Phytoremediation: Using plants to combat a stressed environment
By Amy Johnston
Plants have long been adapting the traits necessary to survive in a wide variety of stressful environments – including areas of high salinity, extreme heat, drought, and freezing temperatures - but now, using genetic modification, scientists have been able to expand the role that plants play in the environment. With the advent of transgenic biotechnology, plants can be enhanced with qualities that not only allow them to flourish in stressed environments but also allow them to be used in the effort to alleviate certain environmental stresses. Phytoremediators, plants that are used to clean-up soil in contaminated areas, can remove heavy metals, arsenic, petroleum, TNT, and many other elements from toxic soil. This paper will review the strategies used to create transgenic phytoremediators, the role these plants play in combating a stressed environment, and the advantages and disadvantages of using plants for bioremediation. Examples of emerging technology in the ever-evolving field of phytoremediation will also be discussed.
Phytoremediation is characterized by the use of vegetative species for in situ treatment of land areas polluted by a variety of hazardous substances. (Sykes et al., 1999) Plants are especially useful in the process of bioremediation because they prevent erosion and leaching which can spread the toxic substances to surrounding areas. (United States Environmental Protection Agency, 2001) There are several types of phytoremediation being used today. These include phytoextraction, which relies upon a plant’s natural ability to take up certain substances (such as heavy metals) from the environment and sequester them in their cells until the plant can be harvested, phytodegredation, a means by which plants convert organic pollutants into a non-toxic form, phytostabilization, where a plant releases certain chemicals that bind to the contaminant to make it less bioavailable and less mobile in the surrounding environment, and phytovolitization, a process through which plants extract pollutants from the soil and then convert them into a gas that can be safely released into the atmosphere. (Bentjen, 2002)
Phytoremediation is a naturally occurring process that was recognized and documented by humans more than 300 years ago. (Lasat, 2000) Since this time, humans have exploited certain plants’ abilities to survive in contaminated areas and to assist in the removal of contaminants from soil. However, true scientific study and development of these plants unique qualities was not conducted until the early 1980’s. (Lasat, 2000) At this time it was recognized that certain species of plants could accumulate high levels of heavy metals from the soil while continuing to grow and proliferate normally. (Lasat, 2000) Although research has been slow and tedious due to scientists’ incomplete understanding of the generalized cellular mechanisms of plants, the advent of new genetic technology has allowed scientists to determine the genetic basis for high rates of accumulation of toxic substances in plants. (Clemens et al., 2002) Using genetic engineering, scientists may soon be able to exploit this characteristic to provide a faster more efficient means of removing contaminants from the soil. Genetic engineering will also be crucial in the creation of transgenic plants that will be able combine the natural agronomic benefits associated with plants (ease of harvest and rapid, expansive growth) with the remediation capabilities of bacteria – a traditional organism used in bioremediation. (Moffat, 1995)
Phytoremediation of heavy metals from the environment serves as an excellent example of the process of plant-facilitated bioremediation and its role in removing environmental stress. Traditionally, when an area becomes contaminated with heavy metals the area must be excavated and the soil removed to a landfill site. (Lasat, 2000) This process is extremely expensive and therefore not entirely appealing in light of the recent discoveries regarding phytoremediation. (Lasat, 2000) Analysts estimate that the cost of cleaning one hectacre of highly contaminated land at a depth of one meter would range from $600,000 to $3,000,000 depending on the extent of the pollution and the toxicity of the pollutant. (Moffat, 1995) The cost of phytoremediation could be as much as 20 times less expensive, making this practice far less prohibitive than previous methods. (Lasat, 2000) The ideal type of phytoremediator is a species that creates a large biomass, grows quickly, has an extensive root system, and must be easily cultivated and harvested. (Clemens et al., 2002) The only problem with this criterion is that natural phytoremediators often lack these qualities. Therefore, scientists have been forced to become very creative in the development of effective transgenic phytoremediators.
