The bacteria and fungi seem to play different roles.
Fungi are often responsible for detoxification reactions
(dehalogenation, hydroxylation, etc.) associated with
small changes in the molecular structure that often result in
solubility increase and a decrease in biological activity.
By comparison, bacteria appear to be involved in
transformations associated in complex metabolic schemes
catalyzed by more specific enzymes. Products may enter
central biochemical pathways to generate energy and new
biomass. For the last 10 years, the potential for biotransformation and biodegradation of Actinomycetes was found very
important (De Schrijver and De Mot 1999). Members of this
group of Gram-positive bacteria, sometimes associated in
consortia, are capable of degrading products as diverse as
organochlorines, s-triazines, carbamates, organophosphates
and organophosphonates, acetanilides, sulfonylureas etc.
The molecular mechanisms of these transformations are
less well known than those that contribute to the degradation
of pesticides by Gram-negative bacteria.
Physiological Bases of Pesticide Degradation
Biodegradation can be studied by using pure strains or
microbial consortia:
(i) Degradation by pure cultures: metabolism and
cometabolism
It has been possible to isolate from soil a large corpus of
microbial strains that show very diverse catabolic
capacities according to different modes of physiological
functioning.
• Degradation of pesticides as the sole source of carbon
and energy: mineralization
When a single microbial strain can mobilize a number
of enzymes sufficient to use a “foreign” substrate as
sole source of carbon and energy, degradation is called
“metabolism” and results in the mineralization of the
chemical. This mode of degradation is functionally
very similar to those which occur for the consumption
of “natural” nutrient sources. It concerns only a limited
number of chemicals. In the soil environment, metabolism may contribute to a stepwise increase of the
microbial community as applications of the chemical
are repeated. This microbial enrichment is the basis of
the phenomenon of “accelerated degradation,” also
called “adaptation” or “acclimatization.” It may be
detrimental for chemicals which are directly applied
to the soil as they show a progressive loss of efficacy.
This phenomenon has been observed in the early 1950s
with 2,4-D (Audus 1951). Since, it seems to occur on a
larger scale and for a growing number of chemicals.
Most recent examples of this behavior are shown by the
insecticide carbofuran and the herbicides atrazine and
isoproturon.
• Degradation of pesticides in the presence of other
sources of carbon and energy
For a large majority of chemicals, it was not possible
to isolate microbial strains capable of using them as
sole sources of carbon and energy. However, it is not
uncommon to isolate degrading strains with the addition in the culture medium of a second substrate
serving as a source of carbon or nitrogen and energy.
This procedure reveals the ability of a number of
microorganisms to transform a pesticide molecule
without being able to drive the metabolic process
at a stage where energy can be derived. These
“failures” may have several causes: lack of or low
catalytic activity of the enzymes, independent enzyme
induction leading to the accumulation of toxic
intermediates, etc. All transformations that are not
directly connected to cell growth and require the
presence of a co-substrate to progress at a significant rate were collectively called “cometabolism”
(Horvath 1972). In the soil environment, fresh or
humidified organic matter is the main source of
co-substrates. The concept of cometabolism is a
generalization proposed by Jensen (1963) of the
“co-oxidation” process previously developed by
Leadbetter and Foster (1959) who observed the oxidation of certain hydrocarbons in the presence of methane
without growth of the responsible strain Pseudomonas
methanica. Cometabolism includes transformations
such as dehalogenations that are not oxidation strictly
speaking. Originally presented as a new microbial
physiological trait, the use of the term cometabolism
was criticized by Hurlbert and Krawieck (1977) who
saw no specific biochemical characteristics but rather
the outcome of dysfunctional enzyme. Nevertheless,
the term was maintained to describe transformations
with particular kinetics, the paradigmatic first-order
law, and/or ecotoxicological consequences when accumulation of hazardous end products occurs.
(ii) Degradation by complex microbial communities.
Primary and ultimate degradation
In 1965, Alexander concluded that degradation
studies conducted with mixed cultures and microbial
communities from natural ecosystems were more appropriate for assessing the inherent biodegradability of
xenobiotic compounds. However, even in the presence
of complex microbial communities, some pesticides are
relatively persistent and/or are not transformed enough
to be further converted by catabolic reactions to provide
the energy needed by the cells. “Primary degradations”
characterize incomplete transformations which result
in the production of dead end products. In contrast,
“ultimate degradations” are those which drive a complete set of transformations enabling mineralization of
16 Applied Microbial Ecology and Bioremediation
691
Fungi are often responsible for detoxification reactions
(dehalogenation, hydroxylation, etc.) associated with
small changes in the molecular structure that often result in
solubility increase and a decrease in biological activity.
