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Vol. 9 No. 3 - July 2003

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

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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