Arsenic is a heavy metal, known to occur naturally in the Earth’s crust, metal ores and sediments (both organic and inorganic forms). It is also found in the form of sulfides, oxides or salts of sodium, copper and iron among others. It exhibits different valences and is mostly encountered as trivalent Arsenite and pentavalent Arsenate, both of which are deemed toxic to humans (1). Naturally, this heavy metal enters groundwater from its natural geological bedrocks and also from arsenic rich geothermal fluids (2). However, human activities like smelting, extraction of metal, burning of petroleum, coal and wood, production of dyes, pharmaceutical industrial waste and pesticides, cause rampant arsenic pollution.
To restore the contaminated environment to their original state, remediation of arsenic contaminated sources is necessary. For this purpose, bioremediation has been found to be the most promising technology. Through bioremediation, indigenous or foreign microbes, capable of assimilating or mineralizing arsenic can treat environment heavily polluted with arsenic.
Presence of Arsenic contamination
Prevalence of Arsenic in groundwater leads to complications, since this water is used for irrigation of agricultural fields, it adversely affects crop. Moreover, arsenic build-up in soils can cause reduction in yield of crops (3). It has proven to be a major cause of environmental pollution, particularly groundwater in many countries like India, China and Bangladesh. When exposed to arsenic pollution, humans suffer from a wide range of diseases like:
- skin and respiratory diseases,
- disorders of nervous and reproductive system and
- disorders of blood and lymphoid system.
More importantly, arsenic has also been established as a potent carcinogen (4). The figure below shows the biogeochemical cycle of Arsenic as different species are exchanged through multiple processes, between the atmosphere, sediments and water.
According to UNICEF, more than 70 countries have shown instances of groundwater contamination with Arsenic, posing a risk to approximately 140 million people. Asia has the highest concentration of high risk individuals. More particularly, in Bangladesh 25-40 million people have been exposed to high levels of up to 50 µg/L arsenic (5). Furthermore, arsenic contamination has affected about 60% of the Indo-Gangetic Basin. This basin (Indus, Ganges and Brahmaputra rivers) sustains agricultural activity of the Indian-Subcontinent and hence needs to be addressed (6).
Microbes used in Bioremediation of Arsenic pollution
The Arsenite and Arsenate forms of arsenic occur most commonly in the aquatic environment. Typically, arsenic is toxic to microorganisms and hence have not evolved specific transporters for its uptake. However, as the arsenate and arsenite forms resemble glycerol and phosphate analogues, which form part of bacterial nutrition, they are assimilated by these transporters in conditions of high arsenic concentration (7). Notably, Escherichia coli uptake arsenate through phosphate transporters, Pit and Pst, and arsenite via the glycerol transporter GlpF (8,9).
The ecology of arsenic remediating microbes has been studied extensivley to understand their mode of action. Typically, microbes employed for bioremediation of arsenic convert the mobile and toxic trivalent form into less toxic and immobile pentavalent state (10). Different types of microbes carry out bioremediation either by detoxification, mobilization or immobilization of the Arsenic. These microbial processes take place via either oxidation, reduction, biosorption or biomethylation processes within the cell (11). With respect to this, several microbes along with specific bioremediation methods they employ have been shown in the table below.
Mechanism of action of microbes in Arsenic Bioremediation
The trivalent arsenite form is more toxic to humans, than arsenate, and hence is the main target species for bioremediation. Microbial bioremediation of arsenic is carried out by unique species of microbes using multiple metabolic pathways. These pathways depend on the way the specific microbe satisfies their nutritional needs. The process of remediation proves to be complex, as environmental factors influence Arsenic’s state in water source. These factors further affect the speciation of arsenic, thus complicating the whole biogeochemical cycling of Arsenic (12). Oxidation of arsenite to arsenate and reduction of arsenate to arsenite by microbiological action affects the speciation and mobility of arsenic (13).
Arsenate enters the cell through transporters facilitating the uptake of phosphate and hence interfere with the energy generating processes dependant on phosphate. Whereas, arsenite enters through the aqua glycerolporins and affects a broad range of cellular processes (12). However, the microbial cells protect themselves against arsenic toxicity by performing detoxification mechanisms.
Three different methods of remediation
The three methods of remediation, oxidation, reduction, biomethylation, differ in the manner microbes utilize arsenic for satisfying their energy requirements. Among these mechanisms, the oxidation mechanism is performed by both heterotrophic and chemoautotrophic bacteria. The arsenite form is used as electron donor, where oxygen acts as an electron acceptor and carbon dioxide as the source of carbon. However, few bacteria such as a facultative chemoautotroph (γ–proteobacteria) use nitrate as an electron acceptor under anaerobic or anoxic growth conditions. Conversely, in the reduction mechanism, arsenate reductase enzymes undertake detoxification of arsenate, such as in Chrysiogenes arsenatis. Or, as seen in Geospirillum asenophilus, arsenate is used as a terminal electron acceptor (10,14).
Another mechanism, methylation is employed for Arsenic detoxification by microbes such as fungi. Many other Prokaryotes produce monomethylarsonic acid or dimethylarsinic acid to protect them from arsenic toxicity. Microbes convert Arsenic into gaseous forms through these compounds, thereby facilitating its removal from the local environment (12). Besides Arsenic metabolism, microbes also adopt non-metabolic techniques like biosorption and bioaccumulation.
Need to develop more efficient microbes
Arsenic pollution effect both soil and groundwater, but is found to affect water sources more significantly leading to dangerous consequences for living beings. The contaminated water sources result in the biomagnification of the heavy metals in food chains and food webs. However, Arsenic threat is majorly presented in the form of Arsenite and Arsenate, the toxic species found to occur abundantly and can cause a host of disorders in humans.
In conclusion future bioremediation techniques needs to consider the complexities of arsenic biogeochemistry in water as well as microbial interactions with this heavy metal. Moreover, there is need to develop efficient microbes capable of assimilating or converting arsenic to non-toxic forms as a technology, which is applicable to a wide range of contaminated water bodies.
Limitations of heavy metal bioremediation
Through bioremediation, the toxic and soluble forms of heavy metal compounds are converted to non-toxic forms, either precipitated from the medium or adsorbed or assimilated into the microbes. However, the major limitation for such processes of mitigating pollutant is that, inspite of their decreased bioavailability, the toxicity of the pollutant is decreased but the metal will not be removed from soil and water (15). Bioavailability is the amount of a compound available for update or absorption into a biological organism.
In the next article, the problem of radioactive pollution from Uranium and solutions offered by bioremediation for mitigating them is explored.
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