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Phytoremediation of Selenium and Other Toxic Trace Elements
Norman Terry and Danika LeDuc
Department of Plant and Microbial Biology, 111 Koshland Hall,
University of California, Berkeley, CA 94720-3102, U.S.A.
Soils and waters in many areas of the world are polluted with toxic
trace elements. These include metalloids such as selenium (Se) and
arsenic (As), as well as heavy metals, e.g., cadmium (Cd), lead (Pb),
chromium (Cr) and mercury (Hg). Because of the acute toxicity of these
elements, there is an urgent need to develop technologies to remove or
detoxify them. Phytoremediation, the use of plants and their associated
microbes, offers an effective, low-cost and sustainable means to achieve
this end. For contaminants present in soils, there are several different
approaches of which three are presented here.
Phytoextraction
utilizes the ability of certain plants to take up contaminants from the
soil and water and accumulate them in their tissues, which can then be
harvested and removed from the site.
Phytovolatilization
makes use of the plant's ability to convert pollutants into volatile
forms, which then escape to the atmosphere (e.g., Se, mercury).
Phytodetoxification
involves the ability of plants to change the chemical species of the
contaminant to a less toxic form, as occurs when plants take up toxic
hexavalent chromium (Cr) and convert it to non-toxic trivalent Cr. For
contaminants present in wastewaters, provide an effective, low-cost
treatment system to remove a wide-range of contaminants from municipal
wastewater as well as effluents from electricity generating facilities
and oil refineries. Although this is not phytoremediation in the sense
used above, plants nevertheless play a critical function in this form of
pollutant removal. In this brief review, we highlight different aspects
of our research on the phytoremediation of Se, as well as some heavy
metal pollutants.
Constructed wetlands
Constructed wetlands are artificial wetlands specifically designed to
improve water quality. Like natural wetlands, they are a complex mixture
of water, sediments, living and dead plant materials, fauna and
microbes. In essence, constructed wetlands act as giant biogeochemical
filters able to remove contaminants present at very low concentrations
from very large volumes of wastewater (e.g., Se from oil refinery
wastewater). The filtration of contaminants that occurs in a wetland
ecosystem takes place mostly in the layer of dead, partially decomposed
plants, known as .fallen litter, and in the fine sediment layer beneath
the litter layer. These two layers provide habitat for microbes and
other organisms able to transform contaminants into less bio available
and therefore less toxic chemical forms. In addition to their role in
generating the fallen litter and fine sediment layers, plants provide
the fixed carbon that supports these microbial populations.
Much
of our research has focused on Se, a pollutant present in wastewater
from industrial and agricultural enterprises. In California, soils on
the west side of the San Joaquin Valley were formed from the natural
weathering of Se-bearing Cretaceous shales of the surrounding California
Coast Range and are therefore enriched in Se. Since these Se- and
salt-laden soils must be heavily irrigated to leach away the salts,
large amounts of subsurface drainage water are produced, which is highly
contaminated with dissolved Se. This drainage-water Se has been shown to
be responsible for mortality, developmental defects, and reproductive
failure in migratory aquatic birds and fish. In an experiment designed
to test whether constructed wetlands could be used to filter Se from
irrigation drainage water, we constructed 10 quarter-acre wetland cells
at the Tulare Lake Drainage District in the San Joaquin Valley.
Specifically, we wanted to know whether varying plant species influences
the efficiency of wetlands for Se removal. Each cell was planted with
one or a combination of the following plant species: sturdy bulrush (Schoenoplectus
robustus
(Pursh)
M.T. Strong), baltic rush (Juncus
balticus
Willd.), smooth cordgrass (Spartina
alterniflora
Loisel.), rabbitfoot grass (Polypogon
monspeliensis
(L.)
Desf.), saltgrass [Distichlis
spicata
(L.)
Greene], cattail [Typha
latifolia
L.],
tule [Schoenoplectus
acutus
(Muhl. ex Bigelow) Á. Löve & D. Löve), and widgeon grass (Ruppia
maritime
L.).
