Introduction heavy metal pollution

Liquid-Liquid Partitioning SPE GPC Concentration

Separation/ Analysis GC HPLC

Use of selective detectors and

multiple analyses are common

(SPE= solid phase extraction, GPC= gel permeation chromatography, GC= gas chromatography, HPLC= high performance liquid chromatography)

Figure 2. Pesticide residue analysis method in environmental samples

(Source: Hawthorne et al. 1994)

Recent Advances in Pesticide Bioremediation

Pesticide degrading catabolic gene and their respective enzymes of microorganisms have been isolated and identified by several researchers. For example lindane (Kumari et al. 2002), endosulfan (Sutherland et al. 2002; Hussain et al. 2007), DDT (Barraga et al. 2007) and monocrotophos degrading microbial genes and enzymes have been (Subhas and Singh 2003; Das and Singh, 2006), isolated and identified. Genetic studies revealed that plasmids are the main place to harbour pesticide catabolising genes in microbial community. Sutherland et al. (2002) had reported Esd gene having sequence homology to monooygenase family which uses reduced flavin, provided by a separate flavin reductase enzyme, as co-substrates in Mycobacterium smegmatis. Esd catalyzes the oxygenation of β-endosulfan to endosulfan monoaldehyde to endosulfan hydroxyether. Esd did not degrade either α-endosulfan or the metabolites of endosulfan and endosulfan sulphate. Wier et al. (2006) have reported that Ese gene of Arthrobacter sp. encoding enzyme from monooxygenase family is capable of degrading both the isomers of endosulfan. After, understanding the gene of interest and enzyme involved, the Superbugs can be created to achieve the desired result at fast rate in short time frame. Lal et al. [25] has reviewed the degradation of HCH and distribution of lin gene in Sphingomonads. S. indicum B90A was found to contain two non-identical linA genes (designated as linA1 and linA2). The linA-encoded HCH dehydrochlorinase (LinA) mediates the first two steps of dehydrochlorination of γ-HCH (Singh, 2008). Besides, genetically modified microbes are used to enhance the capability of pesticide degradation. However, genetically engineered technology for environment use is still controversial because an adverse genotype can be readily mobilized in the environment. In a development of technology following points should be taken care i.e. (i) heterogeneity of contaminant. (ii) concentration of contaminant and its effect on biodegradative microbe, (iii) persistence and toxicity of contaminant, (iv) behaviour of contaminant in soil environment and (v) conditions favourable for biodegradative microbe or microbial population (Singh, 2008). The degradation of persistent chemical compounds by microorganisms in the natural environment has revealed a larger number of enzymatic reactions with high bioremediation potential (Finley et al., 2010). These biocatalysts can be obtained in quantities by recombinant DNA technology, expression of enzymes, or indigenous organisms, which are employed in the field for removing pesticides from polluted sites. The microorganisms contribute significantly for the removal of toxic pesticides used in agriculture and in the absence of enzymatic reactions many cultivable areas would be impracticable for agriculture (Abramowicz, 1995).

Although, significant advances have been made in understanding the roles of plant associated microbial pesticide degradation and application of these processes in field scale bioremediation (Joshi and Juwarkar, 2009; Li et al., 2010; Shi et al., 2011). An exciting alternative to the use of plant-associated bacteria to degrade toxic organic compounds in soil is the use of recombinant DNA technology to generate transgenic plants expressing bacterial enzymes resulting in improved plant tolerance and metabolism of toxic organic compounds in soil. Transgenic plants have been produced for phytoremediation of both heavy metals and organic pollutants (Eapen et al. 2007). Transgenic poplar plantlets expressing bacterial mercuric reductase were shown to germinate and grow in the presence of toxic levels mercury. Arabidopsis thaliana was engineered to express a modified organomercurial lyase (Rugh et al. 1992) and those transgenic plants grew vigorously on a wide range of concentrations of highly toxic organomercurials, probably by forming ionic mercury which should accumulate in the disposable plant tissues. The first report of genetically modified plant for the transformation of xenobiotic contaminants to nontoxic material was reported (French et al. 1999). They previously reported that Enterobacter cloacae PB2 is capable of growth with trinitrotoluene (TNT) as a nitrogen source (Bhatia and Malik, 2011).