is usually done to give information to stakeholder in charge
of the management of the site. This natural way is usually
appropriate for contamination on large pieces of land
without risks for animal and human populations. It is well
accepted as methods for treating fuel components (e.g.,
BTEX) but not for many classes (Atteia and Guillot 2007).
When degrading microorganisms are present but the
pesticide poorly available (case (ii)), the solution to be applied
consists in adding different agents like synthetic surfactants to
improve the availability of the compounds like it is done for
petroleum compounds. This can also be done by inoculating
biosurfactant-producing microbial inoculums which are
producing anionic or nonionic surfactant like rhamnolipid
which were found to enhance the solubility of PAHs and
their subsequent biodegradation (Pei et al. 2010). Recently,
dual application of biosurfactant-producing and pollutantdegrading microbes has been suggested presenting the
advantages of a continuous supply of biodegradable surfactant
and the ability to degrade pollutants (Megharaj et al. 2011).
When degrading microorganisms are present but not
enough active (case (iii)) to clean the contaminated site,
microbial turnover of pesticide degradation by indigenous
microbial populations can be promoted by supplying with
carbon, nutrients such as N and P, and oxygen or by
modifying soil pH or redox potential. Electrobioremediation
a hybrid technology combining bioremediation and electrokinetics was proposed to treat soil contaminated with
hydrocarbons and metal (Chilingar et al. 1997). It consists
in applying to polluted site of a current between electrodes
and uses of microbiological phenomenon for pollutant
degradation. All these techniques consisting in modifying
the proxies of the contaminated environment to favor the
activity of pesticide-degrading populations are grouped
under the generic term of biostimulation.
When pesticide-degrading populations are present but in
too less abundance (case (iv)) or even absent from the
contaminated site, bioaugmentation consisting in inoculating soil with an appropriate pesticide-degrading population can be applied. It is noteworthy that the concept of
“soil activation” which is based on the cultivation of a
degrading-microbial biomass starting from a composite soil
sample collected at the contaminated site and its subsequent
use as an inoculum for the same soil was proposed (Otte
et al. 1994). One of the most frequent problems encountered
with bioaugmentation is the poor competitiveness of the
inoculum to the indigenous members of natural communities
and also the poor adaptability of these inoculums to physicochemical environments, thereby impairing the efficiency
of bioaugmentation treatment.
Finally, it is noteworthy that phytoremediation techniques relying on the use of different plant species to clean
polluted soils can be used as well. Phytoremediation was
mainly used to remove heavy metals from soil, but
nowadays it is also used to treat soil polluted with organic
pollutants such as pesticides. In general, plants can promote
dissipation of organic pollutants by immobilization
(phytostabilization), removal (phytoextraction), degradation
(phytodegradation), and promotion of microbial degradation
(rhizodegradation). As an example, the natural resistance
of corn plants to herbicide atrazine results from hydroxylation by benzoxazinones, conjugation reaction catalyzed by
glutathione-s-transferase, and dealkylation under the control
of P450 monooxygenases. It seems that to some extent, these
different processes are shared by different plant lines, with,
for each of them, differences in the resistance level. Thereby
for the Vetiver grass (Chrysopogon zizanioides Nash),
the resistance to atrazine seems to mainly result from its
conjugation to the glutathione (Marcacci et al. 2005). This
physiological property of the Vetiver grass is one of the traits
explaining its large ecological tolerance. This physiological
feature is of interest to use Vetiver grass in buffer zone
(i.e., grass buffer strip naturally tolerant to different
herbicides) to protect water resources, notably under Mediterranean and subtropical climates. On the contrary, atrazine
hydroxylation and dealkylation are the two dominating
reactions in poplar trees (Burken and Schnoor 1997; Chang
et al. 2005). Recently, several studies were conducted with
the aim of constructing genetically modified plants (GMO)
in which microbial degrading genes are introduced. With
this aim, Wang et al. (2005) showed that it was possible to
transform different models of plants (Medicago sativa,
Nicotiana tabacum, and Arabidopsis thaliana) with the
atzA gene of Pseudomonas sp. ADP coding for the first
enzyme of the atrazine-degrading pathway, catalyzing its
transformation to hydroxyatrazine. In addition, T-DNA
mutant of Arabidopsis thaliana was shown to be able to
phytoaccumulate
14 C-atrazine in presence of sucrose offering potential for phytoremediation (Sulmon et al. 2007).
On the reverse way several studies showed that plant can
be used to establish microbe-assisted phytoremediation
naturally promoting the microbial biodegradation of pesticide via root exudation (Piutti et al. 2002).
Whatever, these different bioremediation techniques can
be applied in situ directly on the contaminated site or ex situ,
after soil excavation and delocalized treatment in specifically built areas or in pilot reactor to protect surrounding
environment from contamination and to protect workers
from pollutant exposure. In situ approaches (bioattenuation,
biostimulation or bioaugmentation) present the advantage of
being cost-effective, but usually they are poorly efficient on
a short term and required long-term treatment to reach
acceptable results. In addition, the efficiency of in situ treatment is spatially variable mainly in reason of natural
variability of soil physicochemical properties. On the contrary, ex situ treatments of contaminated soils in biopiles or
bioreactors are expensive but more effective. This gain in
16 Applied Microbial Ecology and Bioremediation
699
of the management of the site. This natural way is usually
appropriate for contamination on large pieces of land
without risks for animal and human populations. It is well
accepted as methods for treating fuel components (e.g.,
BTEX) but not for many classes (Atteia and Guillot 2007).
