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Vol. 21 No. 3 - July 2015

Arsenic: A dreaded threat to Environment

By: RumaRanjan¹, Atul Kumar Upadhyay¹,   Navin Kumar¹, Arvind Kumar Dubey¹, U. N. Rai¹ and S. N. Pandey²

Arsenic (As) is a toxic metalloid that is pervasive in the environment and causes numerous health problems. Arsenic exists in the environment with different oxidation states (-3, 0, +3, +5), dominantly present in two inorganic forms viz., pentavalent Arsenate [As (V)] and trivalent Arsenite [As (III)]. The volcanic activity is the original source of much of the arsenic in sedimentary rocks, however, in recent time, arsenic release from weathering has been approximately in balance with deposition of arsenic in sediments. Human activities, such as the use of arsenic pesticides, herbicides, crop desiccants, the burning of fossil fuel, the mining and processing of sulfide minerals, soil erosion and leaching have increased the amount of arsenic entering the oceans which have caused additional impacts.  Arsenate is predominantly present in oxygen rich environments while As (III) is the dominant As species in reducing anaerobic environments such as flooded paddy soils. About 150 million people are exposed to As contamination in the World with the largest percentage coming from Asia especially Bangladesh and West Bengal (India). The ministry of water resource, Government of India in 2009 reported that ground water concentration of As in West Bengal, Bihar, Chhattisgarh and Uttar Pradesh was ranged from 0.05-3.773 mg/l, 0.05-1.810 mg/l, 0.05-1.89 mg/l, 0.05-1.31 mg/l respectively. About 30 million people in Bangladesh using Arsenic-contaminated shallow tube wells (STWs) water for drinking and approximately 2.4 million hectare out of 4 million hectare crop fields (mainly paddy fields) are irrigated by As contaminated ground water from 900,000 STWs. Arsenic contaminated ground water leads to the accumulation of As in paddy soils, which poses adverse effects on rice yield and quality.

The biological cycle of arsenic in the surface of ocean involves the uptake of arsenate by plankton, the conversion of arsenate to a number of, as yet unidentified, organic compounds, and the release of arsenite and methylated species into the seawater. Biological demethylation of the methylarsenicals and the oxidation of arsenite through unknown mechanisms serve to regenerate arsenate. Microorganisms play a central role in converting inorganically and organically bound metals to other chemical forms and transporting metals among various compartments of aquatic ecosystems as adsorbed or absorbed As species.

The pathway proceeds anaerobically by arsenate reduction to arsenite followed by several methylation steps producing dimethylarsine and trimethylarsine. Arsenic transformation by microbes may play a critical role in the fate and toxicity of As. Microorganisms are involved in the redistribution and global cycling of arsenic. Arsenic can accumulate and can be subject to various biotransformations including reduction, oxidation, and methylation processes. Methylation through bacteria of inorganic arsenic is coupled to the methane biosynthetic pathway in methanogenic bacteria under anaerobic conditions and this provides a mechanism for arsenic detoxification. The metabolism of As (V) has been studied in many organisms, including mammals, plants and microorganisms. Cyanobacteria, the free-living photosynthetic prokaryotes, are widely distributed in lakes, pond, springs, wetland, streams and rivers, and play a major role in the nitrogen, oxygen and carbon dynamics of many aquatic environments. Cyanobacteria form symbiotic association with more complex biota; for example, the nitrogen-fixing Anabaena species which forms a symbiotic association with the rice plants, is widely distributed in paddy fields. The paddy field ecosystem provides a favorable environment for the growth of cyanobacteria with respect to their requirements for light, water, high temperature and nutrient availability. This could be the reason for more abundant cyanobacterial growth in paddy soils than in upland soils.

Arsenic contamination and  toxicity

Arsenic is causing a worldwide epidemic of poisoning, with tens of thousands of people having developed skin lesions, cancers, and other symptoms like cancer, cerebrovascular disease, diabetes mellitus, and kidney diseases. In seawater, the concentration of As is usually less than 2 µg L-1. The levels of arsenic in unpolluted surface water and ground water are typically from 1-10 µg L-1. In fresh water, the variation is in the range of 0.15-0.45 µg L-1. in thermal water, concentrations of up to 8.5 mg L-1 and 1.8-6.4 mg L-1 has been reported from New Zealand and Japan, respectively. Natural geological sources of As in drinking water are one of the most significant causes of As contamination around the world. The World Health Organization has set a guideline of 10 µg L-1 as the drinking water standard. The concentration of As in different parts of the world has been depicted in table 1.

Table-1. Arsenic concentration in different continents of the world

Rice is the staple food of global population especially of the people in South-East Asian countries where it contributes over 70% of the energy and 50% of protein provided by their daily food intake. In some areas of Bangladesh and West Bengal, which are the worst-affected regions, groundwater As concentration has reached 2 mg l−1, while the WHO provisional guideline value for drinking water is only 0.01 mg L−1. Rice is particularly efficient in arsenic accumulation compared to other cereals as it is generally grown under flooded conditions where arsenic mobility is high. Grain baseline levels of arsenic are 10-fold higher than other cereal grains. A recent study indicated that the concentration of As in rice straw could be up to 92 µg g −1 when rice plants were irrigated with As-contaminated groundwater. The large cultivated area of paddy fields is irrigated with As contaminated ground water. The As is easily taken up by rice plants when irrigated with As contaminated water. Excessive uptake of As by rice plant creates food safety problem as well as the problem of food chain contamination. Thus, being a staple crop worldwide, there is pressing need to develop eco-friendly, sustainable approach to minimize the problems caused by As contamination. The toxicity effects of chronic and acute exposures of arsenic may result in cancer, cardiovascular disease (hypertension and atherosclerosis), neurological disorders, gastrointestinal disturbances, liver disease and renal disease, reproductive health effects, dermal changes, skin lesions to cancer of the brain, liver, kidney and stomach. When air containing arsenic dusts is breathed in, the greater part of the dust particles settle onto the lining of the lungs. Colour pigments that are used in the cosmetic industry in the production of eye shadows frequently contain toxic elements, including arsenic. The skin of the eyelids is very delicate and the application of eye shadows may produce eczemas. Arsenic induced genotoxicity may involve an alteration of the integrity of the cellular genetic material by oxidants or free radical species. A wide range of arsenic toxicity has been determined that depends on As speciation. Generally inorganic arsenic species are more toxic than organic forms to living organisms, including human and other animals. Exposure to As trioxide by ingestion of 70-80 mg has been reported to be fatal for humans.