One cell was left unplanted as a control. On average, the wetland cells
removed approximately 69% of the total Se mass from the inflow. The
presence of plants improved the efficiency of Se removal over the
unvegetated cell but the particular choice of plant species was found to
have no significant effect on Se removal. Rates of biological Se
volatilization on the other hand, varied substantially with the type of
species planted, with the highest rates of biological Se volatilization
being recorded when the wetland cell was planted with rabbitfoot grass.
Most
of the Se filtered out by the wetland cells was trapped in the upper
sediment layers, which could present a serious ecotoxic risk over the
long term. One way of alleviating the buildup of Se in the upper
sediment layers of constructed wetlands is to increase the efficiency by
which wetlands convert the Se into volatile forms. Plants and microbes
are both capable of absorbing inorganic and organic forms of Se and
metabolizing them to volatile forms, e.g., dimethylselenide (DMSe),
which is 500-700 times less toxic than SeO4-2 or
SeO3-2 . In this way Se is removed from the
sediments, and its entry into the food chain diminished. Even though the
volatilized Se may eventually be redeposited in other areas, this is not
a problem in California where much of the state is deficient in Se with
respect to the nutrition of animals, which require Se in low
concentrations.
Genetic modification of plants to enhance phytoremediation
Because of the potential importance of biological Se volatilization to
Se phytoremediation in both wetlands and uplands, research in our
laboratory has focused heavily on genetic engineering strategies to
improve Se volatilization by plants, as well as on increasing the
ability of plants to extract and remove Se from contaminated soil. A
useful approach for increasing phytoremediation efficiency is to
overexpress enzymes catalyzing rate- limiting steps. Tagmount et al.
(2002) showed that S-adenosyl-L-methionine:L-methionine S-methyltransferase(MMT)
is the enzyme responsible for the methylation of selenomethionine to Se-methylselenomethionine
and that this enzyme was rate limiting with respect to the production of
volatile Se. Overexpression of MMTin Arabidopsis and Indian mustard (Brassica
juncea)
increased the rate of Se phytovolatilization approximately twofold over
wild type.
Another, quite different approach is to use hyperaccumulator plant
species as a source of plant genes to enhance phytoremediation. Milk
vetch (Astragalus
bisulcatus)
is a hyperaccumulator that accumulates Se in its leaves to
concentrations in excess of 4000 ppm. However, because it grows very
slowly, it is of limited value for phytoremediation. It is able to
accumulate Se to very high levels partly through the presence of the
gene encoding selenocysteine methyltransferase (SMT). SMT converts the
amino acid, selenocysteine (SeCys), which can cause toxicity when
incorporated into protein, into the non-protein amino acid,
methylselenocysteine (MetSeCys), thereby diminishing Se toxicity.
Transgenic plants overexpressing SMT show enhanced tolerance to Se,
particularly selenite, and produced 3- to 7-fold more biomass than wild
type. SMT plants accumulated up to 4-fold more Se than wild type, with
higher proportions in the form of MetSeCys. Additionally, SMT
Arabidopsis and SMT Indian mustard volatilized Se 2 to 3 times faster
when treated with SeCys and selenate, respectively.
Our
greatest success in using genetically engineered plants for
phytoremediation has been in the phytoextraction of Se from soil with
high levels of Se, boron (B), and salt. Three transgenic lines of Indian
mustard were used in the first transgenic phytoremediation trial in the
U.S. One line overexpressed the enzyme ATP sulfurylase (APS), which
facilitates the reduction of selenate to selenite and is rate limiting
with respect to the production of reduced, organic Se compounds. Indian
mustard plants overexpressing APS have increased tolerance and
accumulation of Se. APS Indian mustard may tolerate metals better
because it has higher concentrations of the thiol, glutathione (GSH),
than wild type.
Glutathione (Y-Glu-Cys-Gly)
plays an important role in heavy-metal detoxification. GSH can directly
form GSH-metal complexes and, as part of the active oxygen-scavenging
system, can protect the plant cell from oxidative stress. GSH is also
the direct precursor of phytochelatins (PCs), which bind, detoxify, and
sequester metal ions to the vacuole. The other two lines tested in the
field trial expressed genes in the GSH synthesis pathway, Y-glutamylcysteine
synthetase (Y-ECS) and glutathione synthetase
(GS). Overexpression of these enzymes in Indian mustard was first shown
to confer increased tolerance to Cd in solution culture. This tolerance
was correlated with 1.5 to 2.5 higher levels of GSH and PCs. In the
field, all three lines accumulated more Se in their shoots than wild
type. In fact, the APS line accumulated over four times as much Se as
wild type. Additionally, the GS line was significantly more tolerant of
the high Se, B, and salt sediment than wild type.
