For instance, Borja et al. (2006) studied the biodegradation
of highly chlorinated PCBs (Aroclor 1260) by a mixed
culture isolated from a contaminated soil after biphenyl
enrichment of the soil. The consortium was immobilized
on cement particles in a three-phase fluidized-bed reactor
running in a discontinuous mode where it was progressively
adapted to PCB by alternated supplies of biphenyl and PCB.
The formed biofilm degraded with a high-efficiency PCBs
added to the reactor: 80 % biodegradation after 1 day and
91 % after 5 days; the important decline of Aroclor the first
day is accounted for by the pollutant adsorption on the
biofilm followed by a progressive PCB degradation.
Biphenyl was used as co-substrate to stimulate PCB
biodegradation in soils and sediments. However, as biphenyl
is itself a pollutant, alternative solutions were proposed,
based in particular on the use of natural compounds such
as carvone extracted from mint (Mentha spicata). This nontoxic ketone (terpenoid) induces in Arthrobacter sp. strain
B1B, Aroclor 1242 cometabolism and is used at a 50 mg
dm
À3 concentration that is too low for the bacterium growth
in the absence of an additional input (fructose). Aroclor 1242
cometabolism in the presence of carvone was demonstrated
in microcosms by a significant disappearance of dominant
congeners, formation of split products from phenylhexadienoate, and production of various chlorobenzoates that
are intermediate compounds of PCB degradation (Gilbert
and Crowley 1997). This important finding suggests that it
would be possible to use plants secreting monoterpene inside
the rhizosphere to achieve in situ bioremediation of
contaminated soils. PCB biodegradation in chronically
polluted soils (about 350 mg kg
À1 of soil, mainly Aroclor
1242 and 1254) was also investigated in the presence (and
absence) of cyclodextrines that increase the availability of
water-insoluble molecules (Fava et al. 1998); the PCBs’ low
solubility was mentioned here above. The soil that contained
microorganisms able to grow on cyclodextrins as the sole
source of carbon and energy was supplemented with biphenyl (4 g kg
À1 of soil) and inorganic nutrients (0.3 NH 4 NO 3 , g
kg
À1 of soil, and 0.04 KH 2 PO 4 , g kg
À1 of soil), N and P
concentrations being very low in the soil. The presence of
cyclodextrins effectively increased the biodegradation level
of some congeners.
Results above presented concern studies conducted in
microcosms on small volumes of soil or sediment. An experiment was conducted in situ to evaluate the aerobic biodegradation of a preexistent PCB contamination in Hudson
River sediments till then submitted to a natural anaerobic
biodegradation (Harkness et al. 1993). It was performed in
steel cylinders of 1.8 m diameter and 5 m high, inserted
in the initially anoxic sediment. Supplementing dioxygen,
inorganic nutrients and biphenyl increased PCB biodegradation (37–55 % of disappearance), the latter being evidenced
by chlorobenzoate production occurring during PCB
biodegradation.
Attempts of bioamplification were also undertaken in
order to increase the PCB aerobic biodegradation. They
essentially focus on improving the efficiency of strains able
to degrade PCBs and amenable to inoculation in polluted
sediments and soils. Several strategies were developed:
(i) Addition of one (several) strain(s) able to degrade PCBs
in a polluted site (seeding) knowing that a given strain
can only attack some congeners.
(ii) Grouping together in a single microorganism the genes
involved in the different degradation steps. For
instance, grouping in the same organism genes ensuring
under aerobiosis the degradation of PCBs and that of
chlorobenzoic acid.
(iii) Using biodegradation capacities of a microorganism by
amplifying its activity through biosurfactant implementation: coupling biostimulation and bioamplification.
Under these conditions, by coupling Arthrobacter sp.
B1B and Ralstonia eutropha H850 addition to that of a
surfactant (sorbitol trioleate), the PCB elimination in a
sediment polluted by Aroclor 1242 reached about 60 %.
