Bernard R. Glick
Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L3G1
This summary focuses on phytoremediation and organic pollutants. See also – Rhizosphere and Phytoremediation – heavy metals.
To avoid the toxicity associated with hazardous chemicals that are present in the environment, researchers have developed strategies that employ plants to degrade, remove or stabilize a range of different compounds from polluted soils (i.e. phytoremediation ). These environmental pollutants may include metals such as lead, zinc, cadmium, selenium, chromium, cobalt, copper, nickel and mercury; inorganic compounds such as arsenic, sodium, nitrate, ammonia and phosphate; radioactive compounds like uranium, cesium and strontium; or organic compounds including chlorinated solvents like trichloroethylene (TCE), explosives such as trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-hexahydrotriazine (RDX), petroleum hydrocarbons such as benzene, toluene and xylene (BTX), polycyclic aromatic hydrocarbons (PAHs), and pesticides such as atrazine and bentazon.
While some organic compounds can be metabolized (remediated) by soil bacteria, in the absence of plants, this process is often slow and inefficient. This notwithstanding, the field of bacterial bioremediation has been expanding. Contaminant-degrading bacteria have been isolated from a wide range of impacted soils. In addition, it has been suggested that these contaminant-degrading bacteria may be found in virtually all soils. Following isolation and characterization of contaminant-degrading bacteria, attempts have been made to inoculate contaminated field soils with the isolates; however as indicated above, this strategy has generally proven to be unsuccessful. This lack of success may be attributed to:
i) the inability of introduced bacterial isolates to compete with existing microflora and microfauna in the soil;
ii) the inability of the bacteria to reach sub-surface contaminants;
iii) the lack of sufficient nutrients in contaminated soils to support bacterial growth;
iv) the low bioavailability of many contaminants;
v) the preferential utilization by the degradative bacteria of carbon compounds other than the contaminant of interest; and
vi) the presence of other toxicants within the soil that may inhibit bacterial growth.
However, in the area around plant roots (the rhizosphere), some organic soil contaminants can be completely degraded and mineralized by plant enzymes through the process of phytodegradation. This process occurs because many plants produce, and secrete to the environment, enzymes that can degrade a wide range of organic compounds. Phytoremediation of organic compounds may occur by phytostabilization (stabilizing pollutants in the soil to make them less bioavailable and therefore less hazardous); phytostimulation (the stimulation of microbial biodegradation in the rhizosphere, sometimes called rhizodegradation); or by phytotransformation, the absorption and degradation of organic contaminants by the plant.
The biodegradation of recalcitrant organic compounds in the soil is often enhanced around the roots of plants. This is a direct consequence of the high level of nutrients (including sugars, amino acids and organic acids) that most plants release (exude) into the soil as root exudates, nutrients that typically support a bacterial concentration in the rhizosphere that is often 100- to 1000-fold greater than the bacterial concentration in the bulk soil. Some rhizosphere bacteria are directly involved in the degradation of the organic soil contaminants while others (plant growth-promoting bacteria) can positively affect plant growth and health, enhancing root development or increasing plant tolerance to various environmental stresses. As a direct consequence of their interaction with plant growth-promoting bacteria, plants grow larger and healthier, and are better able to phytoremediate a range of organic soil contaminants.
Unfortunately, inorganic environmental pollutants cannot readily be degraded. They must either be stabilized in the soil to make them less bioavailable and thereby reduce their spread in the environment; extracted, transported, accumulated and concentrated from the soil into plant roots and/or shoots (phytoextraction); removed from liquid effluents via the use of plant roots (rhizofiltration); or transformed into volatile forms (phytovolatilization). Following phytoextraction, plants may be harvested, dried and converted to ash to recover the concentrated metal. A serious impediment to more effective phytoextraction of metals is the tight binding of metals to soil particles so that often only a small fraction of the metal that is present in the soil can be mobilized and taken up by plant roots.
