“The contamination of the environment with toxic metals has become a worldwide problem, affecting crop yields, soil biomass and fertility, [and] contributing to bioaccumulation in the food chain,” write Gratao, Prasad, Cardoso, Lea, and Azevedo (2005, p. 53). Heavy metals strongly bind to oxygen, nitrogen and sulphur atoms, which in turn can inactivate enzymes (Schutzendubel & Polle, 2002, p.1353). LeDuc and Terry report that more than 50,000 sites in the United States are contaminated with acutely toxic metals and await remediation (2005, p.514). “Currently, $6-8 billion per year is spent for environmental cleanup in the United States,” reports Pilon-Smits, “and $25-50 billion per year worldwide” (2005, p.17). Of this amount, approximately 35% involves metals remediation (Pilon-Smits & Pilon, 2002, p.440). Given the vast queue of sites awaiting remediation, write LeDuc and Terry, the problem demands “low-cost, effective, and sustainable methods to remove [metal contaminants] from the environment or detoxify them” (2005, p.514).
Conventional Soil Remediation Technologies:
The technologies widely used to remediate polluted sites include pneumatic fracturing, soil flushing, solidification/stabilization, vitrification, chemical reduction/oxidation, soil excavation and removal to secured landfills (Gratao, Prasad, Cardoso, Lea, & Azevedo, 2005, p.54), soil washing and reburial, and pump and treat systems (Pilon-Smits & Pilon, 2002, p.440). These methods are cost prohibitive, assert Gratao, Prasad, Cardoso, Lea, and Azevedo, and often generate secondary waste (2005, p.54). Schmidt suggests that while conventional methods are effective for small, highly contaminated sites, they are not applicable to large areas because they require high energy input and expensive machinery (2003, p.1939). Conventional methods, Schmidt continues, also destroy soil structure and decrease soil productivity (p.1939).
Phytoremediation Emerges:
An emerging alternative, reports Pilon-Smits, is phytoremediation, a collection of technologies and techniques that make “use of the naturally occurring processes by which plants and their microbial rhizosphere flora degrade and sequester organic and inorganic pollutants” (2005, p. 16). Phytoremediation does not traffic exclusively in heavy metal contamination, report Gratao, Prasad, Cardoso, Lea, and Azevedo, but is also used to clean up “pesticides and xenobiotics, organic compounds, toxic aromatic pollutants and acid mine drainage” (2005, p.54), but exciting work with phytoremediation in heavy metal cleanup has made this problem emblematic of phytoremediation’s potential.
Phytoremediation is lauded both for its cost-effectiveness and its appeal with the general public. “Because biological processes are ultimately solar-driven,” reports Pilon-Smits, “phytoremediation is on average tenfold cheaper than engineering-based remediation methods such as soil excavation, soil washing or burning, or pump-and-treat systems” (2005, p.17). While admitting that “the science of understanding wetland detoxification mechanisms is in its infancy,” Hansen, Duda, Zayed, and Terry write that constructed wetlands, one application of phytoremediation, are “orders of magnitude lower in cost than other treatment systems” (1998, p.591). And phytoremediation is popular with the public, report Pilon-Smits and Pilon, because plants are aesthetically pleasing (2002, p.440). Phytoremediation is increasingly becoming integrated with landscape architecture, writes Pilon-Smits, as urban remediation sites are now often designed to be safely used by the public as a park or nature area during and after the remediation process (2005, p.30).
Phytotechnology Methods: A Collection of Approaches
Phytoremediation is versatile, proving effective at cleaning up solid, liquid, and gaseous substrates reports Pilon-Smits, but it requires different strategies situationally, particularly in terms of remediating organic pollutants versus inorganic. “Inorganics cannot be degraded,” reports Pilon-Smits, “but they can be phytoremediated via stabilization or sequestration in harvestable plant tissues” (2005, p.16). When inorganic pollutants accumulate in plant tissues they often prove toxic, even lethal, to the plant, writes Pilon-Smits, both directly, by damaging cell structure, and indirectly, by taking the place of essential nutrients (p.24).
