contributed as much as various measures implemented to
reduce pesticide usage.
Whatever the conditions and characteristics of the
environment, it is estimated that less than 5 % of applied
pesticides reach their target, the excess being dispersed in
different environmental compartments (air, soil, water, sediment, etc.) and in foodstuffs. They may involve immediate
or long-term risks and hazards for humans, animals, and
other environmental non-target organisms (NTO). The data
on soil pollution are difficult to collect and still relatively
scarce to have real significance. The compilation of data for
aquatic environments gives a more reliable view of the state
of pollution of aquatic bodies. The results of a survey
conducted in France between 2007 and 2009 were published
in 2011 by the French Ministry in charge of the environment.
Data reported the overall presence of pesticides in surface
and ground waters on the entire territory. Pesticides were
detected at 91 % of the 2,889 monitoring points in rivers
with 21 % showing a total pesticide concentration greater
than 0.5 μg/L. The presence of pesticides was quantified in
70 % of the 2,321 monitoring points in groundwaters. A
little more than 27 % of groundwater points did not meet
water quality requirements in at least 1 year between 2007
and 2009. The pesticides most often encountered in surface
waters were essentially herbicides: glyphosate and AMPA,
its major degraded compound, and deethylatrazine, a metabolite of atrazine banned 10 years ago. Atrazine and its main
dealkylated and hydroxylated metabolites are most commonly found in groundwaters.
16.7.3.2 Pesticide Biodegradation
The Early Studies of Biodegradation:
The Hormone Herbicides
The studies of Mitchell and Hamner on synthetic hormone
herbicides were considered as “the greatest advance in
the history of weeding” (Brian 1976). It is based on two
important observations: the selective biological activity on
mono- and dicotyledonous plants and the persistent action of
synthetic compounds compared to the short-lived natural
auxin, β-indole acetic acid. However, even with synthetic
compounds, it became obvious that the herbicidal activity
decreased over time, especially when the conditions of
moisture, temperature, and organic content of the soil were
more favorable to microbial activity (Brown and Mitchell
1948). It also appeared that the exponential decrease of the
herbicidal activity following a lag phase could be related to
the development of a specific degrading microorganisms
(Audus 1949). The definite conclusion of the involvement
of the microflora in the process of inactivation was supported
by an extension of the herbicidal activity after sterilization of
the soil (Audus 1951) and by the isolation from soil of
microbial species capable of utilizing 2,4-D as the sole
source of carbon and energy. In 1950, Audus isolated a
microbial strain, Bacterium globiforme, capable to degrade
2,4-D from a soil previously perfused with a solution of this
herbicide. Since then, many other microbial strains were
isolated from agricultural soils or even pristine soil with no
previous 2,4-D application. In general, bacteria belonging to
different genera or species were isolated rather than fungi.
Some of them show limited metabolic capabilities that do
not allow them to mineralize 2,4-D.
Principal Biochemical Pathways Involved in Pesticide
Degradation
All the chemicals which are currently authorized for crop
protection all over the world show a large variety of chemical structures. An illustration is given on Table 16.4 for
herbicides which are most commonly used. The degradation
of these chemicals is based on biochemical transformations
controlled by a broad spectrum of enzymes. A detailed
review of different degradation pathways of pesticides is
presented in a database at the University of Minnesota
(http://umbbd.ethz.ch/index.html). This database has been
used to develop a computational metabolic pathway prediction system to derive plausible metabolic pathway of
chemicals for which biodegradation data is currently lacking
(Ellis and Wackett 2012). A limited presentation of the
major degradation pathways and associated enzymes will
be given here. More exhaustive presentations can be found
in the literature (Van Eerd et al. 2003; Scott et al. 2008).
Pesticide degradation involves two stages. A first series
of transformations such as oxidations, reductions, and
hydrolysis contribute to modify the initial structure, and
biological properties, of the parent molecule. Metabolites
are produced which are generally more soluble and less
toxic with some exceptions such as chlorophenols and
chloroanilines. These early changes have the effect of reducing the initial xenobiotic character of the chemicals which,
in the most favorable case, can integrate the central metabolic pathways for further mineralization and synthesis of
microbial biomass. When this metabolic process cannot go
to its completion, the accumulation of transformation
products may result. In the soil, most of these metabolites
do not remain as free solutes. Rather they bind to organic
constituents of the matrix to form high molecular
compounds, the so-called bound residues.
(i) Oxidation Reactions (Fig. 16.21)
Oxidoreductases are a large group of enzymes that catalyze the transfer of electrons from one electron donor
(the reducing agent) to an electron acceptor, NAD (P)
+
,
or a cytochrome for dehydrogenases and reductases,
molecular oxygen for oxidases, quinone, etc. Some
require an additional cofactor. Degradation of glyphosate
into aminomethylphosphonic acid (AMPA) and
glyoxylate is mediated by a dehydrogenase operating
with a flavoprotein as a cofactor. An enzyme of the
686
J.-C. Bertrand et al.
reduce pesticide usage.
