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Kashma N B et.al 2016 ISSN-2321-0524 Research Paper

Role of Microbes in Bioremediation and as Biosensors of Arsenic Pollution in the Environment Kashma N B1 , Geethanjali P A2

Department of Microbiology,Field Marshal K M Cariappa College, Karnataka, India.

Abstract

Arsenic is a toxic heavy metal, noted for its potential toxicity in the environmental contexts. The presence of arsenic in soil and water has become an increasing problem in many countries around the world. The contamination is widely regarded as the largest current calamity of chemical poisoning. Arsenic compounds are widely used as agricultural insecticides, herbicides and wood preservatives. Coal combustion also volatizes and releases arsenic as gases and fine grained aerosols to the atmosphere. A few mining plants remove arsenic contained in ore and release large amounts of arsenic as wastes. The cleaning up of soil and water contaminated with arsenic residues is a global problem today. The biological remediation methods are widely accepted than physical and chemical methods because of the high reagents costs and difficulty to imply it on large scale. The use of bioremediation to remove and mobilize arsenic from contaminated soils and aquifers could be effective and economic ways since a wide range of microorganisms have been found to be successfully degrading this pollutant from the environment. However, certain species of prokaryotes have the ability to use arsenic either through oxidation or reduction process for energy conservation and growth purposes. The present paper includes the various processes involved in microbial transformation of arsenic residues in soil. It also includes various species of microorganisms involved as biosensors to detect the arsenic pollutant in the environment since microorganisms serve best as environmental friendly green technology of future.

Keywords: arsenic; pollution; bioremediation; biosensors; microorganisms.

INTRODUCTION

Arsenic is ubiquitous in the environment, usually being present in small amounts in all rocks, soils, waters, air and biological tissues. Elevated concentrations are found in polluted environment. Shallow ground water in most areas is contaminated with arsenic which causes serious health hazards as reported from many countries of the world, with some countries including Vietnam, West Bengal, Mexico, Canada, Argentina, Bangladesh and USA all severely affected [1]. The biggest known Arsenic (As) calamity occurred in the Bengal Delta (Bangladesh/West Bengal) where millions of people depend on As-rich drinking water in Bangladesh, it is reported that about 2 lakhs people are suffering from arsenicosis ranging from melanosis to skin cancer. In India, the people of the province of West Bengal have been suffering seriously from arsenic problem[2]. A recently discovered case of groundwater contamination in Hanoi (Vietnam) was reported with contamination levels varying from 1 to 3050 mg /L [3]. Arsenic is toxic to all living cells, and in people causes fatal cancers of the lung, liver, kidney, bladder, cirrhosis and gangrene, and on a wider scale seriously damages wildlife in fragile environments. Arsenic, is a trace metalloid and concentrations of As in non-contaminated soils are typically well below 10 mg/ kg. Its presence at elevated concentrations in soils is

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due to both anthropogenic and natural inputs. Anthropogenic sources include mining and smelting processes besides application of As-based insecticides, herbicides, fungicides, wood preservatives, dyestuffs, feed additives etc. Arsenic is also commonly associated with sulphide ore deposits. Other natural sources of As include volcanic activities, windborne soil particles, sea salt sprays and microbial volatilization of As [4].The maximum permissible concentration of arsenic in drinking water recommended by both the EPA and World Health Organization (WHO) is 10 µg/L[5] .

Sources of arsenic in the environment

Soil is a major repository for arsenic. Arsenic is present in coal and at much smaller concentrations in oil.

Combustion of fossil fuels in electrical power plants releases arsenic, which can be deposited onto nearby soil. Arsenic dusts and gases also are released during cement manufacture. Leaching of arsenic from treated lumber can increase soil arsenic near the wood. Rocks high in arsenic release this element as the rock weathers, causing arsenic enrichment of local soil and groundwater. Volcanic emissions and hot springs associated with volcanic activity are another natural source of arsenic. Surface runoff or percolating groundwater from springs having high concentrations of arsenic can increase the soil arsenic content of nearby soils. Soil arsenic normally occurs as negatively charged ions bound to soil particle surfaces. Arsenic solubility and mobility usually increase in very wet or flooded soils. Most surface-deposited arsenic remains in the topsoil, but considerable amounts may have leached into the subsoil, particularly in sandy soils.

Microbial transformation of arsenic in soil

Microorganisms can dissolve different minerals and use them as a source of nutrients and energy. More than 200 minerals that contain arsenic are found in nature. Arsenic occurs mainly as arsenides, sulfides, oxides, arsenates and arsenites. Most of these minerals are found in close association with metals such as Fe, Cu, Co, Ni, Cd, Pb, Ag and Au. Sulfide minerals are also a source of enzyme cofactors such as Mo, Cu, Zn, Mg, and minerals such as apatite can supply phosphorus, required for the synthesis of DNA, RNA, ATP and phospholipids [6].