Organic pollutants, in contrast, are typically manmade and foreign to a plant’s system, continues Pilon-Smits; thus a plant will lack the appropriate transporters for these organic substances, and instead move them through its body by diffusion (2005, p.24). Organic pollutants tend to be less toxic to plants because they tend not to accumulate and are less reactive (Pilon-Smits, p.24). The phytoremediation of organic pollutants “has been summarized as the ‘green liver model’ because of its similarity to mammalian detoxification mechanisms” (Pilon-Smits, 2005, p.28).
Thus phytoremediation, necessarily, comprises a collection of approaches, but all make “use of the naturally occurring processes by which plants and their microbial rhizosphere flora degrade and sequester organic and inorganic pollutants” (Pilon-Smits, 2005, p. 16). “Different phytotechnologies make use of different plant properties and typically different plant species are used for each,” writes Pilon-Smits (2005, p.19). Phytoremediation methods include: natural attenuation, phytodegradation, phytoextraction, phytostabilization, and phytovolatilization.
Natural attenuation is phytoremediation that happens in lieu of more deliberate management. It is the remediation that occurs when a contaminated site goes otherwise untreated. “Because the processes involved in phytoremediation occur naturally, vegetated polluted sites have a tendency to clean themselves up without human interference,” writes Pilon-Smits. “This so-called natural attenuation is the simplest form of phytoremediation and involves only monitoring” (2005, p.19). Natural attenuation is comforting because it is a kind of cleanup that happens by default, but as a deliberate approach it does not maximize time and space in the way that a more closely managed approach would. Pilon-Smits writes, “Natural attenuation is suitable for remote areas with little human use and relatively low levels of contamination” (p.19). Thus, it is most appropriate when the given problem lacks any sense of urgency.
Phytodegradation describes how plants are used to degrade organic pollutants via their enzymatic activities (Pilon-Smits, 2005, p.19). “In phytodegradation,” Pilon-Smits writes, “plant enzymes act on organic pollutants and catabolize them, either mineralizing them completely to inorganic compounds (e.g., carbon dioxide, water and Cl2), or degrading them partially to a stable intermediate that is stored in the plant” (p.28). To this end, a plant species will generally be more effective if it has “large, dense root systems and high levels of degrading enzymes” (Pilon-Smits, p.19-20).
One type of phytodegradation is called phytostimulation or alternately rhizodegradation, whereby organic pollutants are degraded by the microbes plants host in their rhizosphere (Pilon-Smits, 2005, p.19). Rhizosphere remediation occurs without the pollutant near the roots entering the plant (Pilon-Smits, p.22). “Plants release a variety of photosynthesis-derived organic compounds in the rhizosphere that can serve as carbon sources for heterotrophic fungi and bacteria” writes Pilon-Smits (p.22). Plants, in essence, feed tiny organisms near them and, by absorbing these organisms’ byproducts, often can experience synergistic returns. For example, rhizosphere microbes “can promote plant health by stimulating root growth (some microorganisms produce plant growth regulators), enhancing water and mineral uptake, and inhibiting growth of other, [Nitric oxide] pathogenic soil microbes” (Pilon-Smits, p.22). In addition, “Fungal symbiotic associations have the potential to enhance root absorption area, and stimulate the acquisition of plant nutrients including metal ions,” writes Lasat. These “fungal associations were shown to enhance root absorption area up to 47-fold” (2002, p.110). A broad root structure promotes microbial growth and the production of compounds that can promote plant-microbe interactions (Pilon-Smits, p.20). “In rhizosphere remediation,” Pilon-Smits observes, “it is often difficult to distinguish to what extent effects are due to the plant or to the rhizosphere microbes” (p.22).
Bioaugmentation actively cultivates and encourages these rhizospheric remediation effects, either through selection of favorable combinations of vegetation, which together will create a pleasant neighborhood for microbial growth, or by growing large amounts of the desired microbial flora in the lab and introducing them to the soil at the phytoremediation site (Pilon-Smits, 2005, p.23). Important to note is that research has shown that adding native microbes is more effective than adding nonnatives, because established microbial populations outcompete nonnatives (Pilon-Smits, p.23-24).
