Muhammad Faizan Ilyas*, Tahir Ilyas1 & Muhammad Asad Hameed.
Department of Soil Science, University College of Agriculture, University of Sargodha, Pakistan.
1University of Engineering and Technology, Kala Shah Kaku, Pakistan.
Lead contamination is now becoming a global problem. The lead contamination occurs mainly through anthropogenic activities. Many of the developing countries have less availability of technologies required for dealing with the lead contamination. However removal of pollutants through bioremediation is cheap and reliable. If lead is added to the soil, it ultimately becomes the part of food chain. High concentration of lead in environment is dangerous for plants, animals and humans. Methods used to remediate lead contamination in the soil are excavation, stabilization and bioremediation. Bioremediation is economical and environment friendly method. In bioremediation, microorganisms and plants are used for remediation. Soil microbes reduces the mobility and bioavailability of contaminants. Many Pb-resistant bacterial species e.g. Streptoverticillium cinnamoneum, Streptomyces rimosus, Pseudomonas putida and Bacillus sp. are capable of bioremediating the lead contaminated soil. Minimizing the release of lead into the environment is the best measure to control lead intoxication.
*Corresponding Author: Muhammad Faizan Ilyas
Department of Soil Science, University College of Agriculture, University of Sargodha, Pakistan.
Lead contamination, Pb-resistant bacteria, Bioremediation.
Lead (Pb) is found in water, air and soil. It is distributed naturally in biologically inactive form. The use of lead in bearing metals, batteries, gasoline additives, cable covering, explosives, antifouling paints are major source of pollution. Increased number of vehicles caused to increase the use of lead containing fuels (Johnson, 1998).
This metal is hazardous and highly poisonous to microbes, plants, animals and human s (Low et al., 2000). Bio-availability of lead can be hazardous for children and causes mental retardation. Lead may cause lethal effects on muscular, gastrointestinal, neurological and reproductive systems. It may enter into our body through ingestion of food and blood stream. It disturbs the communication between cells and neuronal set up (Klassen and Watkins, 2003). But extensive mining and smelting have resulted in soil contamination which poses risk to human and ecological health. Over 20 000 000 acres of farmland in China have been contaminated by Sn, Cr, Pb and Zn and other heavy metals, accounting for almost one fifth of the total arable farmland (Wang and Ma, 2008).
Three methods are usually employed to remediate heavy metal contamination in the soil; Excavation-physical removal of the contaminated material, stabilization-amendment of the metals in the soil on site, and bioremediation including phytoremediation to uptake the metals from the soil (Usman and Mohamed, 2009). However, the application of first two methods is sometimes restricted due to technological or economical constrains (E. Plionsmits, 2005). It makes use of plants and their rhizospheric microbes to degrade or immobilize pollutants in soils (Wu et al., 2010). Soil microbes play significant roles in the process of bioremediation (Sheng et al., 2008). They can absorb, transform, or degrade heavy metals and they also can reduce the mobility and bioavailability of contaminants (Wu et al., 2010). Microbes in rhizospheric soil can promote plants to accumulate extra heavy metals (Jiang et al., 2008). Bacillus sp. has been identified as a possible candidate for metal sequestration.
Many bacterial species are capable of bioremediating lead by biosoprtion like Streptoverticillium cinnamoneum, Streptomyces rimosus, Pseudomonas putida, Pseudomonas aeruginosa PU21 and Bacillus sp. (Chang et al., 1997; Uslu and Tanyol, 2006). The quantity of lead may be decreased by bacteria to producing complexes between acidic sites of cell wall and lead (Cabuk et al., 2006). The possible means comprise elimination by forming intra and extracellular sequestration, extracellular metal precipitation, a permeable barrier, active transport, dissolution of lead by acid production, enzymatic detoxification, precipitation of lead through the production of organic bases, and biotransformation reactions like volatilization, methylation, oxidation and reduction. Researchers have also found that Bacillus sp. L14 and Solanum nigrum L. could reduce the toxicity of metal by specifically uptaking up to 80.48% of Pb (II) within 24 h of incubation (Guo et al., 2010).
Many coastal sites in the world are contaminated with toxic heavy metals due to anthropogenic activities (Dauvin, 2008). Biologically non-essential and toxic heavy metals viz. lead, cadmium and mercury released in aquatic and terrestrial environment persist indefinitely and eventually accumulate in the food chain and pose serious threat to the biota (Dauvin, 2008; Lombardi et al., 2010). Lead is mutagenic and teratogenic causing deleterious effects on biological systems viz. neurodegenerative impairment, renal failure, reproductive damage and cancer (Watt et al., 2000; Lam et al., 2007). Considering the severe toxic effect on humans, WHO has recommended <10 µg L-1 lead as safe permissible level in the drinking water (Watt et al., 2000). Heavy metal contaminated environments viz. soil, sediments and water create extreme conditions for microbial growth since they may adversely damage DNA, proteins and lipids and also replace essential metal ions from enzymes and proteins (Hartwig et al., 2002).
Microbes possess various resistance mechanisms to withstand toxic levels of metals, which include efflux, reduction, oxidation, precipitation, extracellular sequestration and intracellular bioaccumulation (Borremans et al., 2001; De et al., 2008; Naik and Dubey, 2011). Intracellular metal bioaccumulation and homoeostasis in cell cytosol involves cysteine rich metallothionein proteins.
Metallothioneins play an important role in immobilization of toxic heavy metals thereby protecting their enzyme catalyzed metabolic processes (Liu et al., 2003). Cyanobacterial and bacterial strains have been reported to possess metallothioneins e.g. Synechococcus PCC 7942 (SmtA), Anabaena PCC 7120 and Pseudomonas putida (BmtA) to maintain cytosolic metal homoeostasis (Blindauer et al., 2002). Thus bacteria possessing metallothioneins are an ideal tool for bioremediation of heavy metal contaminated environmental sites.
Minimizing the use of lead is the best measure to control lead intoxication therefore there is a need to develop and implement policies that facilitate the lead phase out at the earliest. Strong measures have been taken to control the use of lead in gasoline successfully in most of the developed countries during last 25 years (Needleman, 2000) and use of lead has been minimized in certain industries. However, strict measures have not yet been taken in most of the developing countries for minimizing the use of lead from industries (Loval and Magda, 1996).
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