Soil is formed from the gradual breaking and weathering of rocks and covers of the landmass of earth as a thin layer. It is a complete ecosystem in itself and its maintenance is of utmost importance for the continuity of life processes of microbes, plants and animals. However, the quality of soil ecosystem is compromised due to increasing human activities resulting in release of pollutants. Furthermore, one such pollutant contributing to soil pollution is the discharge of high concentration of heavy metals (as shown in the figure below). Most noteworthy, Lead occurs most abundantly on Earth, constituting 0.002% of the total crust (1). This article discusses the need for Lead bioremediation in soil. Furthermore, the use of bio-remediation to mitigate the impact has also been discussed.
Lead poisoning is a serious global problem
An ancient element, lead has been used abundantly since prehistoric times and has now accumulated in high concentrations in our environment. Mainly, Lead is used in lead-acid batteries, ammunition, production of brass and bronze alloys and also as a pigment in glass and ceramic products (2). However, lead pollution occurs mainly as a result of combustion of leaded fuel, by-product of battery waste and indiscriminate use of insecticides and herbicides (3). Over the time, increased levels of lead in the soil has Lead to increase in toxicity. When lead enters the food chain, it reduces the quality of food and renders the land useless for agriculture (4). Lead, unlike other heavy metals, does not occur in the biological system of living beings naturally and is highly toxic to humans and other living organisms. It is resistant to corrosion, is non-biodegradable and thus has become a global problem.
In order to restore lead-contaminated soil ecosystem, conventional chemical and physical methods like land filling, chemical fixation of soil and leaching out of heavy metals from the soil through acid treatment are typically undertaken. Conversely, biological processes like bioremediation using microbes is a more beneficial and cost effective method of soil remediation from lead (5). Microbial remediation involves the use of microorganisms which adsorbs, precipitates, oxidises and reduces heavy metals (lead in this case) and are hence more efficient (6).
Microbes capable of bio-remediation of Lead
Microbial bioremediation processes aim at immobilizing the heavy metal Lead in situ, thus reducing their availability or completely removing it from the soil. Research in bioremediation of lead has revealed a wide range of organisms belonging to bacteria and fungi. They are capable of adsorption and sequestering soluble lead compounds and hence extracting them from the soil or water bodies.
For example, Rhodotorula mucilaginosa is a yeast which carries out lead detoxification by the means of Bioadsorption (7). Some of the most frequently found groups of microbes for lead bioremediation are Pseudomonas, Sphingomonas, Ligninolytic fungi and Methylotrophs (5,8). The major bacterial and fungal species involved in the remediation of lead have been determined to be Citrobacter spp, Chlorella vulgaris, Phormidium valderianum, besides fungi like Ganoderma applanatum, Rhizopus arrhizus, Volvariella volvacea and Stereum hirsutum (9,10). The table below summarizes several studies that have isolated microbial species capable of bioremediating lead compounds in soil.
|Country of Research||Author||Microbial species|
|India||Basha & Rajaganesh, 2014 (11)||Escherichia coli, Salmonella typhi, Bacillus licheniformis and Pseudomonas fluorescence|
|India||Das & Kumari, 2016 (12)||Enterobacter sp. and Klebsiella sp.|
|Taiwan||Lee & Chang, 2011 (13)||Spirogyra spp. And Cladophora spp.|
|China||Jiang et al, China(14)||Burkholderia spp.|
Microbial diversity capable of Lead remediation
Mode of action of microbes in bio-remediation of Lead
In addition, bioremediation of lead, like most of other heavy metals, involves the use of microbes to transform toxic materials into harmless or less-toxic compounds. Typically, microbial remediation aims at immobilization of lead ions. Therefore, this is done to control the mobility of ions and reduce the bioavailability. It is also used to remove the Lead ions completely from the soil. When microbes or fungi capable of remediating lead are introduced into the soil, the metal ions bind to the surface of cells of microbes through electrostatic interactions, thereby getting adsorbed. The metal cations are thus retained by the cell.
Remediation of lead can also be understood by determining modes of interactions of lead resistant bacteria with the metal ions in the soil. Above figure shows the typical procedures a cell can adopt, in order to protect itself against lead toxicity. The usual methods of protection adopted by microbes include precipitation of lead as insoluble phosphates outside the cell, adsorption by polysaccharides present on outer surface of cell wall or by polymers which occur inside the cell wall. If it enters the cell, it is inactivated by binding it to the metallothioneins (MT) or exported from the cell through the transporters such as PbrA, CadA, and ZntA, all of which are P (IB2) type ATPase (15).
Besides these, bacteria have also developed techniques for surviving high concentrations of Lead through binding factors or enzymatic reactions consequently, helping in detoxification. Another organism involved in Lead remediation is Pseudomonas aeruginosa, has been known to secrete a lead specific surfactant, Rhamolipid, thereby adsorbing the compound and preventing from entry into the cell (5). These mechanisms of resistance by microbes can be adopted to suit bioremediation purposes, since they involved trapping or precipitation of lead ions and separating it from the soil.
Scope for developing an effective bio-remediation
In addition, lead is a toxic metal, mainly discharged during fuel combustion as well as in the form of mining wastes. As a result, it increases the toxicity of soil and has led to greater pressure on scientists on developing efficient and safe methods of remediation. Microbes adopt a wide range of methods to protect themselves from lead toxicity. As a result, one can adopt microbes with these intrinsic properties for bioremediation.
Despite the discovery of a varied range of microorganisms, an effective bioremediation plan for lead removal from soil does not exist. In conclusion, future research on lead resistance and bioremediation requires developing consortiums capable of complete precipitation of lead. Similarly application of specific surfactants in the soils or identifying its mass production of specific exopolysaccharides for efficient adsorption of lead from the soil is required.
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- Committe on Minerals and Toxic Substances in Diets and Waters for Animals. Mineral Tolerance of Animals. Second. Washington D.C: The National Academic Press; 2006. 510 p.
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- Lee YC, Chang SP. Country of Research Author/ Year Microbe India India Taiwan China Basha & Rajaganesh, 2014 Das & Kumari, 2016 Lee & Chang, 2011 Jiang et al, China Escherichia coli, Salmonella typhi, Bacillus licheniformis and Pseudomonas fluorescence Enterobacter sp. and. Bioresour Technol. 2011;102(9):5297–304.
- Jiang C, Sheng X, Qian M, Wang Q. Isolation and characterization of a heavy metal-resistant Burkholderia sp. from heavy metal-contaminated paddy field soil and its potential in promoting plant growth and heavy metal accumulation in metal-polluted soil. Chemosphere. 2008;72(2):157–64.
- Jarosławiecka A, Piotrowska-Seget Z. Lead resistance in micro-organisms. Microbiol (United Kingdom). 2014;160(PART 1):12–25.