Inorganic pollutants must be treated with other approaches, including phytoextraction and phytostabilization. With phytoextraction, report Gratao, Prasad, Cardoso, Lea, and Azevedo, plants accumulate contaminants in their tissues, which are then harvested (2005, p.55). A sub-type of phytoextraction, called phytomining, involves harvesting the plant parts that absorb the contaminant, processing out the contaminant and then reusing it (p.55). This approach is used most often in the case of valuable metals. Phytoextraction is sometimes aided with soil additives that make contaminants more bioavailable or accessible for extraction by the plant. A plant cannot remediate contaminants that are inaccessible to it, either due to distance or because the contaminant is chemically bound to the soil. Thus, metal chelators such as siderophores, organic acids, and phenolics are sometimes used to release metals from soil particles (Pilon-Smits, 2005, p.23). However, loosening the bonds these contaminants have to the soil can be highly problematic if would-be remediator plants are unable to take up these toxins before they leach into ground water (Lombi, Zhao, Dunham, & McGrath, 2001, p.1925).
In contrast to extraction-based methods, phytostabilization aims to localize pollutants in a soil area, reports Pilon-Smits, to keep them from diffusing and causing problems along a wider area (2005, p.19). “Phytostabilization of metals,” Pilon-Smits and Pilon write, “may simply involve the prevention of leaching through the upward water flow created by plant transpiration, reduced runoff due to above-ground vegetation, and reduced soil erosion via stabilization of soil by plant roots” (2002, p.441). In this way, trees can be used as a hydraulic barrier, whereby the strong force of their transpiration creates sufficient upward water flow to prevent soil contaminants from leaching down or fanning out horizontally (Pilon-Smits, p.18). Fast-transpiring trees such as poplar are popular for phytostabilization because they maintain a strong upward flow of water, which prevents downward leaching (Pilon-Smits, p.20). Similarly, the dense root systems of grasses are used to prevent wind erosion and lateral runoff (Pilon-Smits, p.20).
With phytovolatilization, plants take contaminants into their tissues, and then release them into the air as a volatile gas (Pilon-Smits, p.19 and 29). This is an attractive phytotechnology because it removes pollutants without the need for harvest and disposal (Pilon-Smits, p.29). But, as Pilon-Smits notes, it is important to track the fate of this gas in the atmosphere to be sure remediation is not simply exporting the problem (p.29).
Many of the above strategies may work together on a restoration project simultaneously. Constructed wetlands are a great example because, depending on the design, a constructed wetland may use any and all of the above strategies to remove contaminants. Constructed wetlands “comprise a complex ecosystem of plants, microbes, and sediment that together act as a biogeochemical filter, efficiently removing dilute contaminants from very large volumes of wastewater” write LeDuc and Terry (2005, p.515). Phytoremediation plans of constructed wetlands routinely call for harvesting plant parts to remove sequestered contaminants from a site, but may simultaneously expect the species present in the wetland to volatize contaminants or stabilize them in the soil below (Hansen, Duda, Zayed, & Terry, 1998, p.595), thereby transforming contaminants into immobile or less toxic forms” (LeDuc and Terry, p.515). Constructed wetlands are a promising model because they are relatively cheap to create and maintain, and they filter a wide-range of contaminants (LeDuc and Terry, p.515).
Natural Hyperaccumulators and Other Model Organisms:
Lasat reports that plants perform the work of phytoremediation with mechanisms designed to prepare for potential stress, including mechanisms that tolerate metals (2002, p.113). Schutzendubel and Polle write, “Since plants are sessile organisms and have only limited mechanisms for stress avoidance, they need flexible means for acclimation to changing environmental conditions.” (2002, p.1352). But when it comes to phytoremediation, not all plant species are created equally, and thoughtful species selection is crucial to a remediation project’s success.
