Biodegradation

Introduction

Biodegradation refers to the process where carbon-containing (organic) compounds are broken down into their component compounds. The process involves specialized proteins called enzymes. Bacteria and fungi that have the appropriate enzyme can degrade the particular compound.

This process is beneficial for the microbe, as is provides it with energy that can be used for its growth. From an environmental standpoint, biodegradation is beneficial in lessening the levels of toxic compounds in landfills, oil spills, or other sites of environmental contamination.

Biodegradation can be a natural process, in which microbes already present in the environment have acquired the enzyme(s) necessary to degrade the pollutant of interest. As well, bacteria can be engineered to be able to degrade the toxic compound of interest.

Many compounds can be biodegraded. These include hydrocarbons such as oil, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pharmaceuticals, antibiotics, radioisotopes, and even some heavy metals.

Historical Background and Scientific Foundations

Biodegradation has likely been a feature of the natural environment for millennia. However, the present relevance of biodegradation—the deliberate use of microbes to degrade a desired pollutant—dates back to 1989. That year, the tanker Exxon Valdez spilled nearly 11 million gallons of crude oil into Prince William Sound, Alaska. In remediating the fouled shoreline, fertilizers were used to encourage the growth of bacteria already present that were capable of degrading the oil. This strategy had not been attempted in such a situation previously. The results were encouraging and earned the technique the support of the U.S. Environmental Protection Agency (EPA).

Biodegradation also occur with landfills. The process involves bacteria that can degrade compounds anaerobically (in the absence of oxygen), and characteristically occurs very slowly. Examination of material recovered from landfills after two or three decades has revealed only minor degradation of compounds such as plastic bags, diapers, and aluminum cans. But degradation does occur, so landfills must be equipped with vents to allow the methane that is produced to gas off or to be recovered and used as a source of energy. Recovery or burning of landfill-emitted methane also benefits the atmosphere from the standpoint of global warming. Methane is a greenhouse gas that is about 20 times more potent than another greenhouse gas, carbon dioxide. Accumulation of methane in the atmosphere causes increased retention of heat in a region of the atmosphere called the troposphere.

Another example of biodegradation is composting. The interior of a compost pile can be almost oxygen-free, similar to a landfill. A compost pile is also a good example of the energy liberated during biodegradation. Disturbing a compost pile during cold days reveals steam, which arises due to the heat generated inside the compost pile.

Impacts and Issues

The ability of bacteria and fungi to carry out biodegradation has been exploited in the clean up of tens of thousands of contaminated soil and water sites globally. In the United States these include Superfund sites—seriously contaminated sites that have been abandoned and for which the federal government assumes financial

WORDS TO KNOW

ANABOLISM: The process by which energy is used to build up complex molecules.

CATABOLISM: The process by which large molecules are broken down into smaller ones with the release of energy.

GROUNDWATER: Fresh water that is present in an underground location.

LANDFILL: A low-lying area in which solid refuse is buried between layers of dirt.

SUPERFUND: Legislation that authorizes funds to clean up abandoned, contaminated sites.

responsibility for clean up and restoration under the terms of legislation passed in 1980. As of 2008, over 1,300 sites have been designated for remediation.

Although biodegradation often utilizes naturally occurring microbes that are already adapted to use the target compound as a nutrient source, genetic engineering has been used to adapt other organisms for biodegradation, or to enhance their degradative capability.

One example is the bacterium Deinococcus radiodurans,, which is naturally resistant to radioactivity. Genetic engineering has installed the genes that encode enzymes

responsible for the degradation of toluene and mercury. These compounds are typically present in radioactive nuclear waste, so the genetically engineered version of D. radiodurans has the potential to be used in the biodegradation of radioactive spills.

The genetic engineering approach is still controversial. Critics argue that the release of the genetically engineered organism into the environment might cause the microbes to become dominant. Others argue that once the toxic compound has been degraded, the engineered microbes will be at a disadvantage since maintaining the biodegradative capability requires energy. Organisms that can survive with less energy expenditure will become favored.

See Also Bioremediation; Chemical Spills; Oil Spills; Spill Remediation

BIBLIOGRAPHY

Books

Atlas, Ronald M., and Jim Philip. Bioremediation: Applied Microbial Solutions for Real-World Environment Cleanup. Washington: ASM Press, 2005.

Pisano, Gary P. Science Business: The Promise, The Reality, and the Future of Biotech. Boston: Harvard Business School Press, 2006.

Walker, Sharon. Biotechnology Demystified. New York: McGraw-Hill Professional, 2006.

Web Sites

Biobasics: Government of Canada. “Bioremediation.” February 9, 2006. http://biobasics.gc.ca/english/Viewasp?x=741 (accessed March 25, 2008).

Biodegradation

Biodegradation is the decay or breakdown of materials that occurs when microorganisms use an organic substance as a source of carbon and energy. For example, sewage flows to the wastewater treatment plant where many of the organic compounds are broken down; some compounds are simply biotransformed (changed), others are completely mineralized . These biodegradation processes are essential to recycle wastes so that the elements in them can be used again. Recalcitrant materials, which are hard to break down, may enter the environment as contaminants.

