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Microbial degradation of pesticides

Microbial degradation is a powerful technology for the remediation of polluted sites. This methodology is highly cost-effective and offers remediation of the contaminated soil, sediment, sludge, groundwater, etc. (Parte et al. 2017). Microbes have an innate ability to degrade pesticides which is the basis for degradation of contaminants by microorganisms (bioremediation). Microbes having pesticide degradation potential include bacteria, especially actinomycetes and cyanobacteria, algae, and fungi. The microbes having the ability to degr ade pesticides were isolated from contaminated sites. The major source of these microbes is the soil where these contaminants were applied for crop production or protection purposes. Table 1 summarizes the different microbes involved in the bioremediation of pesticides.

Pesticide degradation strategies by microbes

There are different strategies exhibited by microbes for the degradation of pesticides. They involve: (a) co-metabolism, (b) commensalism and mutualism, (c) catabolism and (d) gratuitous biodegradation. In co-metabolism, the microbes transform the pesticide, which is a non-growth substrate, along with its natural metabolic functions. A non-growth substrate cannot serve as a sole source of carbon and energy for a pure culture of a bacterium, and hence cannot support cell division. Some of the recalcitrant pesticides can be degr aded by co-metabolism. Many microbial cell-bound and extracellular enzymes catalyze the transformation of pesticides. For example. Pseudomonas putida strain PP3 as such cannot metabolize MCA (monochloroacetate), but while metabolizing MCPA (monochloropropionate), the microbe catalyzes dehalogenation of MCA (Tewari et al.

2012). Commensalism is the interaction of two different microbial populations that live together in which one population is benefitted from the interaction while the other is not affected. In mutualism, the interaction between the two species is mutually beneficial. In catabolism, the organic molecule is utilized as a source of nutrition and energy. Sometimes, the microbes get adapted to the use of some pesticides as the sole source of carbon or nitrogen leading to enhanced microbial degr adation of the applied pesticides. This occurs as a result of soil enrichment in microbial species. This trend is recently seen in pesticides like carebofuran, 2,4-D, and atrazine. Biostimulation techniques are usually combined with enhanced biodegradation. Biostimulation is the addition of nutrients, electron donors, or acceptors to stimulate the microbial population existing in natural soil. Bioaugmentation is also paired with enhanced biodegr adation. In this technique, specific microorganisms are introduced for enhanced biodegradation of target molecules. In gratuitous biodegradation, enzymes secreted by microbes are able to degrade the pesticides other than its namral substrate.

Biochemical mechanisms in microbial degradation of pesticides

4.2.1 Oxidation

Oxidation is the first step in the bio transformation of pesticides. These reactions are generally mediated by oxidative enzymes such as cytochrome P450. Cytochrome P450 catalyses monooxygenase reactions resulting in hydroxylation. Along with P450, other enzymes are also involved in microbial

Table 1. Bioremediation of pesticides by microbes.

Microbes involved

Pesticides

Reference

Bacteria

Sphingomonaspaucimobilis

hexachlorocyclohexane

Pal et al, 2005

Sphingobacterium sp.

DDT

Fang et al. 2010

R aeruginosa

endosulfan

Jaysliree and asudevan 2007

Stenotrophomonasmahophiha

Rhodococcuseiythropolis

endosulfan

Kumar et al. 2008

Citrobacteramalonaticus

chlordecone

Chaussonnene et al. 2016

P. aeruginosa

Stenotrophomonasmahophiha B. atrophaeus Citrobacteramolonaticus Acinetobacterlowffii

endosulfan

Ozdal et al. 2016

RaoulteUa sp.

dimethoate

Liang et al. 2010

Proteus vulgaris Vibrio sp. Serratia sp. Acinetobacter sp.

dichlorovos

Agarry et al. 2013

Spingomonas sp

chlorpyrifos

Li et al. 2007

Streptomycetes sp.

chlorpyrifos

Bnceiio et al. 2012

Pseudomonas sp.

profenofos

Malgham et al. 2009

Bacillus sp.

cypermethnn

Sharma et al. 2016

Pseudomonas stutzeri

dichlorovos

Parte et al. 2017

Rhodococcus sp.

acetamipnd

Phugare and Jadhav 2015

Burkholderiacepacia

umdaclopnd

metribuziu

Madliuban et al. 2011

Sphingomonas sp. Arthrobacter sp

Carbofiiran

Kun et al. 2004

Pseudomonas sp.

oxamyl

Rousidou et al. 2016

Pseudomona sp.

carbaryl

Trivedi et al. 2016

Fungi

Phlebiatremellosa

Phlebiabrevispora

PMebiaacanthocystis

heptaclilor

Xiao et al, 2011

Aspergillus niger

endosulfan

Bhalerao and Puranik 2007

MortiereUa sp.

endosulfan

Shimizu 2002

Fusariumoxysporum

Lentitiulaedodes

Penicillium brevicompactum Lecanicillium saksenae

difenoconazole

terbuthylazme

pendimethalin

Shi et al. 2012

Trichodennaviride T. harzianum

pirmncarb

Eapen et al. 2007

Algae

Chlamydomonasreinhardtii

prometryne

Jin et al. 2012

Chlamydomonasreinhardtii

fluroxypyr

Zhang et al. 2011

Chlamydomonasreinhardtii

lsoproturon

Bi et al. 2012

Chlorococcum sp. Scenedesmus sp.

a-endosulfan

Sethunathan et al. 2004

degradation processes. They include peroxidase, polyphenol-oxidase, laccase, tyrosinase, etc. which catalyse the polymerization of various anilines and phenols.

4.2.2 Hydrolysis

The compounds containing amide, carbamate, and ester functional groups are generally metabolized by hydrolytic enzymes. Ester hydrolysis is carried out by esterases and different types of esterases have been reported in Pseudomonas fluorescens.

4.2.3 Carbon-Phosphorus bond cleavage reactions

C-P bonds are seen in organophosphorus compounds and are not easily degraded by different degradation processes. Bacteria such as Escherichia coli are involved in the degradation of C-P bonds through the C-P lyase enzyme.

4.2.4 Conjugation

Pesticide conjugation is a co-metabolic process in which an exogenous or endogenous natural compound is joined to a pesticide. Uridine diphosphate-glucosyl (UDPG) transferase enzyme mediates pesticide-glucose conjugation and pesticide-glucose ester conjugation reactions. Xylosylation, alkylation, acylation, and nitrosation are some of the microbial pesticide conjugation reactions. Conjugated pesticides are generally bound to plant cell walls and are difficult to extract. It is reported that some microbes can mineralize the bound residues and make them bioavailable.

Microbial enzymes in pesticide degradation

Enzymes play a major role in the degradation of pesticides by microbes. These enzymes were produced during different metabolic pathways of plants and microbes present in the soil. Engineered microbes were also used to produce enzymes that act on the pesticides leading to its degradation. Strains of genetically modified bacteria contain enzymes that degrade pesticides belonging to organophosphorus, carbamates, and pyrethroid groups (Javaid et al. 2016). A large number of enzymes such as dehydrogenases, cytochrome P450, dioxigenases, ligninases, etc. are involved in pesticide degr adation. Hydr olysis is one of the major processes of degradation. Phosphotrioesterase is an enzyme involved in the hydrolysis of OP compounds. An enzyme, carbofuran hydrolase, is reported to cany' out hydrolysis of the methylcarbamate linkage of carbamate pesticides. Table 2 summarizes some of the enzymes involved in pesticide degradation.

 
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