Environmental Engineering Reference
In-Depth Information
potential to detoxify or utilize these contaminant by gene
expression of contaminant degradation potential; in other
words, would the number of bacteria that contain enzymes
to degrade contaminants increase? Rhizospheric bacteria
may provide a selective advantage to those plants as far as
the detoxification of deleterious compounds, such as allelo-
pathic compounds. If these processes may also act to protect
plants from xenobiotics, then the processes could be used for
phytoremediation purposes.
A study by Siciliano et al. (2001) examined this issue
further. As was stated in Chap. 11, plant encounter with
allelopathic compounds could lead to an increase in those
bacteria capable of degrading that particular compound, thus
providing protection to the plant that had these bacteria. This
may explain why studies with many compounds show an
increased rate of mineralization and degradation in the rhi-
zosphere that contains higher contaminant concentrations.
Because this relation exists between plants for certain micro-
bial communities, it is interesting to note the influence is
stronger inside and near the root surface.
They (Siciliano et al. 2001) also examined the relative
expression of catabolic genotypes for petroleum hydrocar-
bon degradation, such as
alk
B (alkane monooxygenase),
ndo
B (naphthalene dioxygenase), and for nitroaromatic deg-
radation the genotypes
ntd
A
a
(2-nitrotoluene reductase) and
ntn
M (nitrotoluene monooxygenase) in unplanted soil, rhi-
zosphere soil, and the endophytic part of roots grown at a
contaminated site in California. They report that the
alk
B
and
ndo
B genes were two to four times more prevalent in
bacteria in the root itself than found in the soil and rhizo-
sphere soil, respectively. In addition, they report that the
ntd
A
a
and
ntn
M genes were 7-14 times more prevalent in
the root bacteria than in the soil and rhizosphere soil.
From the perspective of the phytoremediation of
contaminated groundwater, these results present a dilemma
of whether or not to inoculate plants with rhizospheric bac-
teria during plant installation. Studies have indicated, as
mentioned previously, that contaminant remediation is
increased in the soil zone in planted areas relative to
unplanted areas. Will native bacteria colonize the installed
plants, or should inoculants that contain the enzymes for
specific contaminants be added to the soil during planting?
A concern here is that funds will be used to add microbes but
they may not be viable or may simply die off or be
outcompeted.
Although it is generally accepted that there are more
bacteria in planted soils versus unplanted soils, there are
conflicting data about the effect of these bacteria on contam-
inant concentrations. Some research indicates that the rates
of degradation in planted areas relative to unplanted controls
are statistically significant, whereas others state the opposite,
that the difference is not statistically significant. For exam-
ple, Knaebel and Vestal (1994) reported that for plants in
contact with agricultural chemicals, the degradation rates of
these compounds was higher in planted versus unplanted
soils, but the total amount of degradation was not statisti-
cally significant in comparison to unplanted controls. Brandt
et al. (2006) reported that there was little experimental
evidence to suggest that plants such as Vetiver (
Vetiveria
zizanioides
(L.) Nash) added to petroleum-hydrocarbon
contaminated sediments led to an expected enhancement of
the root-associated biodegradation of these compounds pres-
ent in contaminated soils. In fact, they report than both plant
biomass and height were
decreased
in the presence of
contaminated soils.
On the other hand, Nichols et al. (1997) presented evi-
dence to support observations that the presence of plants
leads not only to an increase in microbial numbers relative
to unplanted areas, but the number of contaminant-
degrading bacteria as well. They grew alfalfa (
Medicago
sativa
) and alpine bluegrass (
Poa alpina
) in soil where one
treatment was then contaminated with a mixture of organic
chemicals and other treatment was not. The mixture
consisted of hexadecane, 2,2-dimethylpropyl benzene,
cis
-
decahydronaphthalene, benzoic acid, and pyrene, which are
good analogies for BTEX and PAH groundwater contami-
nation. After 9 weeks of testing, they determined that the
number of organic-chemical degrading bacteria in the rhizo-
sphere of the alfalfa-planted treatment was higher in the
unplanted treatment
that
also was
contaminated, or
4
10
7
/g versus 6
10
6
/g, and in the bluegrass-planted
10
6
/g in the
contaminated versus uncontaminated soils, respectively.
In the early 1990s, intense investigation into the relation
of bacteria and their enzymes systems and their effect on
subsurface contaminants and remediation was in full swing.
One of the first reports of the linkage between bacteria,
contaminant degradation, and plants was an investigation
by Donnelly et al. (1994). They reported that natural plant
organic compounds, in their case flavonoids, also could
support soil bacteria that can degrade PCBs.
An additional study designed to investigate the hypothe-
sis that plant-exudates and plant roots can induce the expres-
sion of genes was performed with the contaminant
naphthalene (Kamath et al. 2004). They reported the induc-
tion of the gene
nah
G by plant-released compounds such as
salicylate, with no expression observed in the presence of
root exudates. In fact, the converse occurred, where
increased root extracts inhibited
nah
G expression. This
suggests that the increased microbial mineralization of
PAHs such as naphthalene in planted versus unplanted
soils may be a consequence not of exposure to root exudates
but of the exposure of the rhizospheric bacteria to the con-
taminant in the root zone.
Because the bacteria in the rhizosphere are associated
with the roots of plants, the distribution of roots becomes
10
7
treatment there was 1
/g versus 1
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