One the major problems faced by microorganisms used in
bioamplification experiments is their elimination by
microorganisms already present in the environment.
In order to overcome this competition, one solution is to
have one microorganism able to use for its growth a substrate not taken up by endogenous communities that consume biphenyl. To reach this objective, the genes bhpABCD
(Fig. 16.32), present in the strain Pseudomonas sp. ENV307,
were inserted via a plasmid into the strain Pseudomonas
paucimobilis 1IGP4 that uses as a growth substrate a nonionic surfactant, Igepal CO-720, water-soluble, and nontoxic
unlike biphenyl (Lajoie et al. 1993). A soil contaminated
by Aroclor 1242 and seeded with the 1IGP4 strain (about
4.10
6 cells∙g
À1 of soil) was added with Igepal CO-720
(1 % w/w). This treatment lead to biodegradation of many
congeners: 60 % biodegradation of 2
0 ,3,4- and 2,5,2
0 ,6
0
congeners, after 24 days.
Several conclusions can be drawn from these experiments
on PCB aerobic biodegradation (Furukawa 2000; Field and
Sierra-Alvarez 2008):
(i) Monochlorinated compounds are growth substrates for
several bacteria species. Generally, biodegradation of
PCBs with more than one chlorine atom proceeds
through cometabolism in the presence of biphenyl
which is then used as a carbon and energy source and
as inducing biodegradation enzymes. The aerobic biodegradation of polychlorinated congeners in the
absence of biphenyl is nevertheless possible. Thus,
Pseudomonas aeruginosa, strain TMU56, isolated
from a soil contaminated for 35 years by a current
transformer fluid, degrades in the absence of biphenyl
2,4-diCB; 2,2
0 ,5,5
0 -tetraCB; and 2,2
0 ,4,4
0 ,5,5
0 -hexaCB
as well as several Aroclor 1442 polychlorinated
congeners (Hatamian-Zarmi et al. 2009).
16 Applied Microbial Ecology and Bioremediation
713
of highly chlorinated PCBs (Aroclor 1260) by a mixed
culture isolated from a contaminated soil after biphenyl
enrichment of the soil. The consortium was immobilized
on cement particles in a three-phase fluidized-bed reactor
running in a discontinuous mode where it was progressively
adapted to PCB by alternated supplies of biphenyl and PCB.
The formed biofilm degraded with a high-efficiency PCBs
added to the reactor: 80 % biodegradation after 1 day and
91 % after 5 days; the important decline of Aroclor the first
day is accounted for by the pollutant adsorption on the
biofilm followed by a progressive PCB degradation.
Biphenyl was used as co-substrate to stimulate PCB
biodegradation in soils and sediments. However, as biphenyl
is itself a pollutant, alternative solutions were proposed,
based in particular on the use of natural compounds such
as carvone extracted from mint (Mentha spicata). This nontoxic ketone (terpenoid) induces in Arthrobacter sp. strain
B1B, Aroclor 1242 cometabolism and is used at a 50 mg
dm
À3 concentration that is too low for the bacterium growth
in the absence of an additional input (fructose). Aroclor 1242
cometabolism in the presence of carvone was demonstrated
in microcosms by a significant disappearance of dominant
congeners, formation of split products from phenylhexadienoate, and production of various chlorobenzoates that
are intermediate compounds of PCB degradation (Gilbert
and Crowley 1997). This important finding suggests that it
would be possible to use plants secreting monoterpene inside
the rhizosphere to achieve in situ bioremediation of
contaminated soils. PCB biodegradation in chronically
polluted soils (about 350 mg kg
À1 of soil, mainly Aroclor
1242 and 1254) was also investigated in the presence (and
absence) of cyclodextrines that increase the availability of
water-insoluble molecules (Fava et al. 1998); the PCBs’ low
solubility was mentioned here above. The soil that contained
microorganisms able to grow on cyclodextrins as the sole
source of carbon and energy was supplemented with biphenyl (4 g kg
À1 of soil) and inorganic nutrients (0.3 NH 4 NO 3 , g
kg
À1 of soil, and 0.04 KH 2 PO 4 , g kg
À1 of soil), N and P
concentrations being very low in the soil. The presence of
cyclodextrins effectively increased the biodegradation level
of some congeners.
