In this activity, students will learn how living organisms have been used traditionally to process materials or to produce certain products or results. Students will also explore how modern bioengineering techniques have been used to mimic natural
46 project learning tree Exploring Environmental Issues: BioTechnology ©AmericAn Forest FoundAtion
background
One man’s trash is another man’s treasure. That adage could not be truer than in the example of
bioremediation. Plants, bacteria, and even fungi
can benefit from metabolizing the concentrated nutrients that are present in some types of waste generated by human activities. Bioremediation is the breakdown of certain contaminants by living organisms in an effort to restore an ecosystem to its natural condition. Often, as in the case of most surface waters, this type of remediation is unplanned and occurs naturally as native organ- isms eat the excess sewage, runoff, industrial by-products, or farm wastes as part of the biogeo-
chemical nutrient cycling process.
Plants can absorb metal contaminants such as arse- nic, cadmium, and lead, or they may be used sim- ply to hold the contaminants in place so chemicals that are toxic do not migrate so fast into ground- water or surface water. Microorganisms, fungi, and invertebrates that are naturally occurring in soil or water can be encouraged to reproduce at higher rates in order to consume toxins that have leaked into an area. For centuries, humans have intentionally influenced the natural processes of nutrient cycling as a means of cleaning up concen- trated contaminants, such as planting sugar beets to remove salt from agricultural fields or adding worms or other macroinvertebrates to compost piles, which will speed the decomposition of agri- cultural waste.
Methods of Bioremediation
Plants are easy to grow and monitor, and they are able to draw out toxins from soil or water as long as the chemicals are within the range of their root system. In addition, plants reduce soil erosion and can trap particles in the soil to slow the migration of contaminants. Often, fungi that are root sym-
bionts catalyze the uptake of certain materials or
may even absorb contaminants directly.1 In places where contaminated substances have permeated deep into the soil beyond the reach of surface plants, naturally occurring soil bacteria have been found to break down pollutants by metabolizing the toxic molecules into inert molecules.
Two primary methods exist by which organisms clean up wastes in a region.2 In the first process, termed hyperaccumulation, a plant absorbs a toxin through its roots and concentrates the sub- stance in its tissues. That uptake and sequestering are most common for plants that are absorbing metals such as arsenic, lead, nickel, copper, cadmi- um, mercury, or even radioactive substances such as strontium or uranium. For example, sunflowers hyperaccumulate strontium and have been used at Chernobyl to help contain radioactive dust. Plants cannot use large quantities of metals for metabo- lism, so why do they absorb those toxic substances from the soil? It is thought that toxin-tolerant plants have adapted to absorb those substances as an evolutionary advantage to outcompete plants that are unable to grow in contaminated soils.3 After the metals have been absorbed into the plant tissue, the plants can be removed and incinerated to fur- ther concentrate the metal for disposal or reuse. The use of plants to control, contain, or hyperac- cumulate a contaminated site is a form of phy-
toremediation. While the plants are growing,
there is a risk that herbivores will ingest plants that have hyperaccumulated a metal, thus reintroduc- ing the metal into the ecosystem through the food chain, so scientists must monitor and control a site that is being restored using this method.
The second process by which toxins are removed from an ecosystem by living organisms is through
metabolism. By using the toxins as a nutrient
source, some bacteria, plants, and fungi can alter the toxic molecule into an inert molecule that is safe to release into the environment. An example of this process is in the cleanup of gasoline or oil spills in the ocean or on land. Specialized, naturally occurring bacteria that live in soil, freshwater, and the oceans consume hydrocarbon chains for cell respiration. The resulting products are those typical of metabolism: carbon dioxide gas and water. Bacteria can also metabolize chlorinated
hydrocarbons and a wide range of organic sol- vents commonly used in industry and agriculture.4 In this case, the organisms chemically alter the pol- lutants rather than simply contain them, so there is no need to recover the organisms after the pollut- ants have been consumed.
Traditional Biotechnology Still Applicable Today
Bioremediation has been a familiar method of disposing of or reducing the quantity of concen- trated waste since humans have lived in sedentary groups. Early sewage systems and compost piles give evidence of rudimentary waste treatment by early civilizations. These practices rely on native bacteria and soil organisms to break down waste through metabolism so the nutrients in the waste can be returned to the surface water or soil in a form that is either less harmful or not harmful. Modern techniques of sewage treatment and land- fill composting promote the same processes that have been used for centuries.