When degrading microorganisms are present but the
pesticide poorly available (case (ii)), the solution to be applied
consists in adding different agents like synthetic surfactants to
improve the availability of the compounds like it is done for
petroleum compounds. This can also be done by inoculating
biosurfactant-producing microbial inoculums which are
producing anionic or nonionic surfactant like rhamnolipid
which were found to enhance the solubility of PAHs and
their subsequent biodegradation (Pei et al. 2010). Recently,
dual application of biosurfactant-producing and pollutantdegrading microbes has been suggested presenting the
advantages of a continuous supply of biodegradable surfactant
and the ability to degrade pollutants (Megharaj et al. 2011).
When degrading microorganisms are present but not
enough active (case (iii)) to clean the contaminated site,
microbial turnover of pesticide degradation by indigenous
microbial populations can be promoted by supplying with
carbon, nutrients such as N and P, and oxygen or by
modifying soil pH or redox potential. Electrobioremediation
a hybrid technology combining bioremediation and electrokinetics was proposed to treat soil contaminated with
hydrocarbons and metal (Chilingar et al. 1997). It consists
in applying to polluted site of a current between electrodes
and uses of microbiological phenomenon for pollutant
degradation. All these techniques consisting in modifying
the proxies of the contaminated environment to favor the
activity of pesticide-degrading populations are grouped
under the generic term of biostimulation.
When pesticide-degrading populations are present but in
too less abundance (case (iv)) or even absent from the
contaminated site, bioaugmentation consisting in inoculating soil with an appropriate pesticide-degrading population can be applied. It is noteworthy that the concept of
“soil activation” which is based on the cultivation of a
degrading-microbial biomass starting from a composite soil
sample collected at the contaminated site and its subsequent
use as an inoculum for the same soil was proposed (Otte
et al. 1994). One of the most frequent problems encountered
with bioaugmentation is the poor competitiveness of the
inoculum to the indigenous members of natural communities
and also the poor adaptability of these inoculums to physicochemical environments, thereby impairing the efficiency
of bioaugmentation treatment.
Finally, it is noteworthy that phytoremediation techniques relying on the use of different plant species to clean
polluted soils can be used as well. Phytoremediation was
mainly used to remove heavy metals from soil, but
nowadays it is also used to treat soil polluted with organic
pollutants such as pesticides. In general, plants can promote
dissipation of organic pollutants by immobilization
(phytostabilization), removal (phytoextraction), degradation
(phytodegradation), and promotion of microbial degradation
(rhizodegradation). As an example, the natural resistance
of corn plants to herbicide atrazine results from hydroxylation by benzoxazinones, conjugation reaction catalyzed by
glutathione-s-transferase, and dealkylation under the control
of P450 monooxygenases. It seems that to some extent, these
different processes are shared by different plant lines, with,
for each of them, differences in the resistance level. Thereby
for the Vetiver grass (Chrysopogon zizanioides Nash),
the resistance to atrazine seems to mainly result from its
conjugation to the glutathione (Marcacci et al. 2005). This
physiological property of the Vetiver grass is one of the traits
explaining its large ecological tolerance. This physiological
feature is of interest to use Vetiver grass in buffer zone
(i.e., grass buffer strip naturally tolerant to different
herbicides) to protect water resources, notably under Mediterranean and subtropical climates. On the contrary, atrazine
hydroxylation and dealkylation are the two dominating
reactions in poplar trees (Burken and Schnoor 1997; Chang
et al. 2005). Recently, several studies were conducted with
the aim of constructing genetically modified plants (GMO)
in which microbial degrading genes are introduced. With
this aim, Wang et al. (2005) showed that it was possible to
transform different models of plants (Medicago sativa,
Nicotiana tabacum, and Arabidopsis thaliana) with the
atzA gene of Pseudomonas sp. ADP coding for the first
enzyme of the atrazine-degrading pathway, catalyzing its
transformation to hydroxyatrazine. In addition, T-DNA
mutant of Arabidopsis thaliana was shown to be able to
phytoaccumulate
14 C-atrazine in presence of sucrose offering potential for phytoremediation (Sulmon et al. 2007).
On the reverse way several studies showed that plant can
be used to establish microbe-assisted phytoremediation
naturally promoting the microbial biodegradation of pesticide via root exudation (Piutti et al. 2002).
Whatever, these different bioremediation techniques can
be applied in situ directly on the contaminated site or ex situ,
after soil excavation and delocalized treatment in specifically built areas or in pilot reactor to protect surrounding
environment from contamination and to protect workers
from pollutant exposure. In situ approaches (bioattenuation,
biostimulation or bioaugmentation) present the advantage of
being cost-effective, but usually they are poorly efficient on
a short term and required long-term treatment to reach
acceptable results. In addition, the efficiency of in situ treatment is spatially variable mainly in reason of natural
variability of soil physicochemical properties. On the contrary, ex situ treatments of contaminated soils in biopiles or
bioreactors are expensive but more effective. This gain in
16 Applied Microbial Ecology and Bioremediation
699