Much research to date has focused on so-called “hyperaccumulator” species that can “accumulate and tolerate greater metal concentrations in shoots than those usually found in non-accumulators, without visible symptoms” (Gratao, Prasad, Cardoso, Lea, & Azevedo, 2005, p.58). Commonly, a species is considered a metal hyperaccumulator if it can accumulate approximately 100-fold higher metal levels than non-accumulator species (Pilon-Smits & Pilon, 2002, p.446). There are somewhere between 400 (Gratao, Prasad, Cardoso, Lea, & Azevedo, p.58) and 500 (Pilon-Smits & Pilon, p.446) reported species of hyperaccumulators. These include at least 45 plant families (Lasat, 2002, p.112), including members of the Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae, and Euphobiaceae families (Gratao, Prasad, Cardoso, Lea, & Azevedo, p.58). Metal hyperaccumulation in these families probably evolved independently, Pilon-Smits and Pilon assert, perhaps as protection against herbivory and pathogens (p.446). Lasat reports that most hyperaccumulating species “accumulate Ni, about 30 accumulate either Co, Cu, and Zn, even fewer accumulate Mn and Cd, and there are no known Pb hyperaccumulators” (p.112). Of these families, Brassicaceae are particularly interesting to researchers because several species in this family have demonstrated the ability to hyperaccumulate more than one metal (Gratao, Prasad, Cardoso, Lea, & Azevedo, p.58).
While phytoremediation is demonstrably cheaper than conventional methods, Gratao, Prasad, Cardoso, Lea, and Azevedo note that it is “not an easy technology that consists of simply planting and growing some hyperaccumulating plants in the metal polluted area” (2005, p.55). Instead, they note, it is “a highly technical strategy, requiring expert project designers with field experience that choose the proper species and cultivars for particular metals and regions” (p.55). Pilon-Smits writes:
Favorable plant properties for phytoremediation in general are to be fast growing, high biomass, competitive, hardy, and tolerant to pollution. In addition, high levels of plant uptake, translocation, and accumulation in harvestable tissues are important properties for phytoextraction of inorganics (2005, p.19).
It goes without saying that plants must be where the pollutant is, writes Pilon-Smits, and be able to act on it, “Therefore, the soil properties, toxicity level, and climate should allow plant growth” (p.17). Incorporating species already growing near a contaminated site ensures local competitiveness and pollutant tolerance (Pilon-Smits, p.21).
But complicating advances using natural hyperaccumulators, Gratao, Prasad, Cardoso, Lea, and Azevedo report that research with these species may have hit a wall. Hyperaccumulators typically have a slow rate of growth, have limited biomass and are limited to their natural habitats (2005, p.55). Additionally, most hyperaccumulator species are metal selective—meaning mixed metal soils can be lethal before the plants can complete any substantive remediation work (Schmidt, 2003, p.1950). A key challenge in phytoremediation today is finding species that can survive sites with mixed contaminants/multiple heavy metals. “[I]t must be considered that metals rarely occur alone in the environment,” write Gratao, Prasad, Cardoso, Lea, and Azevedo, “and an adaptive tolerance may be essential for several metals simultaneously” (p.56). Despite researchers need for versatile, tolerant plant stock, Lombi, Zhao, Dunham, and McGrath found that natural hyperaccumulators have considerable specificity in metal hyperaccumulation (2001, p.1919). “Moreover,” Gratao, Prasad, Cardoso, Lea, and Azevedo write, “the application of hyperaccumulator plants can be further limited because little is known about their agronomic characteristics, pest management, breeding potential and physiology, growing often in remote regions and in certain cases, their habitat is threatened by mining, development and others activities” (p.55).