Results above presented concern studies conducted in
microcosms on small volumes of soil or sediment. An experiment was conducted in situ to evaluate the aerobic biodegradation of a preexistent PCB contamination in Hudson
River sediments till then submitted to a natural anaerobic
biodegradation (Harkness et al. 1993). It was performed in
steel cylinders of 1.8 m diameter and 5 m high, inserted
in the initially anoxic sediment. Supplementing dioxygen,
inorganic nutrients and biphenyl increased PCB biodegradation (37–55 % of disappearance), the latter being evidenced
by chlorobenzoate production occurring during PCB
biodegradation.
Attempts of bioamplification were also undertaken in
order to increase the PCB aerobic biodegradation. They
essentially focus on improving the efficiency of strains able
to degrade PCBs and amenable to inoculation in polluted
sediments and soils. Several strategies were developed:
(i) Addition of one (several) strain(s) able to degrade PCBs
in a polluted site (seeding) knowing that a given strain
can only attack some congeners.
(ii) Grouping together in a single microorganism the genes
involved in the different degradation steps. For
instance, grouping in the same organism genes ensuring
under aerobiosis the degradation of PCBs and that of
chlorobenzoic acid.
(iii) Using biodegradation capacities of a microorganism by
amplifying its activity through biosurfactant implementation: coupling biostimulation and bioamplification.
Under these conditions, by coupling Arthrobacter sp.
B1B and Ralstonia eutropha H850 addition to that of a
surfactant (sorbitol trioleate), the PCB elimination in a
sediment polluted by Aroclor 1242 reached about 60 %.
One the major problems faced by microorganisms used in
bioamplification experiments is their elimination by
microorganisms already present in the environment.
In order to overcome this competition, one solution is to
have one microorganism able to use for its growth a substrate not taken up by endogenous communities that consume biphenyl. To reach this objective, the genes bhpABCD
(Fig. 16.32), present in the strain Pseudomonas sp. ENV307,
were inserted via a plasmid into the strain Pseudomonas
paucimobilis 1IGP4 that uses as a growth substrate a nonionic surfactant, Igepal CO-720, water-soluble, and nontoxic
unlike biphenyl (Lajoie et al. 1993). A soil contaminated
by Aroclor 1242 and seeded with the 1IGP4 strain (about
4.10
6 cells∙g
À1 of soil) was added with Igepal CO-720
(1 % w/w). This treatment lead to biodegradation of many
congeners: 60 % biodegradation of 2
0 ,3,4- and 2,5,2
0 ,6
0
congeners, after 24 days.
Several conclusions can be drawn from these experiments
on PCB aerobic biodegradation (Furukawa 2000; Field and
Sierra-Alvarez 2008):
(i) Monochlorinated compounds are growth substrates for
several bacteria species. Generally, biodegradation of
PCBs with more than one chlorine atom proceeds
through cometabolism in the presence of biphenyl
which is then used as a carbon and energy source and
as inducing biodegradation enzymes. The aerobic biodegradation of polychlorinated congeners in the
absence of biphenyl is nevertheless possible. Thus,
Pseudomonas aeruginosa, strain TMU56, isolated
from a soil contaminated for 35 years by a current
transformer fluid, degrades in the absence of biphenyl
2,4-diCB; 2,2
0 ,5,5
0 -tetraCB; and 2,2
0 ,4,4
0 ,5,5
0 -hexaCB
as well as several Aroclor 1442 polychlorinated
congeners (Hatamian-Zarmi et al. 2009).
16 Applied Microbial Ecology and Bioremediation
713