Phytodetoxification
The
toxicity of the heavy metal, chromium (Cr), is of increasing concern
because of its heavy use in many different industries, including
metallurgy, electroplating, production of paints and pigments, tanning,
wood preservation, Cr chemicals production, and pulp and paper
production. It is especially toxic in its hexavalent form, Cr(VI). In
this form, it is a highly toxic carcinogen, which may cause death to
animals and humans if ingested in large doses. Chromium, in the
trivalent form (Cr(III)), on the other hand, is an important component
of a balanced human and animal diet, and its deficiency disturbs the
metabolism of glucose and lipids in humans and animals. Plants have the
ability to detoxify Cr(VI) by reducing it to nontoxic Cr(III). Using
high energy X-ray absorption spectroscopy, research in our laboratory
showed that many plants including water hyacinth and various vegetable
crop species can convert Cr(VI) to Cr(III) in root and shoot tissues
within hours after uptake. The ability of
E.
crassipes
(water hyacinth) to reduce Cr(VI) to non-toxic Cr(III) could provide a
significant approach for the
in
situ
detoxification of Cr(VI) contaminated wastewater. Water hyacinth has an
excellent potential for the phytoremediation of Cr in wastewater because
(i) it has the ability to absorb and remove toxic Cr(VI) and then reduce
it to nontoxic Cr(III); (ii) it can accumulate Cr in high concentrations
in its tissues, especially roots; (iii) it produces very large amounts
of biomass (106-165 t ha-1 yr-1); and (iv) since
the whole plant can easily be harvested, Cr accumulated in both shoots
and roots can be removed.
Conclusions
Our
research has shown that plant-based technologies for removing and
detoxifying toxic trace elements from contaminated soil and water are
effective, and more importantly, can be improved substantially using
modern scientific approaches. One such approach is genetic engineering.
Laboratory-produced transgenic plants can be effective in field scale
cleanup, as shown by our recent research with transgenic Indian mustard,
which more efficiently removed Se from highly contaminated sediments.
Thus, genetic improvement of plants for phytoremediation has a great
potential for improving environmental quality. Nevertheless, there are
very few examples where genetic
approaches have
reached the stage of practical exploitation. It is clear that further
research and technical implementation is needed to carry
phytoremediation projects beyond the pilot stage. Furthermore, there are
regulatory and public acceptance barriers to overcome in applying
transgenic plant technology to real-world, field situations. Such
constraints have spurred researchers to innovate new methods of creating
transgenic plants that will be more palatable to the public and pose
less potential risk of hybridizing with nearby plants or adversely
affecting wildlife. One such technique is the use of chloroplast
transformation, the use of which prevents the escape of transgenes via
pollen to related weeds and crops.
Unlike upland
phytoremediation, which as yet has relatively limited use in
environmental cleanup, plant-based approaches in wetland ecosystems
(e.g., constructed wetlands) are widely used. The presence of plants in
the wetland ecosystem facilitates an inexpensive, solar-driven treatment
system for industrial wastewater, drainage waters, storm-water
management, and treatment of surface water. Plants confer tolerance
against fluctuations of flow, provide habitats for many wetland
organisms, and create an aesthetic, natural system for cleanup. Aquatic
plants and constructed wetlands have been used to treat municipal
wastewater and remove inorganic nutrients like phosphate and nitrate.
This technique has also been applied to industrial effluents containing
toxic metals or recalcitrant organic pollutants. In all of these
approaches, whether upland or wetland, there is clearly an urgent need
for research aimed at the fundamental understanding of all the physical,
chemical, and biological mechanisms involved. On a final note, more
field demonstration projects are also urgently required to optimize
phytoremediation approaches and to provide recommendations for the
regulators, decision-makers and the general public to convince them of
the suitability of the green approach for environmental cleanup. |
