Microbial Degradation of Pesticides in Tropical Soils
)J' recovered
500
400
300
200
100
not exposed to
PNP
•
parathion
~ f-nitrophenol(PM
o amino parathion
not exposed to
PNP
99
Figure 4.1. Shift in the degradation pathway of parathion from nitro group reduction to hydrolysis
in a flooded soil previously exposed to p-nitrophenol.
parathion, mediated by a constitutive enzyme, is not an energy-yielding reaction,
and proliferation of parathion-hydrolyzing microorganisms actually occurred
during the metabolism of p-nitrophenol.
The first direct evidence for microbial hydrolysis of parathion was obtained
when the resting and growing cells and the cell-free suspension of Flavobacterium
sp., isolated from diazinon-treated rice fields, showed an exceptional capacity
to hydrolyze parathion, but its hydrolysis product, p-nitrophenol, persisted (Sethunathan and Yoshida, 1973b). In yet another study, Pseudomonas sp., also
isolated from a flooded soil but amended with parathion, not only hydrolyzed
parathion but also metabolized p-nitrophenol to form nitrite and carbon dioxide
(Siddaramappa et al., 1973; Sudhakar-Barik et a/., 1976). Even m-nitrophenol,
known for its resistance to biodegradation, was metabolized by Corynebacterium
sp. of flooded soil origin. These observations, together with the extreme instability of various nitrophenols in flooded soils (Sudhakar-Barik and Sethunathan,
1978b), certainly show that flooded soils harbor microorganisms, essentially
bacteria, capable of hydrolyzing parathion and metabolizing related nitrophenols.
In most reported studies with flooded soils, pesticides were applied to the
soils at the time of flooding and then monitored for their disappearance. In this
system, pesticides are exposed to aerobic conditions for a week or more after
flooding before anaerobiosis sets in. But in transplanted rice cultures pesticides
are applied to the standing crop several days after soil submergence, when the
soil is already in a reduced state. The persistence of parathion in the soils was
)J' recovered
500
400
300
200
100
not exposed to
PNP
•
parathion
~ f-nitrophenol(PM
o amino parathion
not exposed to
PNP
99
Figure 4.1. Shift in the degradation pathway of parathion from nitro group reduction to hydrolysis
in a flooded soil previously exposed to p-nitrophenol.
parathion, mediated by a constitutive enzyme, is not an energy-yielding reaction,
and proliferation of parathion-hydrolyzing microorganisms actually occurred
during the metabolism of p-nitrophenol.
The first direct evidence for microbial hydrolysis of parathion was obtained
when the resting and growing cells and the cell-free suspension of Flavobacterium
sp., isolated from diazinon-treated rice fields, showed an exceptional capacity
to hydrolyze parathion, but its hydrolysis product, p-nitrophenol, persisted (Sethunathan and Yoshida, 1973b). In yet another study, Pseudomonas sp., also
isolated from a flooded soil but amended with parathion, not only hydrolyzed
parathion but also metabolized p-nitrophenol to form nitrite and carbon dioxide
(Siddaramappa et al., 1973; Sudhakar-Barik et a/., 1976). Even m-nitrophenol,
known for its resistance to biodegradation, was metabolized by Corynebacterium
sp. of flooded soil origin. These observations, together with the extreme instability of various nitrophenols in flooded soils (Sudhakar-Barik and Sethunathan,
1978b), certainly show that flooded soils harbor microorganisms, essentially
bacteria, capable of hydrolyzing parathion and metabolizing related nitrophenols.
In most reported studies with flooded soils, pesticides were applied to the
soils at the time of flooding and then monitored for their disappearance. In this
system, pesticides are exposed to aerobic conditions for a week or more after
flooding before anaerobiosis sets in. But in transplanted rice cultures pesticides
are applied to the standing crop several days after soil submergence, when the
soil is already in a reduced state. The persistence of parathion in the soils was