strategies.

Metalloid hazards: From plant molecular evolution to mitigation strategies

Metalloids such as boron and silicon are key elements for plant growth and crop productivity. However, toxic metalloids such as arsenic are increasing in the environment due to inputs from natural sources and human activities. These hazardous metalloids can cause serious health risks to humans and animals if they enter the food chain. Plants have developed highly regulated mechanisms to alleviate the toxicity of metalloids during their 500 million years of evolution. A better understanding the molecular mechanisms underlying the transport and detoxification of toxic metalloids in plants will shed light on developing mitigation strategies. Key transporters and regulatory proteins responsive to toxic metalloids have been identified through evolutionary and molecular analyses. Moreover, knowledge of the regulatory proteins and their pathways can be used in the breeding of crops with lower accumulation of metalloids. These findings can also assist phytoremediation by the exploration of plants such as fern species that hyperaccumulate metalloids from soils and water, and can be used to engineer plants with elevated uptake and storage capacity of toxic metalloids. In summary, there are solutions to remediate contamination due to toxic metalloids by combining the research advances and industrial technologies with agricultural and environmental practices.

Insights into conventional and recent technologies for arsenic bioremediation: A systematic review

Arsenic (As) bioremediation has been an economical and sustainable approach, being practiced widely under several As-contaminated environments. Bioremediation of As involves the use of bacteria, fungi, yeast, plants, and genetically modified organisms for detoxification/removal of As from the contaminated site. The understanding of multi-factorial biological components involved in these approaches is complex and more and more efforts are on their way to make As bioremediation economical and efficient. In this regard, we systematically reviewed the recent literature (n=200) from the last two decades regarding As bioremediation potential of conventional and recent technologies including genetically modified plants for phytoremediation and integrated approaches. Also, the responsible mechanisms behind different approaches have been identified. From the literature, it was found that As bioremediation through biosorption, bioaccumulation, phytoextraction, and volatilization involving As-resistant microbes has proved a very successful technology. However, there are various pathways of As tolerance of which the mechanisms have not been fully understood. Recently, phytosuction separation technology has been introduced and needs further exploration. Also, integrated approaches like phytobial, constructed wetlands using As-resistant bacteria with plant growth-promoting activities have not been extensively studied. It is speculated that the integrated bioremediation approaches with practical applicability and reliability would prove most promising for As remediation. Further technological advancements would help explore the identified research gaps in different approaches and lead us toward sustainability and perfection in As bioremediation.

Density Functional Theory and Thermodynamics Modeling of Inner-Sphere Oxyanion Adsorption on the Hydroxylated α-Al2O3(001) Surface

The inner-sphere adsorption of AsO₄^3-, PO₄^3-, and SO₄^2- on the hydroxylated α-Al₂O₃(001) surface was modeled with the goal of adapting a density functional theory (DFT) and thermodynamics framework for calculating the adsorption energetics. While DFT is a reliable method for predicting various properties of solids, including crystalline materials comprised of hundreds (or even thousands) of atoms, adding aqueous energetics in heterogeneous systems poses steep challenges for modeling. This is in part due to the fact that environmentally relevant variations in the chemical surroundings cannot be captured atomistically without increasing the system size beyond tractable limits. The DFT + thermodynamics approach to this conundrum is to combine the DFT total energies with tabulated solution-phase data and Nernst-based corrective terms to incorporate experimentally tunable parameters such as concentration. Central to this approach is the design of thermodynamic cycles that partition the overall reaction (here, inner-sphere adsorption proceeding via ligand exchange) into elementary steps that can either be fully calculated or for which tabulated data are available. The ultimate goal is to develop a modeling framework that takes into account subtleties of the substrate (such as adsorption-induced surface relaxation) and energies associated with the aqueous environment such that adsorption at mineral-water interfaces can be reliably predicted, allowing for comparisons in the denticity and protonation state of the adsorbing species. Based on the relative amount of experimental information available for AsO₄^3-, PO₄^3-, and SO₄^2- adsorbates and the well-characterized hydroxylated α-Al₂O₃(001) surface, these systems are chosen to form a basis for assessing the model predictions. We discuss how the DFT + thermodynamics results are in line with the experimental information about the oxyanion sorption behavior. Additionally, a vibrational analysis was conducted for the charge-neutral oxyanion complexes and is compared to the available experimental findings to discern the inner-sphere adsorption phonon modes. The DFT + thermodynamics framework used here is readily extendable to other chemical processes at solid-liquid interfaces, and we discuss future directions for modeling surface processes at mineral-water and environmental interfaces.