Many human diseases result from the build up of toxic metal in soil, making remediation of these areas crucial in the protection of human health. Lead is one of the most difficult contaminants to remove from the soil, as well as one of the most dangerous. The presence of lead in the environment can have devastating effects on plant growth and can result in serious side effects – including seizures and mental retardation - if ingested by humans or animals. (Lasat, 2000) Both humans and livestock can be exposed to toxic levels of lead through inhalation of particulate matter in the air as well as direct ingestion of contaminated food, water, or dust. (Lasat, 2000) Much of the global lead contamination has occurred as a result of mining and iron smelting activities. (Huang et al., 1996) Phytoremediation of lead contaminated soil involves two of the aforementioned strategies – phytostabilization and phytoextraction. It is believed that a plant’s ability to phytoextract certain metals is a result of its dependence upon the absorption of many metals – such as zinc, manganese, nickel, and copper – to maintain natural function. (Lasat, 2000) Most plants only accumulate these essential elements and prevent all others from entering. However, some plants, termed “hyperaccumulators”, extract and store extremely high concentrations (in excess of 100 times greater than non-accumulator species) of metallic elements. (Lasat, 2000) Research has shown that these hyperaccumulators often do not exclude non-essential metals in the absorption process, thus resulting in plants that can extract high levels (1-2% of their biomass) of pollutants from contaminated soil. (Lasat, 2000) It is believed that plants initially developed this ability to hyperaccumulate non-essential metallic compounds as a means of protecting themselves from herbivorous predators, who would experience serious toxic side effects from ingestion of the hyperaccumulator’s foliage. (Pollard et al., 1997) Lead is one of the non-essential compounds hyperaccumulated by several species of plants including Thalaspi rotundifolium and Brassica juncea. The difficulty with these models, in terms of performing phytoremediation, is that they grow very slowly and have a very low biomass. (Huang et al., 1996) Therefore, the bioremediation process would be extremely slow because the rate of bioremediation is directly proportional to growth rate and the total amount of bioremediation is correlated with a plant’s total biomass. Because no plant yet discovered meets the ideal criteria of an effective phytoremediator (fast growing, deep and extensive roots, high biomass, easy to harvest, hyperaccumulators of a wide range of toxic metals), it is necessary to introduce the hyperaccumulating genes into non-accumulator to make the plants better suited as agents in the phytoremediation process. (Clemens et al., 2002) The authors of a lead absorption study, Huang and Cunningham, cite corn as a perfect phytoremediator due to its large biomass, fast rate of growth, and the existence of extensive genomic knowledge of this crop. The presence of a body of knowledge regarding corn genetics would be helpful in the effective insertion of DNA that codes for hyperaccumulation of the desired contaminant. Other potential model phytoremediators would include various varieties of transgenic trees. Trees are ideal in the remediation of heavy metals because they can withstand higher concentrations of pollutants due to their large biomass, they can accumulate large amounts of the contaminants in their systems because of their size, they can reach an huge area and great depths due to their extensive root systems, they can stabilize an area and prevent erosion, and hence the spread of the contaminant, because of their perennial presence. (Sykes et al., 1999) They can also be easily harvested and removed from the area with minimal risk, effectively taking with them a large quantity of the pollutants that were once present in the soil. (Sykes et al., 1999) The introduction of hyperaccumulating genes - as well as genetic information that would better prepare these species to deal with diverse climatic conditions, up-regulate this hyperaccumulation, or increase the organism’s growth rate and overall size - into these model agricultural species would undoubtedly accelerate the remediation process and result in faster removal of environmental stress from the ecosystem. (Sykes et al., 1999)
The insertion of genes from other plant species is just the beginning of the genetic modification taking place to improve the efficacy of phytoremediators. Mammalian genes, known as metallothioneins, and bacterial genes are also being introduced into plant species, resulting in the creation of novel classes of phytoremediators that have the ability to extract harmful heavy metals from contaminated soil. (Gleba et al., 1999) Because the transfer of these genes is occurring between unrelated species, scientists must first obtain extensive knowledge of the transgenic plant’s native DNA sequences and regulatory regions in order to insert the alien DNA at the appropriate site –one which would contain the appropriate initiation sequences. (Moffat, 1995) Researchers at the University of Georgia first used this technology for phytoremediation in 1996, when they inserted bacterial genes –which coded for an enzyme that converts mercury into an inert form - into Arabidopsis. (Rugh et al., 1996) This experiment remains unique not only because it used bacterial genes to create a transgenic plant species, but also because it incorporates the phytoremediation techniques of phytoextraction, phytodegradation and phytovolitization. (Gleba et al., 1996)
