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Chinese Brake Fern
A Potential Phytoremediator of
Arsenic Contaminated Soil and Water
By: Nandita Singh and Lena Q. Ma
Arsenic, ranking 20th
in abundance in the earth's crust, is a toxic element widely encountered in the
environment and organisms. Arsenic can enter terrestrial and aquatic
environments through both natural process and anthropogenic activity. There are
a number of ways by which the human beings can become exposed to arsenic. The
most important one is through ingestion of arsenic in drinking water or food.
Arsenic is toxic, and long-term exposure to low concentrations of arsenic in
drinking water can lead to skin, lung, bladder, and prostrate cancer. Recently,
the environmental fate and behaviour of arsenic has received increasing
attention due to alarming reports of arsenic poisoning in Southeast Asia (West
Bengal-India, Bangladesh and Vietnam). Large areas of these places use arsenic
contaminated ground water for irrigation of staple crop i.e. rice, consequently
the population is exposed to arsenic. Remediation of arsenic contaminated soils
and waters has thus become a major environmental issue. Current remediation
methods for arsenic contaminated soil like soil removal and washing, physical
stabilization, and/or use of chemical amendments are expensive and disruptive.
Phytoremediation is an emerging technology that employs the use of plants for
clearing the contaminated sites. The concept of using plants to clean up
contaminated sites is not new. About 300 years ago, plants were proposed for
use in the treatment of waste water. In the last decade, extensive research has
been conducted to investigate the mechanism of metal hyper-accumulation in
plants. Plant cultivation and harvesting are inexpensive processes compared
with traditional engineering approaches involving intense soil manipulation.
They also minimize the amount of secondary waste generated compared with soil
heaping, leaching or washing, besides, this technology is environment friendly.
Plant selection
The selection of plant species for
phytoremediation is possibly the single most important factor affecting the
extent of metal removal. Successful phytoextraction of metal from contaminated
soil requires the plants to tolerate the metal. Plants must also be able to
produce sufficient biomass besides accumulating a high concentration of
arsenic. The plants should be responsive to agricultural practices designed to
enhance arsenic accumulation and allow repeated planting and harvesting of
contaminant-laden biomass. Physical characteristics of soil contamination are
also important for plant selection. For example, for remediation of
surface-contaminated soils, shallow rooted species would be appropriate to use,
whereas deep-rooted plants would be the choice for deeper contamination in soil
profile. While studies have been conducted on the phytoextraction of heavy
metals such as cadmium, nickel and Zinc, little is known about the
phytoextraction of arsenic. This is because arsenic hyper-accumulators have
been discovered only recently. Hyper-accumulators are conventionally defined as
species capable of accumulating metals at levels 100 fold greater than those
typically measured in common non-accumulator plants. Arsenic levels exceeding
1000 mg kg-1 in shoots of plants grown on soil containing 100 mg kg-1 of this element are remarkable and would be considered as
hyper-accumulation. Ma et al. (2001) reported the first known arsenic
hyper-accumulator plant - Pteris vittata (Chinese brake fern) - a fern
that can accumulate extremely large concentrations of arsenic in its above
ground biomass (fronds). This discovery is a significant breakthrough for
cleaning up arsenic contaminated sites. A number of other fern species have
also been added to this list by Francesconi et al. (2002), Zhao et al.
(2002) and Mehrag (2003), i.e. Pitrogramma calomelanos (silver fern),
and three ferns from Pteris genus (P. criteca, P. longifolia and P.
umbrosa).
Requisites of an Arsenic -
hyper-accumulator
One requirement that is of great
significance to accumulation of toxic metals is the ability of plants to
tolerate the metals that are extracted from the soil. A variety of tolerance
and resistance mechanisms have evolved, including avoidance or exclusion, which
minimises the toxicity of the metal when accumulated in plant body, and
tolerance, which allows plants to survive while accumulating high
concentrations of metals. Accumulator species have evolved specific mechanisms
for detoxifying high metal levels accumulated in the cells. These mechanisms
allow plants to accumulate extremely high concentration of metals.
Arsenic is found in the
environment as arsenate (As-V) and arsenite (As-III). Arsenate is the dominant
plant available form of arsenic in well aerated soil. Arsenate is a chemical
analogue of macronutrient phosphate. Plants growing on arsenate contaminated
soils will assimilate high levels of arsenate unless they have altered
phosphate transport mechanisms. Thus, arsenic tolerance is inextricably linked
with phosphate nutrition. Plants that adapt to high arsenic levels evolved
tolerance by suppressing the high affinity phosphate - arsenate uptake system
and such a trait could be selected for breeding plants to vegetate arsenate
contaminated sites. Hence, attempts to use plants to remove arsenic from soil
through the process of phytoremediation need to take the multiple effects of
phosphate into consideration.