Dimethylarsinic acid is the causal agent inducing rice straighthead disease

Straighthead disease is a physiological disorder in rice with symptoms of sterile spikelets, distorted husks, and erect panicles. Methylated arsenic species have been implicated as the causal agent of the disease, but direct evidence is lacking. Here, we investigated whether dimethylarsinic acid (DMA) causes straighthead disease and its effect on the transcriptome of young panicles. DMA addition caused typical straighthead symptoms in hydroponic culture, which were alleviated by silicon addition. DMA addition to soil at the tillering to flowering stages induced straighthead disease. Transgenic rice expressing a bacterial arsenite methyltransferase gene gained the ability to methylate arsenic to mainly DMA, with the consequence of inducing straighthead disease. Field surveys showed that seed setting rate decreased with increasing DMA concentration in the husk, with an EC50 of 0.18 mg kg-1. Transcriptomic analysis showed that 364 and 856 genes were significantly up- and down-regulated, respectively, in the young panicles of DMA-treated plants compared with control, whereas Si addition markedly reduced the number of genes affected. Among the differentially expressed genes, genes related to cell wall modification and oxidative stress responses were the most prominent, suggesting that cell wall metabolism is a sensitive target of DMA toxicity and silicon protects against this toxicity.

Molecular mechanisms in phytoremediation of environmental contaminants and prospects of engineered transgenic plants/microbes

Concerns about emerging environmental contaminants have been growing along with industrialization and urbanization around the globe. Among various options for remediating these contaminants, phytotechnology is suggested as a feasible option to maintain the environmental sustainability. The recent advances in phytoremediation, genetic/molecular/omics/metabolic engineering, and nanotechnology are opening new paths for efficient treatment of emerging organic/inorganic contaminants. In this respect, elucidation of molecular mechanisms and genetic engineering of hyperaccumulator plants is expected to enhance remediation of environmental contaminants. This review was organized to offer valuable insights into the molecular mechanisms of phytoremediation and the prospects of transgenic hyperaccumulators with enhanced stress tolerance to diverse contaminants such as heavy metals and metalloids, xenobiotics, explosives, poly aromatic hydrocarbons (PAHs), petroleum hydrocarbons, pesticides, and nanoparticles. The roles of genoremediation and nanoparticles in augmenting the phytoremediation technology are also described in an interrelated framework with biotechnological prospects (e.g., plant molecular nano-farming). Finally, political debate on the preferential use of crops versus non-crop hyperaccumulators in genoremediation, limitations of transgenics in phytotechnologies, and their public acceptance issues are discussed in the policy framework.

Microbial Arsenic Methylation in Soil and Uptake and Metabolism of Methylated Arsenic in Plants: A Review

Arsenic (As) poses a risk to the human health in excess exposure and microbes play an important role in the toxicity of As. Arsenic methylation mediated by microbes is a key driver of As toxicity in the environment and this paper reviews the role of microbial arsenic methylation and volatilization in the biogeochemical cycle of arsenic. In specific, little is presently known about the molecular mechanism and gene characterization of arsenic methylation. The uptake of methylated arsenic in plants is influenced by microbial arsenic methylation in soil, thus enhancing the volatilization of methylated arsenic is a potential mitigation point for arsenic mobility and toxicity in the environment. On the other hand, the potential risk of methylated arsenic on organisms is also discussed. And the directions for future research, theoretical reference for the control and remediation of arsenic methylation, are presented.

Arsenic in cooked rice foods: Assessing health risks and mitigation options

Human exposure to arsenic (As) through the consumption of rice (Oryza sativa L.) is a worldwide health concern. In this paper, we evaluated the major causes for high inorganic As levels in cooked rice foods, and the potential of post-harvesting and cooking options for decreasing inorganic As content in cooked rice, focusing particularly on As endemic areas. The key factors for high As concentration in cooked rice in As endemic areas are: (1) rice cultivation on As-contaminated paddy soils; (2) use of raw rice grains which exceed 200 μg kg^-1 of inorganic As to cook rice; and (3) use of As-contaminated water for cooking rice. In vitro and in vivo methods can provide useful information regarding the bioaccessibility of As in the gastrointestinal tract. Urinary levels of As can also be used as a valid measure of As exposure in humans. Polishing of raw rice grains has been found to be a method to decrease total As content in cooked rice. Sequential washing of raw rice grains and use of an excess volume of water for cooking also decrease As content in cooked rice. The major concern with those methods (i.e. polishing of raw rice, sequential washing of raw rice, and use of excess volume of water for cooking rice) is the decreased nutrient content in the cooked rice. Cooking rice in percolating water has recently gained significant attention as a way to decrease As content in cooked rice. Introducing and promoting rainwater harvesting systems in As endemic areas may be a sustainable way of reducing the use of As-contaminated water for cooking purposes. In conclusion, post-harvesting methods and changes in cooking practices could reduce As content in cooked rice to a greater extent. Research gaps and directions for future studies in relation to different post-harvesting and cooking practices, and rainwater harvesting systems are also discussed in this review.

Arsenic volatilization by Aspergillus sp. and Penicillium sp. isolated from rice rhizosphere as a promising eco-safe tool for arsenic mitigation

Arsenic (As) is a non-threshold human carcinogenic. This element can be volatilized either by nature or anthropogenic sources. In the present study, the analytical performance of an As volatile species trapping system was evaluated to assess the As volatilization promoted by Penicillium sp. and Aspergillus sp., both isolated from rice rhizosphere, and Aspergillus niger sp. considered as a reference. The study was conducted for 60 days (sampling of volatile As species from 1st to 30^th day and from 31st to 60^th day). The efficiency of As-volatilization was associated with the fungal growth. The highest As volatilization occurred from 31st to 60^th day. Penicillium sp., Aspergillus sp. and A. niger were capable of producing 57.8, 46.4, and 5.2% of volatile arsenic species, respectively. The speciation analysis has shown trimethylarsine (TMAs) as the main volatilized As-form, followed by mono- and dimethylarsine (MMAs and DMAs). The results are following the "Challenger pathway". Therefore, the tested fungi isolated from rice rhizosphere have shown promising properties concerning bio-volatilization with potential use for As-mitigation in paddy soils.

Recent advances in arsenic metabolism in plants: current status, challenges and highlighted biotechnological intervention to reduce grain arsenic in rice

Arsenic (As), classified as a "metalloid" element, is well known for its carcinogenicity and other toxic effects to humans. Arsenic exposure in plants results in the alteration of the physiochemical and biological properties and consequently, loss of crop yield. Being a staple food for half of the world's population, the consumption of As-contaminated rice grain by humans may pose serious health issues and risks for food security. In this study, we have described the principal understanding of the molecular basis of arsenic toxicity and accumulation in plant parts. We described the measures for decreasing As accumulation in rice and understanding the mechanism and transport of As uptake, its transport from root to shoot to rice grain, its metabolism, detoxification, as well as the mechanisms lying behind its accumulation in rice grains. There are various checkpoints, such as the tuning of AsV/Pi specific Pi transporters, arsenate reductase, transporters that are involved in the efflux of As to either the vacuole or outside the cell, xylem loading, loading and unloading to the phloem, and transporters involved in the loading of As to grain, that can be targeted to reduce As accumulation in rice grain. Genes/proteins involved in As detoxification, particularly the glutathione (GSH) biosynthesis pathway, phytochelatin (PC) synthesis, and arsenic methyltransferase, also provide a great pool of pathways that can also be castellated for the low As in rice grains. Paddy rice is also used as fodder for animals, enhancing vacuolar sequestration and using constitutive promoters, which may be of concern for animal health. Therefore, using a root-specific promoter and/or converting inorganic arsenic into volatile organic arsenic might be a better strategy for low As in grain. Furthermore, in this review, the other specific approaches, such as bio-remediation, bio-augmentation practices, and molecular breeding, which have great potential to reduce As uptake from soil to rice grains, have also been highlighted.

Arsenic accumulation in rice (Oryza sativa L.) is influenced by environment and genetic factors

Arsenic (As) elevation in paddy soils will have a negative impact on both the yield and grain quality of rice (Oryza sativa L.). The mechanistic understanding of As uptake, translocation, and grain filling is an important aspect to produce rice grains with low As concentrations through agronomical, physico-chemical, and breeding approaches. A range of factors (i.e. physico-chemical, biological, and environmental) govern the speciation and mobility of As in paddy soil-water systems. Major As uptake transporters in rice roots, such as phosphate and aquaglyceroporins, assimilate both inorganic (As(III) and As(V)) and organic As (DMA(V) and MMA(V)) species from the rice rhizosphere. A number of metabolic pathways (i.e. As (V) reduction, As(III) efflux, and As(III)-thiol complexation and subsequent sequestration) are likely to play a key role in determining the translocation and substantial accumulation of As species in rice tissues. The order of translocation efficiency (caryopsis-to-root) for different As species in rice plants is comprehensively evaluated as follows: DMA(V) > MMA(V) > inorganic As species. The loading patterns of both inorganic and organic As species into the rice grains are largely dependent on the genetic makeup and maturity stage of the rice plants together with environmental interactions. The knowledge of As metabolism in rice plants and how it is affected by plant genetics and environmental factors would pave the way to develop adaptive strategies to minimize the accumulation of As in rice grains.

Prospects of genetic engineering utilizing potential genes for regulating arsenic accumulation in plants

The rapid pace of industrial, agricultural and anthropogenic activities in the 20^th century has resulted in contamination of heavy metals across the globe. Arsenic (As) is a ubiquitous, naturally occurring toxic metalloid, contaminating the soil and water and affecting human health in several countries. Several physicochemical methods exist for the cleanup of As contamination but these are expensive and disastrous to microbes and soil. Plant based remediation approaches are low cost and environmentally safe. Hence, extensive biochemical, molecular and genetic experiments have been conducted to understand plants' responses to As stress and have led to the identification of potential genes. The available knowledge needs to be utilized to either reduce As accumulation in crop plants (rice) or to enhance As levels in shoots of hyperaccumulators (Pteris vittata). Gene manipulation using biotechnological tools can be an effective approach to exploit the potential genes (plasmamembrane and vacuolar transporters, glutathione and phytochelatin biosynthetic enzymes, etc.) playing pivotal roles in uptake, translocation, transformation, complexation, and compartmentalization of As in plants. The transgenic plants with increased tolerance to As and altered (increased/decreased) As accumulation have been developed. The need, however, exists to design plants with altered expression of two or more genes for harmonizing various events (like arsenate reduction, arsenite complexation, sequestration and translocation) so as to achieve desirable reduction (crop plants) or increase (phytoremediator plants) in As content. This review sheds light on transgenic approaches adopted to modulate As levels in plants and proposes future directions to achieve desirable results.

Review of interactions between phosphorus and arsenic in soils from four case studies

Arsenic is a non-essential element that poses risks in many environments, including soil, groundwater, and surface water. Insights into the environmental biogeochemistry of As can be gained by comparing As and P reaction processes. Arsenic and P are chemical analogues, and it is proposed that they have similar chemical behaviors in environmental systems. However some chemical properties of As and P are distinct, such as redox reactions, causing the biogeochemical behavior of the two elements to differ. In the environment, As occurs as either As(V) or As(III) oxyanions (e.g., AsO₄^3- or AsO₃^3-). In contrast, P occurs predominantly as oxidation state five plus; most commonly as the orthophosphate ion (PO₄^3-). In this paper, data from four published case studies are presented with a focus on P and As distribution and speciation in soil. The goal is show how analyzing P chemistry in soils can provide greater insights into As reaction processes in soils. The case studies discussed include: (1) soil developed from shale parent material, (2) mine-waste impacted wetland soils, (3) phosphate-amended contaminated soil, and (4) plants grown in biochar-amended, mine-contaminated soil. Data show that while P and As have competitive reactions in soils, in most natural systems they have distinct biogeochemical processes that create differing mobility and bioavailability. These processes include redox reactions and rhizosphere processes that affect As bioavailability. Results from these case studies are used as examples to illustrate how studying P and As together allows for enhanced interpretation of As biogeochemical processes in soils.

A novel fungal arsenic methyltransferase, WaarsM reduces grain arsenic accumulation in transgenic rice (Oryza sativa L.)

Rice (Oryza sativa L.) grown on arsenic-containing soil and water become a primary dietary source of arsenic and pose a significant health risk. Gene modification is an important and practical approach to reduce arsenic accumulation in rice grains. Here, we reported a WaarsM gene of soil fungus Westerdykella aurantiaca, expressed in rice able to convert toxic inorganic arsenicals to methylated arsenic species, therefore, reduce arsenic accumulation in rice grains. In response to arsenic treatment in hydroponics, WaarsM expressing transgenic lines showed a marked increase in arsenic resistance and reduces its accumulation compared to NT. Also, WaarsM expressing transgenic Line 1 evolved ca. 157ng and ca. 43ng volatile arsenicals (mg-1 fresh weight) after 72h of exposure to 25μM AsIII and 250μM AsV, respectively. Transgenic Line 1, grown in soil irrigated with arsenic-containing water accumulates about 50% and 52% lower arsenic than NT in shoot and root, respectively; while arsenic concentration in polished seeds and husk of the transgenic line was reduced by 52% compared to NT. Thus, the present study demonstrates that the expression of WaarsM in rice induces arsenic methylation and volatilization, provides a potential strategy to reduce arsenic accumulation in rice grain.

Regulation of oxidative stress and mineral nutrient status by selenium in arsenic treated crop plant Oryza sativa

The present study was conducted to examine the impact of selenium (Se) on mineral nutrient status and oxidative stress in crop plant Oryza sativa treated with arsenic (As). Scanning electron microscopy (SEM) coupled with Energy dispersive x-ray spectroscopy (EDS) study revealed the morphological deformities in leaf veins along with granular deposition on the leaf surface. The EDS analysis exhibited loss of elements (S, Si, Cl, K, Ca, Fe and Cu) in As(III) treatment in rice roots as compared to untreated root. In the case of As(III) treated shoot, changes in elements content in term of percent atomic weight was K (1.17-0.90%), Cl (1.04-24.75%), Na (0.65-3.52%) and S (0.49-2.52%) when compared with untreated shoot. The result of EDS analysis showed that As limits the concentration of important mineral elements present in the rice root and shoot. Rice plant treated with Se (10µM) and sub lethal dose of As(III) (60µM) showed better growth responses in term of root, shoot length (11.4% and 10.71%, respectively), biomass (11.7%), reduced malonyldialdehyde content (35.14%) and stimulated antioxidant level indicating better As tolerance potential against As. Further, a selenium dependent significant reduction in As accumulation was also observed in root (14.24%) and shoot (23.78%) of rice plant when compared with plant treated with As alone. This study highlights the potential of Se to ameliorate the ecotoxicological risks associated with the As buildup in agricultural land.