Mercury is present in many areas as a result of residual from paper mills, textile plants, gold mining and the chemical industry. (Raskin, 1996) Mercury is a highly reactive element that adopts a liquid form at room temperature. (Raskin, 1996) Because of this characteristic, mercury can easily be converted into a vapor. (Raskin, 1996) This very reactivity, however, prompts mercury to form diatomic molecules. (Raskin, 1996) Unfortunately, in this diatomic form mercury is highly toxic and can cause significant neurological dysfunction when encountered by humans. (Rugh et al., 1996) Diatomic mercury is also dangerous because it has the ability to biomagnify as it ascends the food chain. (Bizily et al., 1999) Therefore, it is necessary to reduce the mercury compound to its elemental form (which is much less toxic), so that it can be volatilized and released into the air. (Raskin, 1996) Because plants do not naturally perform this activity, scientists identified a gene in E. coli that coded for an enzyme responsible for the conversion of mercury in such a manner and inserted a modified form of this bacterial gene (merA9pe) into Arapidopsis thaliana. (Rugh et al., 1996) After extensive tinkering with the promoter sequences that initiated expression of this gene, a transgenic plant was created that had the ability to extract mercury from the soil through hyperaccumulation, reduce diatomic mercury to it elemental form (due to the action of the bacterial gene merA9pe), and then release the less toxic mercury vapor into the air (through natural mechanisms of respiration). (Rugh et al., 1996) While this seems promising, the system is far from perfect. Arabidopsis is an excellent species for research but serves as a poor species for actual remediation due to is small biomass. (Raskin, 1996) Therefore, the merA9pe gene would have to be inserted into another species of plant for efficient phytoremediation to take place. This poses a whole new host of problems because the DNA sequences and regulatory regions of other plant species have not been studied as extensively, making transformation of these species difficult. The final difficulty with this transgenic phytoremediator of mercury is that most environmental regulatory agencies do not accept the release of mercury in a volatilized form as a safe method of bioremediation. (Raskin, 1996) While gaseous elemental mercury is undoubtedly less toxic than the diatomic form, it still poses a risk to the environment. (Raskin, 1996) It has also been noted that the general public might seriously object to fields of transgenic plants that release large clouds of mercury gas. (Moffat, 1995) Such public apprehension is often powerful enough to end, or at least delay the progress of controversial, cutting-edge research and brings into question the feasibility of ever proving that phytovolitization serves as a safe method of phytoremediation.
The most practical examples of phytoremediation to date deal with the extraction of TNT and arsenic from contaminated soil. Arsenic poisoning is a very serious problem globally, but is most deadly in certain areas of India and Bangladesh where more than 112 million people have fallen victim to various levels of arsenic poisoning. (Carlyle, 2002) High levels of arsenic in the water supply are the result of practices used during the Green Revolution of the 1960’s, such as large-scale irrigation and field flooding that have brought arsenic out of the soil and allowed it to enter the water supply. (Dhanker et al., 2002) Arsenic build-up is also a result of byproducts that escape from chemical manufacturing sites. (Dhanker et al., 2002) Scientists, again from the University of Georgia, have developed a transgenic plant to combat this serious problem using the typical research model organism, Arabidopsis. By inserting bacterium genes into this plant, the researchers were able to make it much more tolerant to the presence of arsenic and thus increase its ability to extract arsenic from the soil. (Carlyle, 2002) Because Arabidopsis is not an effective bioremediator, these researchers are now trying to extend this technology to cottonwood trees. (Carlyle, 2002) Cottonwood trees could be extremely effective remediators due to their extensive root systems and high biomass. Once the trees accumulated sufficient quantities of arsenic they could be harvested and removed from the site, effectively removing large quantities of arsenic from affected areas. The permanent presence of trees in these areas would also be effective in preventing the leaching of arsenic from the soil into water sources, the main medium through which humans become exposed.