Whatever the conditions and characteristics of the
environment, it is estimated that less than 5 % of applied
pesticides reach their target, the excess being dispersed in
different environmental compartments (air, soil, water, sediment, etc.) and in foodstuffs. They may involve immediate
or long-term risks and hazards for humans, animals, and
other environmental non-target organisms (NTO). The data
on soil pollution are difficult to collect and still relatively
scarce to have real significance. The compilation of data for
aquatic environments gives a more reliable view of the state
of pollution of aquatic bodies. The results of a survey
conducted in France between 2007 and 2009 were published
in 2011 by the French Ministry in charge of the environment.
Data reported the overall presence of pesticides in surface
and ground waters on the entire territory. Pesticides were
detected at 91 % of the 2,889 monitoring points in rivers
with 21 % showing a total pesticide concentration greater
than 0.5 μg/L. The presence of pesticides was quantified in
70 % of the 2,321 monitoring points in groundwaters. A
little more than 27 % of groundwater points did not meet
water quality requirements in at least 1 year between 2007
and 2009. The pesticides most often encountered in surface
waters were essentially herbicides: glyphosate and AMPA,
its major degraded compound, and deethylatrazine, a metabolite of atrazine banned 10 years ago. Atrazine and its main
dealkylated and hydroxylated metabolites are most commonly found in groundwaters.
16.7.3.2 Pesticide Biodegradation
The Early Studies of Biodegradation:
The Hormone Herbicides
The studies of Mitchell and Hamner on synthetic hormone
herbicides were considered as “the greatest advance in
the history of weeding” (Brian 1976). It is based on two
important observations: the selective biological activity on
mono- and dicotyledonous plants and the persistent action of
synthetic compounds compared to the short-lived natural
auxin, β-indole acetic acid. However, even with synthetic
compounds, it became obvious that the herbicidal activity
decreased over time, especially when the conditions of
moisture, temperature, and organic content of the soil were
more favorable to microbial activity (Brown and Mitchell
1948). It also appeared that the exponential decrease of the
herbicidal activity following a lag phase could be related to
the development of a specific degrading microorganisms
(Audus 1949). The definite conclusion of the involvement
of the microflora in the process of inactivation was supported
by an extension of the herbicidal activity after sterilization of
the soil (Audus 1951) and by the isolation from soil of
microbial species capable of utilizing 2,4-D as the sole
source of carbon and energy. In 1950, Audus isolated a
microbial strain, Bacterium globiforme, capable to degrade
2,4-D from a soil previously perfused with a solution of this
herbicide. Since then, many other microbial strains were
isolated from agricultural soils or even pristine soil with no
previous 2,4-D application. In general, bacteria belonging to
different genera or species were isolated rather than fungi.
Some of them show limited metabolic capabilities that do
not allow them to mineralize 2,4-D.
Principal Biochemical Pathways Involved in Pesticide
Degradation
All the chemicals which are currently authorized for crop
protection all over the world show a large variety of chemical structures. An illustration is given on Table 16.4 for
herbicides which are most commonly used. The degradation
of these chemicals is based on biochemical transformations
controlled by a broad spectrum of enzymes. A detailed
review of different degradation pathways of pesticides is
presented in a database at the University of Minnesota
(http://umbbd.ethz.ch/index.html). This database has been
used to develop a computational metabolic pathway prediction system to derive plausible metabolic pathway of
chemicals for which biodegradation data is currently lacking
(Ellis and Wackett 2012). A limited presentation of the
major degradation pathways and associated enzymes will
be given here. More exhaustive presentations can be found
in the literature (Van Eerd et al. 2003; Scott et al. 2008).
Pesticide degradation involves two stages. A first series
of transformations such as oxidations, reductions, and
hydrolysis contribute to modify the initial structure, and
biological properties, of the parent molecule. Metabolites
are produced which are generally more soluble and less
toxic with some exceptions such as chlorophenols and
chloroanilines. These early changes have the effect of reducing the initial xenobiotic character of the chemicals which,
in the most favorable case, can integrate the central metabolic pathways for further mineralization and synthesis of
microbial biomass. When this metabolic process cannot go
to its completion, the accumulation of transformation
products may result. In the soil, most of these metabolites
do not remain as free solutes. Rather they bind to organic
constituents of the matrix to form high molecular
compounds, the so-called bound residues.
(i) Oxidation Reactions (Fig. 16.21)
Oxidoreductases are a large group of enzymes that catalyze the transfer of electrons from one electron donor
(the reducing agent) to an electron acceptor, NAD (P)
+
,
or a cytochrome for dehydrogenases and reductases,
molecular oxygen for oxidases, quinone, etc. Some
require an additional cofactor. Degradation of glyphosate
into aminomethylphosphonic acid (AMPA) and
glyoxylate is mediated by a dehydrogenase operating
with a flavoprotein as a cofactor. An enzyme of the
686
J.-C. Bertrand et al.