Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 150

consequently a high level of microbiological activity, which tends to increase
the speed and efficiency of the biodegradation of organic substances within the
rhizosphere compared with other soil regions and microfloral communities.
Part of the reason for this is the tendency for plant roots to increase the soil
oxygenation in their vicinity and exude metabolites into the rhizosphere. It has
been estimated that the release of sugars, amino acids and other exudates from
the plant and the net root oxygen contribution can account for up to 20% of plant
photosynthetic activity per year (Foth, 1990), of which denitrifying bacteria,
Pseudomonas spp ., and general heterotrophs are the principal beneficiaries.
In addition, mycorrhizae fungi associated with the roots also play a part in
metabolising organic contaminants. This is an important aspect, since they have
unique enzymatic pathways that enable the biodegradation of organic substances
that could not otherwise be transformed solely by bacterial action. In principle,
rhizodegradation is intrinsic remediation enhanced by entirely natural means,
since enzymes which are active within 1mm of the root itself transform the
organic pollutants, in a way which, clearly, would not occur in the absence
of the plant. Never-the-less, this is generally a much slower process than the
previously described phytodegradation.
Phytovolatilisation
Phytovolatilisation involves the uptake of the contaminants by plants and their
release into the atmosphere, typically in a modified form. This phytoremediation
biotechnology generally relies on the transpiration pull of fast growing trees,
which accelerates the uptake of the pollutants in ground water solution, which
are then released through the leaves. Thus the contaminants are removed from the
soil, often being transformed within the plant before being voided to the atmo-
sphere. One attempt which has been explored experimentally uses a genetically
modified variety of the Yellow Poplar, Liriodendron tulipifera which has been
engineered by the introduction of mercuric reductase gene ( mer A ) as discussed
in Chapter 9. This confers the ability to tolerate higher mercury concentrations
and to convert the metal's ionic form to the elemental and allows the plant to
withstand contaminated conditions, remove the pollutant from the soil and vola-
tise it. The advantages of this approach are clear, given that the current best
available technologies demand extensive dredging or excavation and are heavily
disruptive to the site.
The choice of a poplar species for this application is interesting, since they
have been found useful in similar roles elsewhere. Trichloroethylene (TCE), an
organic compound used in engineering and other industries for degreasing, is a
particularly mobile pollutant, typically forming plumes which move beneath the
soil's surface. In a number of studies, poplars have been shown to be able to
volatilise around 90% of the TCE they take up. In part this relates to their enor-
mous hydraulic pull, a property which will be discussed again later in this chapter.
Acting as large, solar-powered pumps, they draw water out of the soil, taking up
contaminants with it, which then pass through the plant and out to the air.