2,4,6-trinitrotoluene (TNT) is prevalent in a large proportion of the soil surrounding military firing ranges throughout the world. The presence of TNT is hazardous because it presents a risk of detonation, is extremely toxic to humans, and may prevent the growth of vegetation in affected areas, upsetting the balance of the ecosystem. (Boyd et al., 2002) Present remediation strategies include burning, detonation and burial of the toxic material. (Boyd et al., 2002) These methods are not optimal because they create a large amount of useless ash and present serious problems for the atmosphere due to associated release of other toxic chemicals. (Boyd et al., 2002) In order to combat these problems scientists have again turned to bacterial genes. The bacteria studied seem to be extremely effective in the remediation of explosives. However, they lack the large biomass and ability to withstand adverse environmental conditions – two qualities necessary for effective bioremediation. (Boyd et al., 2002) Therefore, to obtain the best characteristics of both plants and bacteria, and thus the best bioremediators, scientists have created transgenic plants that contain the bacterial genes that have proven effective in the degradation of explosives. (Boyd et al., 2002) Transgenic tobacco plants that have both the ability to withstand high concentrations of TNT as well as degrade this contaminant have been created using this strategy. (Strand, 2002) These plants are currently being field-tested. Meanwhile, researchers are attempting to introduce these genes to other plant models – such as trees – that would be more efficient phytoremediators. Similar techniques are currently being employed to create aquatic and land grasses that would be able to clean-up petroleum spills and other man-made disasters, a source of considerable stress to the environment today. (Shaw, 2000)
The previous discussion illuminates many of the advantages and disadvantages of transgenic phytoremediation. The primary advantages of using plants in bioremediation are that phytoremediation is more cost-effective, more environmentally friendly, and more aesthetically pleasing (United States Environmental Protection Agency, 2001) than conventional methods, which are usually expensive and environmentally disruptive. (Bizily et al., 1999) Plants also offer a permanent, in situ, nonintrusive, self-sustaining method of soil contaminant removal. (Strand, 2002) It is also important to note that accumulated contaminants can be removed much more easily through the harvest of plants than from the soil itself. Phytoextraction will enable scientists to reclaim and recycle useable materials, including a wide variety of precious metals, from the soil. (Moffat, 1995) The potential economic benefits of this practice are extremely high and extremely attractive to scientists and businessmen alike. (Moffat, 1995) Phytoextraction is also energetically favorable because only solar energy must be present to maintain the system. (Bizily et al., 1999) Finally, the greatest advantage of this technology is that it utilizes the inherent agronomic benefits of plants. (Abdulla, 2002) These benefits include high biomass, extensive root systems that both stabilize the ecosystem to prevent contaminant spread through leaching as well as reach a large volume of contaminated soil, and a greater ability to withstand adverse environmental conditions and interspecies competition than bacteria. (Abdulla, 2002)
As extensive as these benefits are, the possible costs of using plants for bioremediation should not be ignored. Some of the concerns that have been voiced in response to phytoremediation include its slow speed in comparison to mechanical methods such as soil excavation, the climatic restrictions that may limit growing many species of plants, and the unknown long-term environmental costs. (Sykes, 1999) Potential danger might exist for animals that live in the areas in which phytoremediators are grown, especially if these animals typically feed on this species of plant being used for phytoremediation. (Moffat, 1995) Concerns have also been raised regarding the potential for contaminants to move up the food chain more quickly; a problem that could occur if toxic materials are sequestered in consumable sources, such as plants. (Kirchner, 2001) Finally, issues with the disposal of these toxic materials still remain. Once contaminants have been extracted from the soil by the plants we are still faced with the dilemma of what to do with these contaminants. It seems that the end result remains the same - especially in light of strong opposition to phytovolitization – and would involve the removal of contaminants to a landfill location where the plants would eventually biodegrade and the contaminants could enter the soil once again. (Kirchner, 2001) The largest barrier to the advancement of phytoremediation, however, may be public opposition to genetic modification in general. Because all natural hyperaccumulator species are small in size, genetic modification must be used to introduce this technology to other species or to increase the biomass of the natural hyperaccumulators in order to create effective phytoremediators. This public opposition stems from the same fears that surround the issue of genetic modification of crops, and includes concerns regarding decreased biodiversity, the entry of potentially harmful genes into products consumed by humans, and the slippery slope created by introducing and transferring novel, foreign DNA between non-related species.
I would argue that the benefits of using phytoremediation to restore balance to a stressed environment seem to far outweigh the costs. However, as with all new technology, it is important to proceed with caution. Because genetic modification has entered the quest to remove contaminants from the ecosystem relatively recently, it is necessary to conduct longitudinal studies to determine the true costs and benefits of this technology to the ecosystem as a whole, before such technology is applied on a larger scale.
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