Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 71

c. Substances that are highly toxic to microorganisms are not easily degraded.

d. Compounds with a high oxidation state are resistant to further oxidation and

hence degrade slowly.

The most important degraders in soil are within the genuses Arthrobacter , Aspergillus ,
Alcaligenes , Bacillus , Corynebacterium , Flavobacterium , Fusarium , Nocardia , Penicillium , Pseudomonas ,
and Trichoderma . Alcaligenes and Pseudomonas are very good to degrade chloroaromatic
pesticides (Hoostal et al. 2002). Fungus Phanerochaete chrysosporium is used efficiently to
degrade persistent pesticides such as DDT or lindane.

3.4.2 Organophosphorus Pesticides

Organophosphorus pesticides form a major and most widely used group accounting for
more than 36% of the total world market. The organophosphorus pesticides are degraded
by bacteria such as Pseudomonas sp., Agrobacter , Arthrobacter sp., Flavobacterium sp., fungi,
Fusarium , Aspergillus niger , Geobacillus caldoxylosilyticus , Actinomycetes , Streptomycetes , etc.
Organophosphorus pesticides undergo microbial degradation via hydrolysis and/or
microbial cleavage. Enzymes such as phosphatase, esterase, hydrolyase, and oxygenase
participate actively in the degradation of organophosphorus pesticides.
Parathion : On microbial hydrolysis by enzyme oxidases or hydrolyases, parathion is
decomposed to p-nitrophenol and ionic diethylthiophosphate. Parathion, in the presence
of microorganisms, also undergoes transformation to form amino parathion.
Methyl parathion : It gets hydrolyzed on soil and also in flooded soil by Pseudomonas sp.
and Flavobacterium sp. to p-nitrophenol. The optimum enzymatic activity of hydrolyases
occurs at pH range 7.5-9.5.
Malathion : Malathion on degradation in the presence of Pseudomonas strain forms diethyl
phosphorothioate, which is subsequently converted to dimethyl malate or salt of succinic
acid.
Chlorpyrifos and quinalphos : As these organophosphorus pesticides are less soluble, they
are difficult to degrade. In the presence of lipid-producing microorganisms, chlorpyrifos
is degraded to 3,5,6-trichloro-2-pyridinol (Gilani et al. 2010). The half-life period of chlor-
pyrifos ranges from 34 to 46 days (Swati and Singh 2002). Microbial degradation contrib-
utes significantly to the dissipation of chlorpyrifos in fresh water but is inhibited in sea
water (Bondarenko et al. 2004). The degradation of chlorpyrifos occurs by nonspecific and
noninducible enzyme systems produced in high pH soils. Quinalphos is rapidly degraded
in flooded soils, besides, other kind of degradation of quinalphos also occurs by green
algae, Chlorella vulgaris and Scenedesmus , and blue algae, Synechococcus .
Monocrotophos (MCP) : MCP is most susceptible to bacterial degradation. Monocrotophos
on microbial degradation forms valeric acid and acetic acid in addition to dimethyl phos-
phate, which is not used by the bacteria as a sole source of phosphorus. Degradation of
MCP occurs by Bacillus, Azospirillum lipoferum , Arthrobacter , and Pseudomonas.
Dimethoate : The microbial degradation by fungi and bacteria is the means of disappear-
ance of dimethoate from soil, as it is used as a source of carbon and energy or source of
phosphorus by Aspergillus sydowii , A. niger, A. flavus, , and Fusarium oxysporum . Dimethoate
on microbial hydrolysis forms methylamine, phosphoric acid, and other products.
Fenitrothion : By microbial degradation, like methyl parathion, fenitrothion undergoes
hydrolytic cleavage to form 4-nitro-m-cresol in addition to phosphoric acid.
Coumaphos : By microbial degradation, coumaphos undergoes hydrolysis as: