reflecting stabilizing residues of atrazine on the organic
matter during humidification (Barriuso et al. 1997). Later
on several reports further confirmed that addition of
simple or complex carbohydrates, thus creating a
nitrogen-limited environment, stimulates the degradation
of atrazine being viewed as a nitrogen source for
degrading microorganisms (Udikovic-Kolic et al. 2012).
Massive organic amendments often have the effect of
stimulating the fraction of the most opportunistic high
growth microorganisms (r-strategy). More limited, but
more frequent, carbon inputs would avoid this nutritional
discrimination. This carbon delivery can be achieved by
plants which are known to release up to 20 % of their
photosynthates as root exudates in their rhizosphere. This
allows microorganisms to grow in the rhizosphere where
their abundance can reach up to an order of magnitude
higher than that it is in the non-rhizosphere soil. From
these considerations originate the idea of developing
microbe-assisted phytostimulation (Anderson et al.
1994), in which plant is not playing part of pesticide
biodegradation but acts as a source of carbon that can
be used by rhizospheric microorganisms as co-substrates
to degrade pesticides (Alvey and Crowley 1996; Piutti
et al. 2002). Thus, a significant increase in the degradation of atrazine applied at concentrations far exceeding
the recommendations dose, in the presence of other
herbicides such as metolachlor and trifluralin, was
obtained by cropping contaminated soil with Kochia
scoparia (L) Roth, an annual plant commonly found on
pesticide site production and tolerant to different
herbicides (Perkovich 1996). Moreover, K. scoparia has
also been successfully used to promote microbial degradation of other pesticides such as metolachlor and lindane
(Singh 2003). Other plants like Pennisetum clandestinum
or Zea maize have also been shown to enhance the activity of atrazine-degrading-microbial populations in their
rhizosphere (Piutti et al. 2002; Singh et al. 2004). Conversely, atrazine-degrading-microbial activity was not
improved in the rhizosphere of several grasses such as
Sorghum vulgare, Lolium perenne, Festuca arundinacea,
and Agropyron desertorum (Anderson et al. 1995). The
impression left by the literature on the subject is that of
a certain disparity of situations from which it seems
difficult to draw general conclusions and guidance on
the involved processes.
• Bioaugmentation approach for atrazine bioremediation
Those that have been first implemented in the mid 1990s
are based on the use of pure strains or microbial consortia
able to degrade atrazine. Most of the bioaugmentation
studies carried out in microcosms incubated under laboratory conditions have concluded to an immediate
increase of degrading capacity of atrazine-contaminated
soil in response to inoculation of degrading inoculums
(Rousseaux et al. 2003). Only few bioaugmentation
studies have been done at field scale. Indeed atrazinecontaminated soil from the Purdue Agricultural Experiment Station (Indiana, USA) was treated ex situ by
bioaugmentation leading to a 20-fold increase of initial
atrazine-degradation rate (Grigg et al. 1997). In another
study, bioaugmentation carried out in simulated wetland
sediment was shown to be successful (Runes et al. 2001).
These studies also showed the major limitations inherent
to the use of living microorganisms for bioremediation
which are related to the characteristics of applied atrazine-degrading-microbial strains, the physicochemical
and microbiological characteristics of receptor environment, and the composition, preparation, storage, and
application of the inoculums.
• Inoculum characteristics
A first distinction is clear: it is that which differentiates
bacterial and fungal inoculums. The last being a priori
good candidates for bioaugmentation application, especially because these microorganisms can be produced in
large amount by solid-state fermentation using cheap
plant by-products (Durand 2003). Fungi species are
known to produce a wide range of enzymes such as
peroxidases, laccases, and phenoloxidases. These
enzymes presenting strong oxidizing capabilities and
having a wide substrate range are involved in the conversion of lignin and of its derivatives but also of various
compounds including several pesticides. Thus, several
studies have shown that, in a liquid medium,
Phanerochaete chrysosporium was able to transform
atrazine to its N-dealkylated derivative (Mougin et al.
1997). In addition, it was shown that inoculation of
atrazine-contaminated
soil
with
Phanerochaete
chrysosporium contributes to atrazine dissipation but
did not have any effect on s-triazine ring (Hickey et al.
1994). Indeed, inoculation of Phanerochaete
chrysosporium to soil microcosms led to the production
of N-dealkylated derivatives and to the accumulation of
bound residues (Entry et al. 1996). If Phanerochaete
chrysosporium inoculation leads effectively to atrazine
dissipation from contaminated soil, accumulation of Ndealkylated metabolites in treated soils is not to
recommend.
From this point of view, bacteria whose biochemical
activity tends to orientate the flow of carbon and of
electrons to biosynthetic and energetic metabolisms are
more environmentally friendly. Indeed, a wide range of
pure bacterial strains able to mineralize atrazine (i.e.,
transformation of atrazine to simple compounds such as
CO 2 and NH 4 ) are now available (for review, see
Udikovic-Kolic et al. 2012). It has to be underlined that
this has not always been the case since Pseudomonas sp.
ADP, the first known strain to mineralize atrazine, was
16 Applied Microbial Ecology and Bioremediation
701
matter during humidification (Barriuso et al. 1997). Later
on several reports further confirmed that addition of
simple or complex carbohydrates, thus creating a
nitrogen-limited environment, stimulates the degradation
of atrazine being viewed as a nitrogen source for
degrading microorganisms (Udikovic-Kolic et al. 2012).
Massive organic amendments often have the effect of
stimulating the fraction of the most opportunistic high
growth microorganisms (r-strategy). More limited, but
more frequent, carbon inputs would avoid this nutritional
discrimination. This carbon delivery can be achieved by
plants which are known to release up to 20 % of their
photosynthates as root exudates in their rhizosphere. This
allows microorganisms to grow in the rhizosphere where
their abundance can reach up to an order of magnitude
higher than that it is in the non-rhizosphere soil. From
these considerations originate the idea of developing
microbe-assisted phytostimulation (Anderson et al.
1994), in which plant is not playing part of pesticide
biodegradation but acts as a source of carbon that can
be used by rhizospheric microorganisms as co-substrates
to degrade pesticides (Alvey and Crowley 1996; Piutti
et al. 2002). Thus, a significant increase in the degradation of atrazine applied at concentrations far exceeding
the recommendations dose, in the presence of other
herbicides such as metolachlor and trifluralin, was
obtained by cropping contaminated soil with Kochia
scoparia (L) Roth, an annual plant commonly found on
pesticide site production and tolerant to different
herbicides (Perkovich 1996). Moreover, K. scoparia has
also been successfully used to promote microbial degradation of other pesticides such as metolachlor and lindane
(Singh 2003). Other plants like Pennisetum clandestinum
or Zea maize have also been shown to enhance the activity of atrazine-degrading-microbial populations in their
rhizosphere (Piutti et al. 2002; Singh et al. 2004). Conversely, atrazine-degrading-microbial activity was not
improved in the rhizosphere of several grasses such as
Sorghum vulgare, Lolium perenne, Festuca arundinacea,
and Agropyron desertorum (Anderson et al. 1995). The
impression left by the literature on the subject is that of
a certain disparity of situations from which it seems
difficult to draw general conclusions and guidance on
the involved processes.
• Bioaugmentation approach for atrazine bioremediation
Those that have been first implemented in the mid 1990s
are based on the use of pure strains or microbial consortia
able to degrade atrazine. Most of the bioaugmentation
studies carried out in microcosms incubated under laboratory conditions have concluded to an immediate
increase of degrading capacity of atrazine-contaminated
soil in response to inoculation of degrading inoculums
(Rousseaux et al. 2003). Only few bioaugmentation
studies have been done at field scale. Indeed atrazinecontaminated soil from the Purdue Agricultural Experiment Station (Indiana, USA) was treated ex situ by
bioaugmentation leading to a 20-fold increase of initial
atrazine-degradation rate (Grigg et al. 1997). In another
study, bioaugmentation carried out in simulated wetland
sediment was shown to be successful (Runes et al. 2001).
These studies also showed the major limitations inherent
to the use of living microorganisms for bioremediation
which are related to the characteristics of applied atrazine-degrading-microbial strains, the physicochemical
and microbiological characteristics of receptor environment, and the composition, preparation, storage, and
application of the inoculums.
• Inoculum characteristics
A first distinction is clear: it is that which differentiates
bacterial and fungal inoculums. The last being a priori
good candidates for bioaugmentation application, especially because these microorganisms can be produced in
large amount by solid-state fermentation using cheap
plant by-products (Durand 2003). Fungi species are
known to produce a wide range of enzymes such as
peroxidases, laccases, and phenoloxidases. These
enzymes presenting strong oxidizing capabilities and
having a wide substrate range are involved in the conversion of lignin and of its derivatives but also of various
compounds including several pesticides. Thus, several
studies have shown that, in a liquid medium,
Phanerochaete chrysosporium was able to transform
atrazine to its N-dealkylated derivative (Mougin et al.
1997). In addition, it was shown that inoculation of
atrazine-contaminated
soil
with
Phanerochaete
chrysosporium contributes to atrazine dissipation but
did not have any effect on s-triazine ring (Hickey et al.
1994). Indeed, inoculation of Phanerochaete
chrysosporium to soil microcosms led to the production
of N-dealkylated derivatives and to the accumulation of
bound residues (Entry et al. 1996). If Phanerochaete
chrysosporium inoculation leads effectively to atrazine
dissipation from contaminated soil, accumulation of Ndealkylated metabolites in treated soils is not to
recommend.
From this point of view, bacteria whose biochemical
activity tends to orientate the flow of carbon and of
electrons to biosynthetic and energetic metabolisms are
more environmentally friendly. Indeed, a wide range of
pure bacterial strains able to mineralize atrazine (i.e.,
transformation of atrazine to simple compounds such as
CO 2 and NH 4 ) are now available (for review, see
Udikovic-Kolic et al. 2012). It has to be underlined that
this has not always been the case since Pseudomonas sp.
ADP, the first known strain to mineralize atrazine, was
16 Applied Microbial Ecology and Bioremediation
701