98
N. Sethunathan et al.
chemically and later microorganisms attacked the products of chemical decomposition.
Soil microorganisms implicated in diazinon degradation can be classified
into three categories (see Sethunathan, 1972) with respect to their ability to
degrade diazinon:
1. As sole carbon source (Flavobacterium sp.).
2. By cometabolism (Arthrobacter sp., Corynebacterium sp., Pseudomonas melophthora, Streptomyces sp., and Trichoderma viride).
3. By synergism (Arthrobacter sp. + Streptomyces sp.).
4.3.2.2. Parathion
Parathion and its methyl analog are probably the most widely used organophosphorus insecticides in agriculture. Among the organophosphates, parathion
is perhaps the most resistant to chemical hydrolysis, especially in acidic and
neutral conditions; but under alkaline conditions it is readily hydrolyzed (Faust
and Gomaa, 1972). Sorption-catalyzed hydrolysis of parathion has been demonstrated in soil samples from Israel, but at very slow rates (Saltzman et al.,
1974; Yaron, 1975).
Parathion is relatively short-lived in most environments despite its resistance
to chemical hydrolysis, simply because microbial degradation constitutes the
major means of detoxication of parathion-polluted environments (Sethunathan
et al., 1977). The first established pathway for microbially mediated metabolism
of parathion was via nitro group reduction to aminoparathion, but recent evidence
suggests significant biological hydrolysis of parathion to p-nitrophenol and diethylthiophosphoric acid in natural ecosystems such as flooded soils, lake sediments, and water, especially after repeated additions (Sudhakar-Barik and Sethunathan, 1978a).
The degradation of parathion in four Philippine soils proceeded more rapidly
under flooded than under upland conditions (Sethunathan and Yoshida, 1973c).
The principal pathway of parathion after the first addition to a flooded soil was
nitro group reduction. But after a second addition, substantial hydrolysis of
parathion occurred in addition to nitro group reduction, while after the third
addition the pathway shifted essentially to hydrolysis (Sudhakar-Barik et al.,
1979). The degradation pattern for all successive additions of parathion followed
first-order kinetics, but kinetic constants indicated that hydrolysis proceeded at
a faster rate than nitro group reduction. The population of parathion-hydrolyzing
microorganisms increased considerably after repeated additions of the insecticide.
Furthermore, parathion was converted to aminoparathion in a flooded soil not
exposed to p-nitrophenol previously; but, in a flooded soil pretreated twice with
p-nitrophenol, parathion was readily hydrolyzed (Fig. 4.1). The hydrolysis of
N. Sethunathan et al.
chemically and later microorganisms attacked the products of chemical decomposition.
Soil microorganisms implicated in diazinon degradation can be classified
into three categories (see Sethunathan, 1972) with respect to their ability to
degrade diazinon:
1. As sole carbon source (Flavobacterium sp.).
2. By cometabolism (Arthrobacter sp., Corynebacterium sp., Pseudomonas melophthora, Streptomyces sp., and Trichoderma viride).
3. By synergism (Arthrobacter sp. + Streptomyces sp.).
4.3.2.2. Parathion
Parathion and its methyl analog are probably the most widely used organophosphorus insecticides in agriculture. Among the organophosphates, parathion
is perhaps the most resistant to chemical hydrolysis, especially in acidic and
neutral conditions; but under alkaline conditions it is readily hydrolyzed (Faust
and Gomaa, 1972). Sorption-catalyzed hydrolysis of parathion has been demonstrated in soil samples from Israel, but at very slow rates (Saltzman et al.,
1974; Yaron, 1975).
Parathion is relatively short-lived in most environments despite its resistance
to chemical hydrolysis, simply because microbial degradation constitutes the
major means of detoxication of parathion-polluted environments (Sethunathan
et al., 1977). The first established pathway for microbially mediated metabolism
of parathion was via nitro group reduction to aminoparathion, but recent evidence
suggests significant biological hydrolysis of parathion to p-nitrophenol and diethylthiophosphoric acid in natural ecosystems such as flooded soils, lake sediments, and water, especially after repeated additions (Sudhakar-Barik and Sethunathan, 1978a).
The degradation of parathion in four Philippine soils proceeded more rapidly
under flooded than under upland conditions (Sethunathan and Yoshida, 1973c).
The principal pathway of parathion after the first addition to a flooded soil was
nitro group reduction. But after a second addition, substantial hydrolysis of
parathion occurred in addition to nitro group reduction, while after the third
addition the pathway shifted essentially to hydrolysis (Sudhakar-Barik et al.,
1979). The degradation pattern for all successive additions of parathion followed
first-order kinetics, but kinetic constants indicated that hydrolysis proceeded at
a faster rate than nitro group reduction. The population of parathion-hydrolyzing
microorganisms increased considerably after repeated additions of the insecticide.
Furthermore, parathion was converted to aminoparathion in a flooded soil not
exposed to p-nitrophenol previously; but, in a flooded soil pretreated twice with
p-nitrophenol, parathion was readily hydrolyzed (Fig. 4.1). The hydrolysis of