Temporal dynamics of total and free-living nitrogen-fixing bacterial community abundance and structure in soil with and without history of arsenic contamination during a rice growing season

Despite the fact that the nitrogen (N) fixers act as the key regulator of ecosystem process, a detailed study of their abundance, diversity, and dynamics in arsenic (As)-contaminated rice fields is missing so far. DNA extracted from soil followed by 16S rRNA and nifH gene-based real-time qPCR, clone library analysis, and DNA sequencing were used to examine the status of the total and diazotrophic communities in two agricultural fields with and without arsenic contamination history during one rice cultivation season. In general, higher nifH and 16S rRNA gene copy numbers were observed in rice growing soils with lesser As than that with higher As. Elevated levels of 16S rRNA and nifH genes in soil is directly associated with total and nitrogen fixers abundance in the agricultural land without As contamination history through the cultivation period, but the copy number of 16S rRNA gene was decreased, and the nifH gene remained unchanged in the As-contaminated land. Additionally, Canonical Correspondence Analysis (CCA) indicated the possible suppression of nifH gene abundance by soil pH, phosphate, and As content. Increased abundance of total and Acidobacterial lineages in low As-containing soil and the detection of several uncultured groups among nifH gene sequence in higher frequency indicated the presence of novel nifH bearing bacterial groups. Conversely, the abundance of copiotrophic Proteobacterial lineages gradually increased in soil with higher As. Herein, our study demonstrated that the dynamics of free-living nitrogen-fixing bacterial communities were perturbed due to As contamination in agricultural land.

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.

Arsenic methylation by a genetically engineered Rhizobium-legume symbiont

BACKGROUND AND AIMS

Arsenic (As) is one of the most widespread environmental contaminants. The aim of our study was to test a novel bioremediation system based on the symbiosis between leguminous plant and genetically engineered rhizobia.

METHODS

The arsenite [As(III)] S-adenosylmethionine methyltransferase gene (CrarsM) from the alga Chlamydomonas reinhardtii was inserted into the chromosome of Rhizobium leguminosarum bv. trifolii strain R3. The As methylation ability of the recombinant Rhizobium was tested under free living conditions and in symbiosis with red clover plants. Arsenic speciation was determined using high-performance liquid chromatography-inductively coupled plasma mass spectrometry.

RESULTS

Under free-living conditions, CrarsM-recombinant R. leguminosarum gained the ability to methylate As(III) to methylated arsenicals, including methylarsenate [MAs(V)], dimethylarsenate [DMAs(V)] and trimethylarsine oxide [TMAs(V)O]. Red clover plants were inoculated with either control (non-recombinant) or CrarsM-recombinant R. leguminosarum and exposed to 5 or 10 μM arsenite. No methylated As species were detected in red clover plants inoculated with control R. leguminosarum. In contrast, all three methylated species were detected in both the nodules and the shoots when the recombinant Rhizobium established symbiosis with red clover, accounting for 74.7-75.1% and 29.1-42.4% of the total As in the two plant tissues, respectively. The recombinant symbiont also volatilized small amounts of As.

CONCLUSIONS

The present study demonstrates that engineered rhizobia expressing an algal arsM gene can methylate and volatilize As, providing a proof of concept for potential future use of legume-rhizobia symbionts for As bioremediation.

Arsenic Methylation and its Relationship to Abundance and Diversity of arsM Genes in Composting Manure

Although methylation is regarded as one of the main detoxification pathways for arsenic (As), current knowledge about this process during manure composting remains limited. In this study, two pilot-scale compost piles were established to treat manure contaminated with As. An overall accumulation of methylated As occurred during 60 day-composting time. The concentration of monomethylarsonic acid (MMA) increased from 6 to 190 μg kg^-1 within 15 days and decreased to 35 μg kg^-1 at the end of the maturing phase; while the concentration of dimethylarsinic acid (DMA) continuously increased from 33 to 595 μg kg^-1 over the composting time. The arsM gene copies increased gradually from 0.08 × 10⁹ to 6.82 × 10⁹ copies g^-1 dry mass over time and correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed the high abundance and diversity of arsM genes. Many of these genes were related to those from known As-methylating microbes, including Streptomyces sp., Amycolatopsis mediterranei and Sphaerobacter thermophiles. These results demonstrated that As methylation during manure composting is significant and, for the first time, established a linkage between As biomethylation and the abundance and diversity of the arsM functional genes in composting manure.

