Mercury bioremediation processes to combat mercury contamination
Mercury bioremediation processes as mercury occur naturally in the environment and are found in both elemental inorganic and organic forms. It generally occurs in two oxidation states, Hg+1 and Hg+2, they are commonly found as:
- elemental mercury,
- mercuric chloride,
- mercuric sulfide (cinnabar ore),
- and methylmercury.
Soil sediments actively adsorb the ionic form of mercury, while iron oxides adsorb mercury ions in neutral soils. Furthermore, organic matter adsorbs mercury ions in acidic environments. However, in the absence of organic matter, mercury becomes mobile and evaporates or can leach on to groundwater (1). Pollution of groundwater with mercury results in accumulation and biomagnification in living systems, posing serious risks to humans. So it is important to mitigate the pollution and mercury bioremediation is one of the solutions.
Furthermore, many countries have taken preventive measures to reduce exposure to toxic forms of mercury. Yet, there is persistent pollution of groundwater requiring urgent intervention in the form of remediation. Thus, bioremediation, application of living organisms to remove pollutants from the environment in a safe and sustainable manner is a solution for cleaner groundwater.
The above figure shows the global distribution of emissions of mercury. Majority of which originates from East and Southeast Asia. In 2013, there were about 1,960 tonnes of mercury emissions (2). In addition, Figure 2 shows the contribution of major industries towards mercury emissions. Most noteworthy of these are gold mining and coal-burning industries. Typically, mercury enters aquatic environments either through the atmosphere or through anthropogenic sources. Subsequently, inorganic forms of mercury in water are converted to methylmercury due to the action of microbes and are absorbed by animals and fish. Therefore, the consumption of contaminated fish by humans results in exposure to toxic levels of mercury (1) resulting in autoimmune diseases, fatigue, loss of hair, depression, insomnia, memory loss, among others (3,4).
Mercury present in the environment is removed from the cycle when it becomes part of the ocean sediment or lake sediment. This is mainly as a result of its association with mineral compounds in the sediments. Figure 3 shows the complex biogeochemical cycle of mercury, shifting from one form to another in air, water, and soil. As the figure shows, the abiotic factors influence the cycle (oceans, soil, and air) as well as biotic factors such as microbes and marine life.
Microbes involved in mercury bioremediation
Microbes are able to survive in adverse conditions, especially in environments with contamination of heavy metals. They are able to do so by developing resistance against toxic substances through metabolic processes. These processes can either be the transformation of the valence state of metal ions, extracellular precipitation, or volatilization (5). Mercury present in soil or water can be detoxified by microbes by undergoing reduction. Especially relevant are species such as Pseudomonas, Escherichia, Bacillus, and Clostridium, which are involved in detoxification of mercury through methylation.
As a consequence, they produce volatile methylated compounds that mobilize mercury (6). A similar reduction of mercury has also been exhibited by Shewanella oneidensis (a dissimilatory metal-reducing bacterium), Geobacter sulfurreducens and Geobacter metallireducens (7). In a unique case, the microbial fauna of Arctic circle have also been found to possess merA genes, responsible for the reduction of Hg+2 to its volatile elemental form. In addition, these microbial populations included algae Fucus sp. and Desmarestia sp. and thick photosynthetic microbial masses (8).
Mode of action of microbes on mercury bioremediation
Mercury-resistant bacteria have unique metabolic properties to transform toxic mercury into non-toxic forms. The extent of mercury concentration determines the proportion of mercury resistant bacteria in contaminated environments (9). In addition, marine bacteria typically eliminate mercury from their surroundings by facilitating the binding of the mercury with thiols to reduce toxicity. They also inhibit the entry of mercury into the cell through the selectively permeable membrane. Another important mechanism of action adopted by mercury resistant bacteria is associated with the mer operon. The genes merA and merB are two functionally important genes harbored by the operon which code for mercuric ion reductase and organomercurial lyase enzyme respectively. Furthermore, these enzymes reduce toxic methylmercury into nontoxic volatile elemental form and together provide broad-spectrum resistance to mercury (10).
Besides naturally occurring microbes, several genetically engineered microbes have been designed with mercury resistance properties by introducing the mer operon. For example, Deinococcus geothemalis has been modified by supplying it with mer genes from E. coli which facilitates the reduction of Hg2+. Similarly, Cupriavidus metallidurans MSR33 has been supplied with merB and merG genes for regulation of mercury biodegradation. Also, Pseudomonas has been supplied with pMR68 plasmid containing mer genes making the strain resistant to mercury (11). Subsequently, the introduction of mer genes inappropriate microbes imparts them with the ability to bioremediate mercury in the natural environment and hence, prove advantageous for large scale cleaning up.
Mercury bioremediation as the most effective solution
Mercury possesses the ability to form amalgamations with other metals, thus, it finds wide application in the industrial processes. Due to its large scale use, the risk of exposure to this toxic metal is increasing rapidly, hence its elimination is crucial to ensure the health of the environment. The mercury bioremediation using microbes is a suitable method, due to its cost-effectiveness and ability to restore the quality of the ecosystem in situ. Mercury resistant bacteria is a promising solution since they passively release the non-toxic forms of Hg into the environment. Also, they do not pose the problem of a buildup of contaminated biomass. The successful introduction of mer operon in several microbial species opens to the possibility of developing new and practical solutions of large scale remediation. There should be further research in testing these strains for ex-situ bioremediation and effectively developing a standardized remediation plan.
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