For phytoremediation of metals it
is necessary for the plants to continually accumulate and detoxify metals in
their system. Studies on arsenate toxicity have shown that plant species not
resistant to arsenic suffer considerable stress upon exposure, with symptoms
ranging from inhibition of root growth to death. Once being taken up by plants,
arsenic may be toxic to plants in a number of ways including reduction of
arsenate (As-V) to arsenite (As-III), which then attacks proteins. In general,
plants employ several extra-cellular and intracellular mechanisms to detoxify
heavy metals. External mechanism include exudation of substances from roots,
which binds metals. Whereas, internally the plants alternate the influx/efflux
of metal ions to reduce metal concentration in cell and bind it in a non-toxic
form to transport it to vacuole where detoxification takes place. Since the
immobilised metals are less toxic than the free ions, binding of arsenic to
phytochelatins is considered to be a part of the detoxifying mechanism.
Chinese Brake Fern as Arsenic
Hyper-accumulator
Chinese brake fern accumulates
large amount of arsenic in the fronds. The fern has a staggering ability to
extract and concentrate arsenic from the soil. On one contaminated site with
38.9 mg kg-1 of arsenic in the soil, the fern's fronds had 7,526 mg kg-1
of arsenic, and under experimental conditions where soil was loaded with
arsenic, the fern accumulated 22,630 mg kg-1 (2.3%) of the heavy
metal. Even where the arsenic concentration in the soil is low, the fern will
seek it out and take it up: a soil on the University of Florida campus with
just 0.47 mg kg-1 produced a fern with 136 mg kg-1 of
arsenic in its fronds.
Furthermore, the bioaccumulation
factor, defined as the ratio of fronds arsenic concentration to soil arsenic
concentration, is greater than 10. This ratio is held for non-contaminated
soils (6 mg kg-1 arsenic) and highly contaminated soils (1500 mg kg-1 arsenic). This fern also has an efficient root to fronds
transport mechanism of the metalloid leading to most arsenic being concentrated
in the fronds. This character is in contrast to those of many other arsenic
tolerant plants, which achieve arsenic tolerance mainly through reduced uptake
of arsenic by suppression of phosphate/ arsenate uptake system. The fern is
capable of taking up a range of inorganic and organic arsenic species including
arsenate and arsenite, with upto 93% of the arsenic concentrated in the fronds.
This capability to hyper-accumulate arsenic is till date, unique. The ability
of Chinese brake fern (Pteris vittata) to take up high concentrations of
arsenic and sequester into its above ground portions when grown in arsenic rich
soil implies that the fern has highly effective arsenic scavenging mechanisms.
The tolerance and hyper-accumulation ability of this fern is considered as a
constitutive property. Even though significant progress has been made in
understanding physiological basis of tolerance to arsenic in higher plants,
there remains a considerable uncertainty about the mechanisms by which Chinese
brake fern hyper-accumulates arsenic.
In addition to its remarkable
arsenic accumulating capability, Chinese brake fern has numerous desirable
characteristics that make it ideal for phytoremediation of arsenic contaminated
soil and water. They are versatile and hardy, have a large biomass, fast
growing, easy to reproduce and are perennial plants. The above ground biomass
can be harvested season after season until the site is cleaned up. Chinese
brake fern are found in most of the habitats. They are resistant to adverse
soil characteristics - disturbed sites, sites impacted by human activity, and
areas with limestones. Chinese brake fern are common in South Africa, U.S.A.,
Madagascar, Asia, Japan, Malaysia, and Australia. The promise appears to be
high for subtropical areas where this fern will thrive. Once the plants are
established, concentrations of the heavy metal in the fronds will be high, and
they can be harvested periodically for disposal in some safe facility. In
addition, ferns will be an attractive addition to the landscape.
The research group led by Dr. Lena
Ma is currently focusing on the mechanisms of arsenic uptake, translocation,
distribution and detoxifications by Chinese brake fern. They are also involved
in gathering knowledge of the precise biochemical and detoxification processes
at play in P vittata, which are fundamental to the possible use of this
plant for phytoremediation application.
The authors Dr Nandita Singh
(Scientist NBRI Lucknow, India) is a Fulbright Visiting Scholar and Dr. Lena Q.
Ma is Professor, at Soil and Water Science Department, University of Florida,
Gainesville, USA |