Medical geology in the framework of the sustainable development goals

Exposure to geogenic contaminants (GCs) such as metal(loid)s, radioactive metals and isotopes as well as transuraniums occurring naturally in geogenic sources (rocks, minerals) can negatively impact on environmental and human health. The GCs are released into the environment by natural biogeochemical processes within the near-surface environments and/or by anthropogenic activities such as mining and hydrocarbon exploitation as well as exploitation of geothermal resources. They can contaminate soil, water, air and biota and subsequently enter the food chain with often serious health impacts which are mostly underestimated and poorly recognized. Global population explosion and economic growth and the associated increase in demand for water, energy, food, and mineral resources result in accelerated release of GCs globally. The emerging science of "medical geology" assesses the complex relationships between geo-environmental factors and their impacts on humans and environments and is related to the majority of the 17 Sustainable Development Goals in the 2030 Agenda of the United Nations for Sustainable Development. In this paper, we identify multiple lines of evidence for the role of GCs in the incidence of diseases with as yet unknown etiology (causation). Integrated medical geology promises a more holistic understanding of the occurrence, mobility, bioavailability, bio-accessibility, exposure and transfer mechanisms of GCs to the food-chain and humans, and the related ecotoxicological impacts and health effects. Scientific evidence based on this approach will support adaptive solutions for prevention, preparedness and response regarding human and environmental health impacts originating from exposure to GCs.

Arsenic Transport in Rice and Biological Solutions to Reduce Arsenic Risk from Rice

Rice (Oryza sativa L.) feeds ∼3 billion people. Due to the wide occurrence of arsenic (As) pollution in paddy soils and its efficient plant uptake, As in rice grains presents health risks. Genetic manipulation may offer an effective approach to reduce As accumulation in rice grains. The genetics of As uptake and metabolism have been elucidated and target genes have been identified for genetic engineering to reduce As accumulation in grains. Key processes controlling As in grains include As uptake, arsenite (AsIII) efflux, arsenate (AsV) reduction and AsIII sequestration, and As methylation and volatilization. Recent advances, including characterization of AsV uptake transporter OsPT8, AsV reductase OsHAC1;1 and OsHAC1;2, rice glutaredoxins, and rice ABC transporter OsABCC1, make many possibilities to develop low-arsenic rice.

Arsenic and other heavy metal accumulation in plants and algae growing naturally in contaminated area of West Bengal, India

The present study was conducted to quantify the arsenic (As) and other heavy metal concentrations in the plants and algae growing naturally in As contaminated blocks of North-24-Pargana and Nandia district, West Bengal, India to assess their bioaccumulation potential. The plant species included five macrophytes and five algae were collected from the nine selected sites for estimation of As and other heavy metals accumulated therein by using Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS). Results revealed that maximum As concentration (117mgkg(-1)) was recorded in the agricultural soil at the Barasat followed by Beliaghat (111mgkg(-1)) sites of North-24-Pargana. Similarly, concentration of selenium (Si, 249mgkg(-1)), lead (Pb, 79.4mgkg(-1)), chromium (Cr, 138mgkg(-1)) was also found maximum in the soil at Barasat and cadmium (Cd, 163mgkg(-1)) nickel (Ni, 36.5mgkg(-1)) at Vijaynagar site. Among the macrophytes, Eichhornia crassipes found more dominating species in As contaminated area and accumulate As (597mgkg(-1)) in the shoot at kanchrapara site. The Lemna minor found to accumulate maximum As (735mgkg(-1)) in the leaves at Sonadanga and Pistia stratiotes accumulated minimum As (24.5mgkg(-1)) in the fronds from Ranaghat site. In case of diatoms, maximum As (760mgkg(-1)) was accumulated at Kanchrapara site followed by Hydrodictiyon reticulatum (403mgkg(-1)) at the Ranaghat site. High concentration of As and other heavy metal in soil indicates long term effects of irrigation with contaminated ground water, however, high concentration of heavy metals in naturally growing plants and algae revealed their mobilization through leaching and possible food chain contamination. Therefore, efficient heavy metal accumulator macrophytes Eichhornia crassipes, Lemna minor, Spirodela polyrhiza may be exploited in removing metals from contaminated water by developing a plant based treatment system. However, As accumulator algal species may be used as a bioresource for understanding algae mediated As detoxification and bioindication studies.

Arsenic Methylation in Arabidopsis thaliana Expressing an Algal Arsenite Methyltransferase Gene Increases Arsenic Phytotoxicity

Arsenic (As) contamination in soil can lead to elevated transfer of As to the food chain. One potential mitigation strategy is to genetically engineer plants to enable them to transform inorganic As to methylated and volatile As species. In this study, we genetically engineered two ecotypes of Arabidopsis thaliana with the arsenite (As(III)) S-adenosylmethyltransferase (arsM) gene from the eukaryotic alga Chlamydomonas reinhardtii. The transgenic A. thaliana plants gained a strong ability to methylate As, converting most of the inorganic As into dimethylarsenate [DMA(V)] in the shoots. Small amounts of volatile As were detected from the transgenic plants. However, the transgenic plants became more sensitive to As(III) in the medium, suggesting that DMA(V) is more phytotoxic than inorganic As. The study demonstrates a negative consequence of engineered As methylation in plants and points to a need for arsM genes with a strong ability to methylate As to volatile species.