Microorganisms have evolved different mechanisms of nutrient uptake from minerals. The primary mechanism is connected with colonization and involves physical penetration of the rock surface causing disaggregation of the mineral. Exopolysaccharides (EPS) produced by microorganisms also play an important role in the mineral colonization process. EPS can serve as substrates for other (heterotrophic) bacteria, which produce organic agents that promote the chemical dissolution of the underlying minerals [7].

Burkholderia fungorum (ATCC BAA-463) growing in a phosphorus-limited medium could release arsenic from apatite through the production of gluconic acid [8].

Bio oxidation of Arsenic containing ores

The bio oxidation process is used as a pre treatment, whereby the desired metal remains in the rock, but the mineral structure is converted to allow better penetration of chemicals that can solubilise it. A good example of this type of bio oxidation is the BIOX™ process, which was the first such method for the pre treatment of refractory gold sulphide ores such as arsenopyrite [9].During the bio oxidation process, iron, arsenic, and sulphur are removed from gold deposits so that the gold that remains in the mineral is more easily extracted by subsequent treatment with cyanide. Bio oxidation has the advantages of being less expensive and safer for the environment. Another technology, developed for the treatment of concentrates not suitable for commercial smelting due to their content of deleterious elements, such as arsenic, is BioCOP™ [10]. The BioCOP™ process utilizes thermophilic microorganisms operating at temperatures of up to 80 °C to leach copper sulfide mineral concentrates. A semi-commercial demonstration plant employing thermophiles to solubilize a copper–arsenic concentrate, was successfully implemented by Alliance Copper at Chuquicamata in Chile [11]. The use of Acidithiobacillus ferrooxidans for the partial removal of arsenic from galena (PbS)

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concentrate was demonstrated by Makita et al. Galena and arsenopyrite were totally oxidized, but only 22–

33 % of arsenic from the original content in the mineral was retained in the solution. Oxidized galena formed an anglesite (PbSO4) concentrate and the rest of the bio oxidized arsenic was precipitated in the form of amorphous compounds[12].

Biological treatment of Arsenic contaminated waters

Numerous bacteria, fungi, yeasts and algae are able to transform Arsenic compounds by oxidation, reduction, demethylation and methylation. Microbial reduction of AsV to AsIII is known to occur by dissimilatory reduction and detoxification activities of microbes. The recovery of arsenic from mine waters is a major environmental issue that has to be addressed by the mining industry. Various physical and chemical remediation methods have been proposed, depending on the geochemistry of the contaminated waters. Most of these procedures are by use of powerful oxidants such as potassium permanganate, hydrogen peroxide or ozone. However, the use of chemical oxidants has limitations, mainly associated with side effects such as environmental pollution with these oxidizing compounds. Microbiological methods represent an environmentally harmless and economically viable alternative.

Specific arsenite oxidation mediated either by arsenite-oxidizing bacterial strains or by natural bacterial consortia in arsenic-contaminated wastewaters (100 mg/l) was proposed by Simeonova et al.,[13]. Their approach was based on As(III) oxidation mediated by Herminiimonas arsenicoxydans immobilized on Ca- alginate beads. Lievremont et al., [14] described a two stage process in which arsenite oxidation is performed by H. arsenicoxydans immobilized on chabazite, followed by arsenate adsorption on kutnahorite, a low cost adsorbent. A similar two-step approach was also presented by Mokashi and Paknikar [15], who utilized arsenite oxidation mediated by Microbacterium lacticum immobilized on brick pieces packed in a glass column, followed by the removal of As(V) with zero valent iron or activated charcoal (an arsenate adsorbent). Biological As (III) oxidation systems are efficient in the laboratory over a wide range of arsenite concentrations, but the effective removal of the As (V) produced by this process is required before in situ implementation is possible.

Some proposed methods for the microbiological treatment of arsenic-containing water also include the use arsenate-reducing bacteria. As (III) is removed by precipitation or complexion with sulfide.

Desulfomicrobium Ben-RB and Desulfotomaculum auripigmentum can concomitantly reduce As(V) to As(III) and S(VI) to S(II), which leads to the formation of both intracellular and extracellular arsenic trisulfide precipitates[16]. Luo et al., reported the use of an ethanol-fed sulfate-reducing bioreactor system to remove arsenic, selenium and sulfate from neutral to alkaline mine wastewater in the presence of iron. In landfill sites, the induction of reducing conditions in groundwater has the potential to immobilize arsenic [17]. A study conducted by Battaglia-Brunet et al., showed that indigenous bacteria are able to participate in arsenic removal from mine water under both anaerobic and aerobic conditions. Using the indigenous microbial population, 99 % of As (III) and 91 % of As (V) removal was observed in the column experiment.