Current techniques encourage bacteria, algae, snails, worms, and other invertebrates to ingest nutrient-rich sewage or landfill waste by aerating the wastewater or soil while providing moisture or substrates for organisms to cling to while they eat and reproduce. In addition to using native organ- isms, artificial selection of plants to restore agri- cultural fields and the selection of bacterial strains to consume a particular toxin such as cyanide or sulfur are common cleanup procedures. Scientists also encourage populations of native soil bacteria and microorganisms to flourish in order to clean up highly contaminated sites such as the tailing
piles from mining operations, brownfields, or
industrial waste burial sites. Using fertilizers or chemicals such as chelating agents, electron
acceptors, or electron donors, scientists can
help local bacteria metabolize the waste at those sites to minimize the spread of contamination into groundwater wells and surface water.5
Modern Biotechnology Used to Deal with Modern Problems
Scientists have been experimenting with encourag- ing the growth of certain native bacteria and other soil or water microorganisms in situ by fertilizing regions that have been contaminated by oil spills or other types of petroleum-based pollutants. The addition of nitrate or sulfate fertilizers or electron acceptors (such as phosphorus, oxygen, or car- bon) can remove the factors that would normally limit the growth and reproduction of certain oil- consuming species. If nutrient or electron or both acceptors are made available through the use of
injection wells, the microorganism population can be stimulated to bloom and consume the contami- nant while this food source is abundant. This form of bioremediation is termed biostimulation. The latest techniques in bioremediation involve the use of genetically modified or transgenic
species to target the cleanup needs of a particular
substance in a specific region. The introduction of naturally occurring strains of microorganisms or genetically engineered microorganisms to assist in chemical cleanups is called bioaugmentation, which is often used in conjunction with biostimu- lation. By isolating and copying metabolic genes from bacteria that produce enzymes that can break down certain chemicals, scientists can insert the gene of interest into another species that can more adequately address the demands of quantity or drainage in an area. The species that receives the genes for a particular enzyme may be another bacterium, or any other organism that is suitable for growth in the contaminated region, such as an annual or perennial plant.
An example of bioaugmentation is the insertion of a mercury-absorption gene that is found in bacte- ria into a fast-growing tree species that has a deep root system such as willow, cottonwood, or poplar trees.6 The mercury is absorbed from soil or water in a highly toxic form; then mercury vapor, which is significantly less toxic, is released from leaf sur- face areas. In the past, bacteria have been used to perform this cleanup process, but the vapor release rate is slow because of the low surface area and the slow rate of diffusion of the concentrated vapor in soil. Trees have helped speed up the rate of mercury vaporization, keeping the more toxic contaminants from spreading farther into the soil and groundwater.
In the past century, applications of bioremediation have expanded with advances in microbiological techniques and the use of genetic recombina-
tion.7 Although we have increased our consump- tion and waste production exponentially, and although—as a society—we are realizing the extent of the damage we have already inflicted on terres- trial and aquatic ecosystems, there is hope in the form of living organisms that the environment may be returned to its original conditions.
48 project learning tree Exploring Environmental Issues: BioTechnology ©AmericAn Forest FoundAtion
endnotes
1. R. S. Boyd and S. N. Martens, “The Significance of Metal Hyperaccumulation for Biotic
iIteractions,” Chemoecology 8 (1998): 1–7. 2. E. Philon-Smits and J. Freeman,
“Environmental Cleanup Using Plants: Biotechnological Advances and Ecological Considerations,” Frontiers in Ecology and the
Environment 4, no. 4 (2006): 203–10.
3. M. N. V. Prasad, “Emerging Phytotechnologies for Remediation of Heavy Metal Polluted and Contaminated Soil and Water,” Hyderabad, India, December 29, 2006, http://wgbis.ces. iisc.ernet.in/energy/lake2006/programme/ programme/proceedings/lc2.htm.
4. “Advances in Remediation of Contaminated Water and Soil Systems II,” session held at the 2006 Western Pacific Geophysics Meeting, Beijing, China, July 24–27, www.agu.org/ meetings/wp06/wp06-sessions/wp06_H24B. html.
5. S. A. Thomas, “Mushrooms: Higher Macrofungi to Clean Up the Environment,” Battelle
Environmental Issues, Fall 2000.
6. D. Glass, “Current Trends in
Phytoremediation,” International Journal of
Phytoremediation 1, no. 1 (1999): 1–8.
7. D. R. Lovley, “Cleaning Up with
Genomics: Applying Molecular Biology to Bioremediation,” Nature Reviews Microbiology 1, no. 1 (October 2003): 35–44.