parent compounds but the mobility and bioavailability of the reaction products are
variable, depending on the size of the molecule to which the residue is attached. For
example, the methylation of the hydroxyl group in PCP by fungi produces a volatile
methoxy derivative, tetrachloroanisole (Cserjsei and Johnson 1972). This is a rare
example of reaction products being much more volatile, and therefore, more mobile
in the environment than the parent compound. Activated transformation products
can react together to form polymers. For example, the hydrolysis of the phenylurea
herbicides produced chlorinated anilines that readily dimerized to form azobenzene
and other condensation products (Bartha and Pramer 1970).
The covalent attachment of pesticide residues to soil humus also effectively
immobilized the pesticide residues to the soil matrix and greatly reduced their
bioavailability and movement through the soil profile. Bound pesticide residues
perhaps may be slowly released during turnover of organic matter. Microbial
population and abiotic mechanisms in soil often transform parent pesticide residues
in humus to intermediate compounds that are subsequently incorporated into soil
organic components, and this phenomenon is often noticed in the case of
polyaromatic hydrocarbons, polychlorinated biphenyls, pentachlorophenol, etc.
(Bossert et al. 1984; Chauhan et al. 2008). Polymerization of various phenolic
compounds was found less toxic after their copolymerization with natural soil
components such as syringic acid (Bollag et al. 1988). Some pesticide residues
could be immobilized to soil organic material via “oxidative coupling.” In this
process, the parent compound is enzymatically transformed by oxidation to a
reactive intermediate, which rapidly reacts with soil organic matter. For example,
laccase and peroxidase enzymes can catalyze the oxidative coupling of oxidized
2,4-dichlorophenol with fulvic acid and humic acids (Nannipieri and Bollag 1991).
It has been suggested that the covalently attached residues are not bioavailable or
mobile and, therefore, are effectively detoxified.
2.9 Genetic Engineering of Microbes to Enhance
Degradation of Pesticides
The production of extracellular enzymes by soil microorganisms can be enhanced by
genetic modification to degrade residual pesticides in the soil (Scott et al. 2008;
Sindhu et al. 2010a; Bass and Field 2011; Riya and Jagapati 2012). Various
measures can be taken to reduce the stress of bacteria that are constantly exposed/
stressed by pesticides available under soil conditions. Various mechanisms involved
in pesticide detoxification include: (1) increasing the copy number of genes that
allow the organism to produce more protective enzymes such as esterases, glutathione transferases, and other oxidases; (2) reducing the number of receptors that bind
to pesticides; and (3) mutating single genes that lead to pesticide resistance. When
different pesticides are used sequentially in the field, bacteria may adapt to or
develop resistance to other pesticides, leading to the development of strains with
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
53
variable, depending on the size of the molecule to which the residue is attached. For
example, the methylation of the hydroxyl group in PCP by fungi produces a volatile
methoxy derivative, tetrachloroanisole (Cserjsei and Johnson 1972). This is a rare
example of reaction products being much more volatile, and therefore, more mobile
in the environment than the parent compound. Activated transformation products
can react together to form polymers. For example, the hydrolysis of the phenylurea
herbicides produced chlorinated anilines that readily dimerized to form azobenzene
and other condensation products (Bartha and Pramer 1970).
The covalent attachment of pesticide residues to soil humus also effectively
immobilized the pesticide residues to the soil matrix and greatly reduced their
bioavailability and movement through the soil profile. Bound pesticide residues
perhaps may be slowly released during turnover of organic matter. Microbial
population and abiotic mechanisms in soil often transform parent pesticide residues
in humus to intermediate compounds that are subsequently incorporated into soil
organic components, and this phenomenon is often noticed in the case of
polyaromatic hydrocarbons, polychlorinated biphenyls, pentachlorophenol, etc.
(Bossert et al. 1984; Chauhan et al. 2008). Polymerization of various phenolic
compounds was found less toxic after their copolymerization with natural soil
components such as syringic acid (Bollag et al. 1988). Some pesticide residues
could be immobilized to soil organic material via “oxidative coupling.” In this
process, the parent compound is enzymatically transformed by oxidation to a
reactive intermediate, which rapidly reacts with soil organic matter. For example,
laccase and peroxidase enzymes can catalyze the oxidative coupling of oxidized
2,4-dichlorophenol with fulvic acid and humic acids (Nannipieri and Bollag 1991).
It has been suggested that the covalently attached residues are not bioavailable or
mobile and, therefore, are effectively detoxified.
2.9 Genetic Engineering of Microbes to Enhance
Degradation of Pesticides
The production of extracellular enzymes by soil microorganisms can be enhanced by
genetic modification to degrade residual pesticides in the soil (Scott et al. 2008;
Sindhu et al. 2010a; Bass and Field 2011; Riya and Jagapati 2012). Various
measures can be taken to reduce the stress of bacteria that are constantly exposed/
stressed by pesticides available under soil conditions. Various mechanisms involved
in pesticide detoxification include: (1) increasing the copy number of genes that
allow the organism to produce more protective enzymes such as esterases, glutathione transferases, and other oxidases; (2) reducing the number of receptors that bind
to pesticides; and (3) mutating single genes that lead to pesticide resistance. When
different pesticides are used sequentially in the field, bacteria may adapt to or
develop resistance to other pesticides, leading to the development of strains with
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
53