Microbial Degradation of Pesticides in Tropical Soils
97
of2,4,5-T, although according to earlier reports (Alexander, 1965) an additional
chlorine attached to its benzene ring rendered it more resistant than 2,4-D to
biological degradation.
4.3.2. Organophosphorus Insecticides
Hydrolysis, at ester linkages in particular, constitutes the major means of
decomposition of several organophosphorus pesticides in soil and water systems.
This hydrolysis can be chemical, biological, or both. Distinguishing between
chemical and biological roles in such hydrolytic reactions has been difficult.
Consequently, hydrolysis of chemically unstable organophosphates in natural
ecosystems was attributed essentially to chemical action. But more recently,
explicit evidence has been provided for microbially mediated hydrolysis of at
least two important members of this group, diazinon and parathion, in tropical
flooded soils and in isolated cultures of bacteria.
4.3.2.1. Diazinon
Diazinon, a widely used insecticide in rice culture, is chemically stable at
neutral and alkaline pH and is readily hydrolyzed under acid conditions as in
acid soils (Sethunathan and MacRae, 1969). The degradation of diazinon via
hydrolysis proceeded fairly rapidly in nonsterile soils with near-neutral pH under
flooded conditions (Sethunathan and MacRae, 1969). But its hydrolysis product
with the pyrimidine ring persisted, and consequently only 0.4-0.7% of ring_ 14 C
in diazinon was recovered as 14C02 from 50-day flooded soils (Sethunathan and
Yoshida, 1969), due to the inhibition of oxygenase-mediated ring cleavage reactions under predominantly anaerobic conditions (Williams, 1977).
The behavior of diazinon in flooded soils previously treated with diazinon
provided further insight into the role of microorganisms in its degradation. Diazinon persisted for only 15 days in a tropical flooded field soil that had been
treated previously with the insecticide and over 60 days in a flooded soil with
no previous history of diazinon application (Sethunathan, 1972). Likewise, diazinon disappeared completely within 5 days of its incubation with water from
diazinon-treated rice fields. More in-depth studies showed that populations of
microorganisms capable of hydrolyzing diazinon increased upon its repeated
additions to rice fields. A Flavobacterium sp., isolated from diazinon-treated
rice field soils, readily hydrolyzed diazinon and then mineralized the pyrimidinyl
moiety to carbon dioxide in a mineral salts medium supplemented with the
insecticide (Sethunathan and Yoshida, 1973b). Undoubtedly, microorganisms
accelerated the hydrolysis of diazinon in flooded rice paddies, although according
to an earlier concept (Kearney and Helling, 1969), diazinon was first hydrolyzed
97
of2,4,5-T, although according to earlier reports (Alexander, 1965) an additional
chlorine attached to its benzene ring rendered it more resistant than 2,4-D to
biological degradation.
4.3.2. Organophosphorus Insecticides
Hydrolysis, at ester linkages in particular, constitutes the major means of
decomposition of several organophosphorus pesticides in soil and water systems.
This hydrolysis can be chemical, biological, or both. Distinguishing between
chemical and biological roles in such hydrolytic reactions has been difficult.
Consequently, hydrolysis of chemically unstable organophosphates in natural
ecosystems was attributed essentially to chemical action. But more recently,
explicit evidence has been provided for microbially mediated hydrolysis of at
least two important members of this group, diazinon and parathion, in tropical
flooded soils and in isolated cultures of bacteria.
4.3.2.1. Diazinon
Diazinon, a widely used insecticide in rice culture, is chemically stable at
neutral and alkaline pH and is readily hydrolyzed under acid conditions as in
acid soils (Sethunathan and MacRae, 1969). The degradation of diazinon via
hydrolysis proceeded fairly rapidly in nonsterile soils with near-neutral pH under
flooded conditions (Sethunathan and MacRae, 1969). But its hydrolysis product
with the pyrimidine ring persisted, and consequently only 0.4-0.7% of ring_ 14 C
in diazinon was recovered as 14C02 from 50-day flooded soils (Sethunathan and
Yoshida, 1969), due to the inhibition of oxygenase-mediated ring cleavage reactions under predominantly anaerobic conditions (Williams, 1977).
The behavior of diazinon in flooded soils previously treated with diazinon
provided further insight into the role of microorganisms in its degradation. Diazinon persisted for only 15 days in a tropical flooded field soil that had been
treated previously with the insecticide and over 60 days in a flooded soil with
no previous history of diazinon application (Sethunathan, 1972). Likewise, diazinon disappeared completely within 5 days of its incubation with water from
diazinon-treated rice fields. More in-depth studies showed that populations of
microorganisms capable of hydrolyzing diazinon increased upon its repeated
additions to rice fields. A Flavobacterium sp., isolated from diazinon-treated
rice field soils, readily hydrolyzed diazinon and then mineralized the pyrimidinyl
moiety to carbon dioxide in a mineral salts medium supplemented with the
insecticide (Sethunathan and Yoshida, 1973b). Undoubtedly, microorganisms
accelerated the hydrolysis of diazinon in flooded rice paddies, although according
to an earlier concept (Kearney and Helling, 1969), diazinon was first hydrolyzed