As a result of the testing of numerous plants, several that are naturally able to accumulate large amounts of metal per unit of plant biomass have been identified and are being studied for possible use in the phytoremediation of metallic contaminants. These plants are called hyperaccumulators and are often found growing in soils with elevated metal concentrations. A practical limitation of using hyperaccumulators is that many of the plants that are most effective at removing metals from the soil, such as Thlaspi caerulescens (Alpine pennycress) and Alyssum bertolonii, are small, containing only a low level of biomass, and they are slow growing, thus reducing their potential for metal phytoextraction from soil (on a large scale) in the field. Moreover, the growth of metal-resistant metal-accumulating plants that are capable of hyperaccumulating metals can be severely inhibited when the concentration of available metal in the contaminated soil is very high. This results in a decrease in plant biomass and, thereby, in the efficiency of phytoremediation.
To be effective for the remediation of metal polluted soils, plants must be tolerant to one or more metals, highly competitive, fast growing, and produce a high aboveground biomass. Because of their high biomass and extensive root system, some species of trees (e.g. poplar) have been considered to be attractive for phytoremediation; however, metal accumulation by trees is generally low.
Finally, a convergence of phytoremediation and bacterial bioremediation strategies has led to a more successful approach to remediation of contaminants, particularly organic compounds. Bacteria-assisted phytoremediation, both with bacteria already present in the soil and with bacteria deliberately introduced by seed inoculation, has been investigated in a number of laboratory, greenhouse and field studies. In this regard, phytoremediation is most effective when the introduced bacteria can both degrade the soil contaminant(s) and promote the growth of plants.
Given the above mentioned considerations, it is currently possible to develop phytoremediation strategies to clean up a large number of the sites contaminated with organic compounds (this process may require several field seasons depending on the particular plant, soil, bacteria, contaminants and climate involved). On the other hand, phytoremediation is not yet a practical approach for the removal of inorganic compounds from contaminated soil environments.
References and Additional Readings
Applied Bioremediation and Phytoremediation. 2004. A. Singh and O.P. Ward, Eds., Springer, Berlin, 281 pages.
Barac, T., Taghavi, S., Borremans, B., Provoost, A., Oeyen, L., Colpaert, J.V., Vangronsveld, J. and van der Lelie, D. 2004. Engineered endophytic bacteria improve phytoremediation of water-soluble, volatile, organic pollutants. Nat. Biotechnol. 22: 583-8.
Gamalero, E., Berta, G. and Glick, B.R. 2009. Effects of plant growth promoting bacteria and AM fungi on the response of plants to heavy metal stress. Can. J. Microbiol. 55: 501-514.
Gamalero, E. and Glick, B.R. 2010. Bacterial ACC deaminase and IAA: interactions and consequences for plant growth in polluted environments. In: “Handbook of Phytoremediation”, I.A. Golubev, Ed., Nova Science Publishers, NY, in press.
Glick, B.R., Cheng, Z., Czarny, J. and Duan, J. 2007. Promotion of plant growth by ACC deaminase-containing soil bacteria. Eur. J. Plant Pathol. 119: 329-339.
Glick, B.R., Patten, C.L., Holguin, G. and Penrose, D.M. 1999. “Biochemical and Genetic Mechanisms Used by Plant Growth Promoting Bacteria’, Imperial College Press, London, UK, 270 pages.
Glick, B.R. 2010. Using soil bacteria to facilitate phytoremediation. Biotechnol. Adv. 28: 367-374.
Phytoremediation: Methods and Reviews. 2007. N. Willey, Ed., Humana Press, Totowa, NJ, 478 pages.
Phytoremediation and Rhizoremediation: Theoretical Background. 2006. M. Mackova, D. Dowling and T. Macek, Eds., Springer, Dordrecht, The Netherlands, 300 pages.
Phytoremediation of Toxic Metals: Using Plants to Clean Up the Environment. 2000. I. Raskin and B.D. Ensley, Eds., Wiley-Interscience, NY, 304 pages.
Pilon-Smits, E. Phytoremediation. 2005. Annu. Rev. Plant Biol. 56: 15-39.
Pilon-Smits, E. and Freeman, J.L. 2006. Environmental cleanup using plants: biotechnological advances and ecological considerations. Front. Ecol. Environ. 4: 203-10.
Weyens, N., van der Lelie, D., Taghavi, S. and Vangronsveld, J. 2009. Phytoremediation: plant endophyte partnerships take the challenge. Curr. Opin. Biotechnol. 20: 248-254.