Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 289

a long-term, residual source of addition groundwater

contamination.

12.2.1 Microbial, Fungal, and Root-Zone
Processes

It was observed that the root zone, or rhizosphere, is
characterized by greater numbers and species diversity of
microbes compared to areas that do not have plants (Paul and
Clark 1989). A possible explanation for this difference is
that the plant-root-released organic matter can support
microbial metabolism, and this metabolism in turn may
increase the bioavailability of essential minerals required
by the plants. There are two general classifications of this
observed association between plant roots and
microorganisms. Microbes, in particular fungi, that are
found to colonize the exterior parts of subsurface plant
structures are called ectomycorrhizae. Fungi that colonize
the interior of the plant roots within the cortical cells are
called endomycorrhizae. Bacteria that colonize the interior
of plants are called endophytes. The endomycorrhizae
can be further divided into the arbuscular and ericoid
mycorrhizae (Vos ´ tka et al. 2006). The arbuscular
mycorrhizae (AM) are present in more than 80% of vascular
plants. They do not form fruiting bodies and, therefore, their
presence is not always noticeable. The AM fungi have been
show to be present in PAH- and PCB-contaminated soils.
The ericoid mycorrhizae (ERM) are mostly ascomycetous
fungi that inhabit the roots of Ericaceae , and are not wide-
spread. Ectomycorrhizae (ECM) fungi can occur widely in
trees. They consist of basidiomycetous and ascomycetous
fungi; unlike the endomycorrhizae that produce invisible
spores, these produce fruiting bodies that appear as
mushrooms. These fungi appear to be able to use some
fraction of lignin as well as cellulose as growth substrates.
Therefore, they appear to be able to degrade more recalci-
trant organic compounds, such as the multiple ringed (

Fig. 12.3 Increased microbial activity in the rhizosphere of plants (R)
relative to unplanted sediments (S). MPN is the Most Probable Number,
an indication of the number of colony-forming units of bacteria grow-
ing in the culture medium. The 1-sigma standard deviation for the
samples is shown.

the absorption potential of the soil (Brigmon et al. 1998).
The increased number of microorganisms associated with
plants does not necessarily mean a de facto increase in
contaminant degradation potential, as the biodegradation of
a particular class of xenobiotic compounds is ultimately
determined by (1) contaminant bioavailability and (2) the
appropriate enzyme expression for contaminant metabolism.
One of the first studies of the effect of enhanced degrada-
tion of chemicals associated with the rhizosphere was
performed by Hsu and Bartha (1979). In their experimental
design, rather than using a soil substrate for their test plants,
which would confound their study on account of competing
factors such as root sorption, they grew the plants in an
aqueous medium (hydroponically). A study by Westover
et al. (1997) suggested that with the herbaceous plants they
studied ( caespitose , Festuca idahoensis , and Poa secunda ),
plant composition may determine somewhat the degree of
community structure of rhizospheric bacteria and fungi. The
degree of this influence and its distance of regulation from
the plant root will be examined below.
Along a similar line of reasoning, does the presence of
contaminants induce in the rhizospheric bacterial
populations an increase in the genetic capability to degrade
these contaminants? The assumption is that plants that are
exposed to contaminant compounds would have a selective
advantage if

> 2)

PAHs.
The effect of these rhizospheric organisms on
contaminants released to the subsurface is threefold. First,
these rhizosphere microbes have an increased potential to
degrade xenobiotic compounds (Fig. 12.3 ) because many
plant exudates are structurally analogous to contaminant
compounds (Fletcher and Hegde 1995; Reilley et al. 1996).
Second, organic exudates from growing roots also stimulate
cometabolic processes, which result in the fortuitous biotic
degradation of chlorinated solvents. Jordahl et al. (1997)
found that the numbers of benzene-, toluene-, and xylene-
biodegrading microbes were higher in soil samples if poplar
trees were present compared to adjacent soils (Fig. 12.3 ).
Third, the presence of roots in a subsurface often devoid of
sedimentary organic matter prior to planting will increase the rhizospheric community increased its