Amelioration of arsenic toxicity in rice: Comparative effect of inoculation of Chlorella vulgaris and Nannochloropsis sp. on growth, biochemical changes and arsenic uptake

The present study was conducted to assess the responses of rice (Oryza sativa L. var. Triguna) by inoculating alga; Chlorella vulgaris and Nannochlropsis sp. supplemented with As(III) (50µM) under hydroponics condition. Results showed that reduced growth variables and protein content in rice plant caused by As toxicity were restored in the algae inoculated plants after 7d of treatment. The rice plant inoculated with Nannochloropsis sp. exhibited a better response in terms of increased root, shoot length and biomass than C. vulgaris under As(III) treatment. A significant reduction in cellular toxicity (thiobarbituric acid reactive substances) and antioxidant enzyme (SOD, APX and GR) activities were observed in algae inoculated rice plant under As(III) treatment in comparison to uninoculated rice. In addition, rice treated with As(III), accumulated 35.05mgkg(-1)dw arsenic in the root and 29.96mgkg(-1)dw in the shoot. However, lower accumulation was observed in As(III) treated rice inoculated with C. vulgaris (24.09mg kg(-1)dw) and Nannochloropsis sp. (20.66mgkg(-1)dw) in the roots, while in shoot, it was 20.10mgkg(-1)dw and 11.67mgkg(-1)dw, respectively. Results demonstrated that application of these algal inoculum ameliorates toxicity and improved tolerance in rice through reduced As uptake and modulating antioxidant enzymes. Thus, application of algae could provide a low-cost and eco-friendly mitigation approach to reduce accumulation of arsenic in edible part of rice as well as higher yield in the As contaminated agricultural field.

MDI Biological Laboratory Arsenic Summit: Approaches to Limiting Human Exposure to Arsenic

This report is the outcome of the meeting "Environmental and Human Health Consequences of Arsenic" held at the MDI Biological Laboratory in Salisbury Cove, Maine, August 13-15, 2014. Human exposure to arsenic represents a significant health problem worldwide that requires immediate attention according to the World Health Organization (WHO). One billion people are exposed to arsenic in food, and more than 200 million people ingest arsenic via drinking water at concentrations greater than international standards. Although the US Environmental Protection Agency (EPA) has set a limit of 10 μg/L in public water supplies and the WHO has recommended an upper limit of 10 μg/L, recent studies indicate that these limits are not protective enough. In addition, there are currently few standards for arsenic in food. Those who participated in the Summit support citizens, scientists, policymakers, industry, and educators at the local, state, national, and international levels to (1) establish science-based evidence for setting standards at the local, state, national, and global levels for arsenic in water and food; (2) work with government agencies to set regulations for arsenic in water and food, to establish and strengthen non-regulatory programs, and to strengthen collaboration among government agencies, NGOs, academia, the private sector, industry, and others; (3) develop novel and cost-effective technologies for identification and reduction of exposure to arsenic in water; (4) develop novel and cost-effective approaches to reduce arsenic exposure in juice, rice, and other relevant foods; and (5) develop an Arsenic Education Plan to guide the development of science curricula as well as community outreach and education programs that serve to inform students and consumers about arsenic exposure and engage them in well water testing and development of remediation strategies.

Biochar Soil Amendment Effects on Arsenic Availability to Mountain Brome ()

Biochar is a renewable energy byproduct that shows promise for remediating contaminated mine sites. A common contaminant at mine sites is arsenic (As). In this study, the effects of biochar amendments to a mine-contaminated soil on As concentrations in mountain brome ( Nees ex Steud.) were investigated. In the biochar-amended soil, mountain brome had greater root biomass and decreased root and shoot As concentrations. X-ray absorption near-edge structure spectroscopy results showed that arsenate [As(V)] is the predominant species in both the nonamended and biochar-amended soils. Soil extraction tests that measure phosphate and arsenate availability to plants failed to accurately predict plant tissue As concentrations, suggesting the arsenate bioavailability behavior in the soils is distinct from phosphate. Results from this study indicate that biochar will be a beneficial amendment to As-contaminated mine sites for remediation.