Genetic Engineering Advances Phytoremediation:
Many lines of phytoremediation research are turning to genetic engineering to surmount these challenges. Lasat reports that, “ Brown et al. (1995) proposed the transfer of the hyperaccumlator phenotype from small and slow growing hyperaccumulator species to fast growing, high biomass-producing nonaccumulator plants” (2002, p.115). To this end, Lasat continues, “[B]iotechnology offers the opportunity for direct gene transfer, thus circumventing limitations imposed by sexual incompatibility” (p.115). Direct gene transfer is accomplished by inserting a foreign piece of DNA from any organism, from bacteria to mammals, into the plant cell genome. After propagation, the transgenic plant matures and the foreign gene is inherited by its offspring (Pilon-Smits & Pilon, 2002, p.441-442). The foreign DNA can be inserted either with a particle gun or via a soil bacterium called Agrobacterium (Pilon-Smits & Pilon, p.442). “The diversity and adaptability of microorganisms allows them to thrive in harsh, toxic environments where higher plants are unable to grow,” write LeDuc and Terry. “As such,” they continue, “microbes represent a potential reservoir of important genes involved in metal detoxification. Highly efficient phytoremediating plants could be generated that overexpress microbial genes” (2005, p.517). This work is exciting, Pilon-Smits and Pilon report, because it typically increases metal accumulation potential by 2- to 3-fold (p.451).
Even more encouraging, Lasat reports that studies indicate metal tolerance is regulated by just a few major genes (2002, p.115), perhaps as little as 1 to 3 (Pilon-Smits & Pilon, 2002, p.447). Furthermore, metal accumulation, tolerance, and plant productivity are not necessarily linked, and thus could be manipulated separately. It should be possible then to breed or genetically engineer a plant with high metal tolerance and metal accumulation as well as high productivity (Pilon-Smits & Pilon, p.447).
Risks and Limitations of Phytoremediation:
However, genetic engineering also has its risks and limitations. Gratao, Prasad, Cardoso, Lea, and Azevedo note that “phytoremediation technology is still in its early development stages and full scale applications are still limited” (2005, p.61). And while phytoremediation works, the often complex biological reasons why it works remain to be discovered (Pilon-Smits, 2005, p.21), making the selection of traits with genetic engineering a challenge. “Phytoremediation efficiency is still limited by a lack of knowledge of many basic plant processes and plant-microbe interactions,” writes Pilon-Smits. “There is also a need for more phytoremediation field studies to demonstrate the effectiveness of the technology and increase its acceptance” (p.30).
But limiting the use of field trials is public unease about transgenic plant genes spreading into the gene pool of wild relatives (Gratao, Prasad, Cardoso, Lea, & Azevedo, 2005, p.60) and thriving due to their metal tolerance or general weedy nature (Pilon-Smits & Pilon, 2002, p.450). Another concern is that sequestration of contaminants in plants could put wildlife and humans at higher risk for exposure (Gratao, Prasad, Cardoso, Lea, & Azevedo, p.60) due to accumulation in edible plant parts or volatilization dispersal (Pilon-Smits & Pilon, p.450). Further, the safe disposal of sequestered contaminants presents challenges (Gratao, Prasad, Cardoso, Lea, & Azevedo, p.60). “More research is needed to improve combustion techniques of contaminated biofuels,” asserts Schmidt, “and the production of marketable products from contaminated biomass for industrial use (mainly oils and fibers) should be tested,” but to date the challenges of properly redirecting waste products remain very real (2003, p.1952). Soberingly, Gratao, Prasad, Cardoso, Lea, and Azevedo recommend that genetic engineering of phytoremediators be studied further “to determine the true costs and benefits of this technology to the ecosystem as a whole, before it is to be applied to a larger scale” (p.61). More generally, phytoremediation, both of the conventional and genetically engineered varieties, is severely limited by the bioavailability of contaminants at a site. Put simply, writes Pilon-Smits, “For plants and their associated microbes to remediate pollutants, they must be in contact with them and able to act on them” (2005, p.21). “If the contamination runs too deep, or the concentration of toxic compounds is too high,” Gratao, Prasad, Cardoso, Lea, and Azevedo add, “then plants alone cannot efficiently remediate the soil” (2005, p.54). This limitation due