These results formed the basis for the development of an in situ passive treatment process that significantly improved the quality of the discharge effluent of mine waters [18].

Mycorrhizal fungi may be important and efficient in the revegetation or phytostabilisation of As-polluted sites. The mine-site ericoid mycorrhizal fungus Hymenoscyphus ericae showed an approximately 90%

enhanced efflux of As in the form of As III when hydroponically-grown on the host. The authors suggested that the mine site fungus acts as an As filter to maintain low As concentrations in plant tissues, while improving P nutrition of the host plant [19].

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Microbes as Biosensors of Arsenic Contamination

During the last two decades, a number of arsenic biosensors have been developed based on both whole-cell- and cell-free-based biosensors. The bacteria which live in a community called a biofilm, can clean up contaminated areas by removing the arsenic from soil or drinking water and even in the cold. The enzyme from these new strains of bacteria can be developed into an arsenic biosensor to be used in polluted environments. This would warn when traces of arsenic are escaping from areas like mine workings, industrial chemical facilities, or even laboratories, alerting us before pollution manages to get into watercourses or drinking water supplies. To date, a number of biosensors for the detection of arsenic involving the coupling of biological engineering and electrochemical techniques has been developed.

Biosensors have the advantages of sensitivity, specificity, simplicity, low manufacturing cost, low detection limit, fast response time, ease of use, portability and ability to furnish continuous real-time signals [20].

Protein or DNA Based Biosensors

Sensors using nucleic acids for the recognition and monitoring of toxic compounds are useful because many toxins, such as arsenic, have high affinity for nucleic acids or for DNA binding proteins and specific sequences can be detected rapidly and at low cost [21]. A variety of proteins have been used for sensing arsenicals. Most protein-based biosensors developed for As (III) or As (V) are based on inhibition. An amperometric biosensor was developed to study the inhibition of acetyl cholinesterase by As (III)[22].

Whole Cell-Based Biosensors

A whole cell-based biosensor is an analytical device that integrates whole cells, which provides high selectivity, with a physical transducer to generate a measurable signal proportional to the concentration of analytes. Whole cell biosensors have the advantages of specificity, low cost, ease of use, portability and the ability to furnish continuous real-time signals [23]. The ars operon of E. coli plasmid R773is widely used.

The biosensors using luciferase (luxAB) in a fusion with the arsenic resistance operon used the expression of firefly luciferase controlled by the regulatory region of the ars operon of Staphylococcus aureus plasmid pI258 in recombinant plasmid pTOO21, with S. aureus RN4220, Bacillus subtilis BR151 or E. coli MC1061 as host strains [24]. A luminescence-based bacterial sensor strain, Pseudomonas fluorescens OS8 (pTPT31), was developed in 2002 for arsenite detection in soil extracts [25]. The sensor strain with pTPT31 of Pseudomonas appeared to have a useful detection range similar to that of chemical methods. Recently, a compact portable biosensor for measurement of As(III) in water using E. coli strain 1598 has been developed. It carries the plasmid pPROBE-ArsR-ABS and luminiscizes in response to As(III) and As(V) [26].An eukaryotic biosensor utilizing an arsenic-responsive fungal gene has been constructed [27].

Aspergillus niger and Saccharomyces cerevisiae appears to be more selective for sensing As (III). These fungi biosensor reliably detects both arsenite and arsenate in the range of 1.8–180 µg/L, exhibiting a low arsenic detection limit.

CONCLUSION

Exposure to arsenic is a global public health problem due to its ubiquity in the environment and its association with numerous human diseases. The development of biosensors for arsenic detection is of great value and interest not only to scientists, but to the public, as well. As discussed above, the performance of a number of microorganisms for bioremediation purposes and as biosensors could be improved. Their application has limitations in the detection time, action and specificity. Most microbes as clean-up agents and biosensors are still in a proof-of-principle or research phase and not close to commercial use. The future investigations should be based on effective use of indigenous as well as genetically modified organisms and mycorrhizae as arsenic tolerant and efficient hyper accumulators.

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ACKNOWLEDGEMENT

The authors are very grateful to Principal, Field Marshal K.M.Cariappa College, Madikeri, and Mangalore University for providing necessary facilities and support in carrying out the present study.

REFERENCES

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