Integrated phytobial remediation for sustainable management of arsenic in soil and water

Arsenic (As), cited as the most hazardous substance by the U.S. Agency for Toxic Substance and Disease Registry (ATSDR, 2005), is an ubiquitous metalloid which when ingested for prolonged periods cause extensive health effects leading to ultimate untimely death. Plants and microbes can help mitigate soil and groundwater As problem since they have evolved elaborate detoxification machineries against this toxic metalloid as a result of their coexistence with this since the origin of life on earth. Utilization of the phytoremediation and bioremediation potential of the plants and microbes, respectively, is now regarded as two innovative tools that encompass biology, geology, biotechnology and allied sciences with cutting edge applications for sustainable mitigation of As epidemic. Discovery of As hyperaccumulating plants that uptake and concentrate large amounts of this toxic metalloid in their shoots or roots offered new hope to As phytoremediation, solar power based nature's own green remediation. This review focuses on how phytoremediation and bioremediation can be merged together to form an integrated phytobial remediation which could synergistically achieve the goal of large scale removal of As from soil, sediment and groundwater and overcome the drawbacks of the either processes alone. The review also points to the feasibility of the introduction of transgenic plants and microbes that bring new hope for more efficient treatment of As. The review identifies one critical research gap on the importance of remediation of As contaminated groundwater not only for drinking purpose but also for irrigation purpose and stresses that more research should be conducted on the use of constructed wetland, one of the most suitable areas of application of phytobial remediation. Finally the review has narrowed down on different phytoinvestigation and phytodisposal methods, which constitute the most essential and the most difficult part of pilot scale and field scale applications of phytoremediation programs.

Omics and biotechnology of arsenic stress and detoxification in plants: current updates and prospective

Arsenic (As), a naturally occurring metallic element, is a dreadful health hazard to millions of people across the globe. Arsenic is present in low amount in the environment and originates from anthropogenic impact and geogenic sources. The presence of As in groundwater used for irrigation is a worldwide problem as it affects crop productivity, accumulates to different tissues and contaminates food chain. The consumption of As contaminated water or food products leads to several diseases and even death. Recently, studies have been carried out to explore the biochemical and molecular mechanisms which contribute to As toxicity, accumulation, detoxification and tolerance acquisition in plants. This information has led to the development of the biotechnological tools for developing plants with modulated As tolerance and detoxification to safeguard cellular and genetic integrity as well as to minimize food chain contamination. This review aims to provide current updates about the biochemical and molecular networks involved in As uptake by plants and the recent developments in the area of functional genomics in terms of developing As tolerant and low As accumulating plants.

Earth Abides Arsenic Biotransformations

Arsenic is the most prevalent environmental toxic element and causes health problems throughout the world. The toxicity, mobility, and fate of arsenic in the environment are largely determined by its speciation, and arsenic speciation changes are driven, at least to some extent, by biological processes. In this article, biotransformation of arsenic is reviewed from the perspective of the formation of Earth and the evolution of life, and the connection between arsenic geochemistry and biology is described. The article provides a comprehensive overview of molecular mechanisms of arsenic redox and methylation cycles as well as other arsenic biotransformations. It also discusses the implications of arsenic biotransformation in environmental remediation and food safety, with particular emphasis on groundwater arsenic contamination and arsenic accumulation in rice.

Interactive effects of different inorganic As and Se species on their uptake and translocation by rice (Oryza sativa L.) seedlings

There is a lack of information on the interactive relationship of absorption and transformation between two inorganic arsenic (As) species and two inorganic selenium (Se) species in rice grown under hydroponic condition. Interactive effects of inorganic As (As(III)) and (As(V)) and Se (Se(IV)and Se(VI)) species on their uptake, accumulation, and translocation in rice (Oryza sativa L.) seedlings were investigated in hydroponic culture. The results clearly showed the interactive effects of inorganic As and Se on their uptake by rice. The presence of Se reduced the sum of As species in the rice shoots regardless of Se speciation. If Se is present as Se(IV), then is it is accompanied by a corresponding increase of the sum of As species, but if Se is present as Se(VI), then there is no change in the sum of As species in rice roots. These effects are observed regardless of initial As speciation. When the rice plants are exposed to Se(IV), the presence of As increases the sum of Se species in the roots, and decreases the sum of Se species in the corresponding shoots. This effect is more pronounced for As(III) than for As(V). There is no effect on Se during exposure to Se(VI). Co-existence of As also increased SeMet in rice roots.

High arsenic in rice is associated with elevated genotoxic effects in humans

Arsenic in drinking water may cause major deleterious health impacts including death. Although arsenic in rice has recently been demonstrated to be a potential exposure route for humans, there has been to date no direct evidence for the impact of such exposure on human health. Here we show for the first time, through a cohort study in West Bengal, India, involving over 400 human subjects not otherwise significantly exposed to arsenic through drinking water, elevated genotoxic effects, as measured by micronuclei (MN) in urothelial cells, associated with the staple consumption of cooked rice with >200 μg/kg arsenic. Further work is required to determine the applicability to populations with different dietary and genetic characteristics, but with over 3 billion people in the world consuming rice as a staple food and several percent of this rice containing such elevated arsenic concentrations, this study raises considerable concerns over the threat to human health.