By comparison, bacteria appear to be involved in
transformations associated in complex metabolic schemes
catalyzed by more specific enzymes. Products may enter
central biochemical pathways to generate energy and new
biomass. For the last 10 years, the potential for biotransformation and biodegradation of Actinomycetes was found very
important (De Schrijver and De Mot 1999). Members of this
group of Gram-positive bacteria, sometimes associated in
consortia, are capable of degrading products as diverse as
organochlorines, s-triazines, carbamates, organophosphates
and organophosphonates, acetanilides, sulfonylureas etc.
The molecular mechanisms of these transformations are
less well known than those that contribute to the degradation
of pesticides by Gram-negative bacteria.
Physiological Bases of Pesticide Degradation
Biodegradation can be studied by using pure strains or
microbial consortia:
(i) Degradation by pure cultures: metabolism and
cometabolism
It has been possible to isolate from soil a large corpus of
microbial strains that show very diverse catabolic
capacities according to different modes of physiological
functioning.
• Degradation of pesticides as the sole source of carbon
and energy: mineralization
When a single microbial strain can mobilize a number
of enzymes sufficient to use a “foreign” substrate as
sole source of carbon and energy, degradation is called
“metabolism” and results in the mineralization of the
chemical. This mode of degradation is functionally
very similar to those which occur for the consumption
of “natural” nutrient sources. It concerns only a limited
number of chemicals. In the soil environment, metabolism may contribute to a stepwise increase of the
microbial community as applications of the chemical
are repeated. This microbial enrichment is the basis of
the phenomenon of “accelerated degradation,” also
called “adaptation” or “acclimatization.” It may be
detrimental for chemicals which are directly applied
to the soil as they show a progressive loss of efficacy.
This phenomenon has been observed in the early 1950s
with 2,4-D (Audus 1951). Since, it seems to occur on a
larger scale and for a growing number of chemicals.
Most recent examples of this behavior are shown by the
insecticide carbofuran and the herbicides atrazine and
isoproturon.
• Degradation of pesticides in the presence of other
sources of carbon and energy
For a large majority of chemicals, it was not possible
to isolate microbial strains capable of using them as
sole sources of carbon and energy. However, it is not
uncommon to isolate degrading strains with the addition in the culture medium of a second substrate
serving as a source of carbon or nitrogen and energy.
This procedure reveals the ability of a number of
microorganisms to transform a pesticide molecule
without being able to drive the metabolic process
at a stage where energy can be derived. These
“failures” may have several causes: lack of or low
catalytic activity of the enzymes, independent enzyme
induction leading to the accumulation of toxic
intermediates, etc. All transformations that are not
directly connected to cell growth and require the
presence of a co-substrate to progress at a significant rate were collectively called “cometabolism”
(Horvath 1972). In the soil environment, fresh or
humidified organic matter is the main source of
co-substrates. The concept of cometabolism is a
generalization proposed by Jensen (1963) of the
“co-oxidation” process previously developed by
Leadbetter and Foster (1959) who observed the oxidation of certain hydrocarbons in the presence of methane
without growth of the responsible strain Pseudomonas
methanica. Cometabolism includes transformations
such as dehalogenations that are not oxidation strictly
speaking. Originally presented as a new microbial
physiological trait, the use of the term cometabolism
was criticized by Hurlbert and Krawieck (1977) who
saw no specific biochemical characteristics but rather
the outcome of dysfunctional enzyme. Nevertheless,
the term was maintained to describe transformations
with particular kinetics, the paradigmatic first-order
law, and/or ecotoxicological consequences when accumulation of hazardous end products occurs.
(ii) Degradation by complex microbial communities.
Primary and ultimate degradation
In 1965, Alexander concluded that degradation
studies conducted with mixed cultures and microbial
communities from natural ecosystems were more appropriate for assessing the inherent biodegradability of
xenobiotic compounds. However, even in the presence
of complex microbial communities, some pesticides are
relatively persistent and/or are not transformed enough
to be further converted by catabolic reactions to provide
the energy needed by the cells. “Primary degradations”
characterize incomplete transformations which result
in the production of dead end products. In contrast,
“ultimate degradations” are those which drive a complete set of transformations enabling mineralization of
16 Applied Microbial Ecology and Bioremediation
691