Arsenic contamination affects the plant growth and crop yield. Its accumulation in food is harmful to animals and humans. Arsenite [As(III)] and arsenate [As(V)] are the phytoavailable forms of inorganic As in soil solution. Arsenate is taken up by plants via phosphate transporters in the plasma membrane of root cells, and is rapidly reduced to arsenite once inside the cytoplasm. Since arsenate and phosphate behave as analogues with respect to their uptake, arsenate toxicity is linked to phosphorus nutrition, and high levels of phosphate can mitigate arsenate toxicity. Recent studies have provided experimental evidence that As induced generation of free radicals and oxidative stress can cause cell damage and cell death through activation of oxidative sensitive signaling pathways. Exposure to As causes reduced root elongation and branching, leaf chlorosis, and the shrinking and even necrosis of the aerial parts of plants. Arsenic can also induce the production of reactive oxygen species (ROS) in plants. The overproduction of reactive oxygen species (ROS) is commonly triggered by heavy metals in animal and plant tissues. ROS are produced basically in all cell compartments, associated with electron-transport chains in chloroplasts, mitochondria, and peroxisomes. Overproduction of ROS is very harmful because it can affect proteins, lipids, and DNA, giving rise to lipid peroxidation, membrane leakage, enzyme inactivation, and DNA breaks or mutations which can compromise cell viability.

Bioremediation of As

Mitigation of environmental As contamination is a vital requirement in many parts of the world. Various technologies are in place to clean up arsenic or to reduce arsenic exposure from contact with arsenic contaminated soil and water. At the present time, there appears to be no cost-effective method for the in situ cleaning of arsenic contaminated soils and groundwater. Besides, the physicochemical technologies such as chemical precipitation, activated alumina, reverse osmosis, etc. are currently used for the ex-situ clean-up of arsenic contaminated water (groundwater, drinking water, industrial wastewater etc). But these technologies have significant drawbacks such as high cost, generation of high volumes of toxic sludge and brine and low water recovery. Bioremediation is the process of environmental biotechnology which uses microorganisms and plants to clean the environment. Bioremediation strategies have been proposed as an attractive alternative owing to their low cost and high efficiency. Recent studies show that the strains of organisms like algae, cyanobacteria, bacteria, fungi and yeast isolated from contaminated soils, water and contaminated sludge have excellent capability of removing significant amounts of metals. It is reported that microbes accumulate high concentration of As and also detoxify As through methylation process and convert inorganic As (iAs) to organic As (oAs) in methylated form of As which is less toxic and volatile form. Microorganisms such as Pseudomonas sp., Synechocysis sp., Ostreococcustauri, Bacillus Sp., Chlorella sp and cyanobacteria play an important role in As remediation. These microorganisms breakdown the toxins during their metabolic activity. Cyanobacteria occur naturally in such places where toxins are abundant and they can be removed through methylation and any other detoxification method. Toxins are just like their food and these microbes are able to remove toxins in large quantity. In addition, cyanobacteria have As detoxification mechanism through methylation to produce organic arsenicals, which are less toxic than inorganic As i.e., methylarsenate [MAs(V)], dimethylarsenate [DMAs(V) or cacodylate], trimethylarsine oxide [TMAsO(V)], and the final product of the methylation pathway, volatile trimethylarsine [TMAs (III)]. As methylation is supposed to play an important role in As cycling among terrestrial, aquatic and atmospheric surroundings.

Phytoremediation, a plant-based green technology, has received attention as an effective, low cost, eco-friendly clean-up option for As-contaminated areas. The Chinese brake fern (Pterisvittata) has been reported to hyper-accumulate arsenic to extremely high concentrations, up to 23,000 µg arsenic/g, in its shoots (fronds). This fern actually thrives on arsenic, doubling its biomass in one week when subjected to 100 ppm arsenic. The striking feature of P. vittata is its remarkable capacity to transport arsenic from roots to shoots. It can accumulate up to 95% of the arsenic in the above-ground tissue. A number of other fern species of the family Pteridaceae have also been identified as As hyper-accumulators. Arsenic hyper-accumulation appears to involve enhanced arsenate uptake by the phosphate transporters. Recent studies have shown that the duckweed Spirodelapolyrrhiza, Hydrillaverticillata and several Azolla species have moderate amount of As accumulation and tolerance. A number of duckweeds e.g. Wolffiaglobosa is a strong accumulator of As and submerged macrophyteHydrillaverticillata is also reported to be a good As accumulator. Some species of submerged macrophytes, such as Callitrichestagnalis and Myriophyllumpropinquum, which play important structural and functional roles in aquatic ecosystems have shown tremendous potential to accumulate As and are therefore potential phytofiltrators of As-contaminated water.  

¹Plant Ecology and Environment Science Division, CSIR-NBRI, Rana Pratap Marg, Lucknow-226001, India

²Department of Botany, University of Lucknow, Lucknow-226007, India

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This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.