to bioavailability can potentially open a gap between what phytoremediation can reasonably accomplish and what policy-makers and the public expect. “If only a fraction of the pollutant is bioavailable, but the regulatory cleanup standards require that all of the pollutant is removed,” writes Pilon-Smits, “phytoremediation is not applicable by itself” (p.17). This is particularly challenging at older sites, where the pollutants in aged soil tend to be more recalcitrant and difficult to phytoremediate (Pilon-Smits, p.22). As a compromise, Pilon-Smits suggests that remediation targets be pegged specifically to a site’s cache of bioavailable contaminants—those usually regarded as the most dangerous band of contaminants expressly because they are more readily accessible to living organisms—rather than total level of contaminants, many of which are stable in the soil and thus pose a lesser threat, that way phytoremediation may enjoy more widespread use (Pilon-Smits, p.31). Another matter of concern is the considerable length of time phytoremediation can take to reach cleanup targets. “With the plant materials currently available, years or decades are needed to clean up a contaminated site” (Lombi, Zhao, Dunham, & McGrath, 2001, p.1919), limiting its applicability (Pilon-Smits, 2005, p.17). Long cleanup times may increase opportunities for collateral wildlife and human exposure to contaminants, may present continuity problems in monitoring and maintenance, and may simply exceed policy-makers and the public’s patience for results. Future of Phytoremediation: Unease from the general public about the use of transgenic plants in the field and the regulatory barriers that have sprung from that sentiment, LeDuc and Terry report, “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” (2005, p.518). One promising result is a method called chloroplast transformation, which prevents the escape of transgenes via pollen to related weeds and crops (LeDuc and Terry, p.518). Genetically engineering chloroplast genomes of higher plants, write Ruiz, Hussein, Terry, and Daniell, “offers several advantages over nuclear transformation” (2003, p.1345). Advantages include: very high levels of transgene expression (up to 46% [weight for weight]) of total protein; uniparental plastid gene inheritance (in most crop plants) that prevents pollen transmission of foreign DNA, the absence of gene silencing and positioning effect, the ability to express multiple genes in a single transformation event, the ability to express bacterial genes without codon optimization, integration via a homologous recombination process that facilitates targeted transgene integration, and sequestration of foreign proteins in the organelle, which prevents adverse interactions with the cytoplasmic environment (Ruiz, Hussein, Terry, & Daniell, p.1345). Besides early indications that chloroplast engineering is highly effective at improving phytoremediation processes, perhaps its greatest advantage is that it helps quell fears that transgenic plants will interbreed with wild relatives. Hopes are high regarding the potential of chloroplast engineering in phytoremediation research, in part because similar work has demonstratively conferred “insect resistance, herbicide resistance, disease resistance, drought tolerance, and expression of edible vaccines, monoclonals, and biopharmaceuticals” (Ruiz, Hussein, Terry, & Daniell, p.1345). And yet, regardless of the course that new phytoremediation research takes, for phytoremediation to have any meaningful effect on the world’s widespread soil contamination problems, undeniably, researchers must continue to find ways to safely move their trials out of the lab and into field use. References Gratao, P. L., Prasad, M. N. V., Cardoso, P. F., Lea, P. J., & Azevedo, R. A. A. (2005). 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Pilon-Smits, E. (2005). Phytoremediation. Annual Review of Plant Biology, 56, 15–39. Ruiz, O. N., Hussein, H. S., Terry, N., & Daniell, H. (2003). Phytoremediation of Organomercurial Compounds via Chloroplast Genetic Engineering. Plant Physiology, 132, 1344–1352. Schmidt, U. (2003). Enhancing Phytoextraction: The Effect of Chemical Soil Manipulation on Mobility, Plant Accumulation, and Leaching of Heavy Metals. Journal of Environmental Quality, 32, 1939–1954. Schutzendubel, A., & Polle, A. (2002). Plant responses to abiotic stresses: heavy metal-induced oxidative stress and protection by mycorrhization. Journal of Experimental Botany, 53(372), Antioxidants and Reactive Oxygen Species in Plants Special Issue, 1351-1365.
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