Biodegradation is a microbial process that occurs when all of the nutrients and physical conditions involved are suitable for growth. Temperature is an important variable; keeping a substance frozen can prevent biodegradation. Most biodegradation occurs at temperatures between 10 and 35°C. Water is essential for biodegradation. To prevent the biodegradation of cereal grains in storage, they must be kept dry. Foods such as bread or fruit will support the growth of mold if the moisture level is high enough. The microorganisms need energy plus carbon, nitrogen, oxygen, phosphorus, sulfur, calcium, magnesium, and several metals to grow and reproduce. The oxidation of organic substances to carbon dioxide and water is an exothermic (heat-releasing) process. For each mole of oxygen used as electron acceptor (oxidant), about 104 kilocalories (435 kJ) of energy is potentially available. All organisms make use of only part of this energy. The rest is lost as heat. This can be seen in composting when the compost becomes hot. Biodegradation can occur under aerobic conditions where oxygen is the electron acceptor and under anaerobic conditions where nitrate, sulfate, or another compound is the electron acceptor.

Bacteria and fungi, including yeasts and molds, are the microorganisms responsible for biodegradation. Environmental managers want to use biodegradation when it is needed and prevent it when preservation is important. Chemicals are commonly used to treat wood in buildings and other structures to prevent biodegradation. Wooden posts and pilings are treated with creosote or copper compounds to prevent rotting. Compounds that inhibit biodegradation are often added to automobile antifreeze solutions, aircraft deicer formulations, and other products to preserve the original qualities of the product. These products and chemicals can enter the environment and become contaminants. The inhibitors have a negative effect when the product becomes a waste and is to be biodegraded. For example, biodegradation of aircraft deicer formulations in airport runoff is often inhibited because of the benzotriazoles that are present to preserve the formulation.

see also Bioremediation; Solid Waste.

Bibliography

Alexander, Martin. (1994). Biodegradation and Bioremediation. New York: Academic Press.

Gibson, David T., ed. (1984). Microbial Degradation of Organic Compounds. New York: Marcel Dekker.

Internet Resources

Kansas State University. Great Plains/Rocky Mountain Hazardous Substance Research Center Web site. Available from http://www.engg.ksu.edu/HSRC.

Larry Eugene Erickson and Lawrence C. Davis

Biodegradable

Biodegradable substances are those that can be decomposed quickly by the action of biological organisms, especially microorganisms . The term is a process by which materials or compounds are broken down to smaller components, and all living organisms participate to some degree. Foods, for instance, are degraded by living creatures to release energy and chemical constituents for growth. In the sense that the term is usually used, however, it has a more restricted meaning. It refers specifically to the breakdown of undesirable toxic and waste materials or compounds to harmless or tolerable ones. When breakdown results in the destruction of useful objects, it is referred to as biodeterioration.

Although the term has been in common use for only two or three decades, processes of biodegradation have been known and used for centuries. Some of the most familiar are sewage treatment of human wastes, composting of kitchen, garden and lawn wastes, and spreading of animal waste on farm fields. These processes, of course, all mitigate problems with common and ubiquitous byproducts of civilization. The variety of objectionable wastes has greatly increased as human society has become more complex. The waste stream now includes items such as plastic bottles, lubricants, and foam packaging. Many of the newer products are virtually non-biodegradable, or they degrade only at a very slow rate. In some instances biodegradability can be greatly enhanced by minor changes in the chemical composition of the product. Biodegradable containers and packaging have been developed that are just as functional for many purposes as their non-degradable counterparts.

Advances in the science of microbiology have greatly expanded the potential for biodegradation and have increased public interest. Examples of new developments include the discovery of hitherto unknown microorganisms capable of degrading crude oil, petroleum hydrocarbons , diesel fuel, gasoline , industrial solvents, and some forms of synthetic polymers and plastics . These discoveries have opened new approaches for cleansing the environment of the accumulating toxins and debris of human society. Unfortunately, the rate at which the new organisms attack exotic wastes is sometimes quite slow, and dependent on environmental conditions. Presumably, microorganisms have been exposed to common wastes of a very long time and have evolved efficient and rapid ways to attack and use them as food. On the other hand, there has not been sufficient time to develop equally efficient means for degrading the newer wastes.

Research continues on the surprising capabilities of the new microbes emphasizing opportunities for genetic control and manipulation of the unique metabolic pathways that make the organisms so valuable. The potential for biodegradation can be improved by increasing the rates at which wastes are attacked, and the range of environmental conditions in which degrading organisms can thrive. The advantages of biological cleanup agents are several. Non-biological techniques are often difficult and expensive. The traditional way of removing petroleum wastes from soil , for instance, has been to collect and incinerate it at high temperatures. This is very costly and sometimes impossible. The prospect of accomplishing the same thing by treating the contaminated soil with microorganisms without removing it from its location has much appeal. It should have less destructive impact on the contaminated site and be much less expensive.

[Douglas C. Pratt ]

RESOURCES

BOOKS

King, R. B., G. M. Long and J. K. Sheldon. Practical Environmental Bioremediation. Boca Raton, Florida: CRC Press, Inc., 1992.

Sharpley, J. M. and A. M. Kaplan. Proceedings of the Third International Biodegradation Symposium. London: Applied Science Publishers, 1976.

Biodegradable

The term biodegradable is used to describe substances that are capable of being broken down, or decomposed, by the action of bacteria, fungi, and other microorganisms. Temperature and sunlight may also play a role in the decomposition of biodegradable substances. When substances are not biodegradable, they remain in the environment for a long time, and, if toxic, may pollute the soil and water, causing harm to plants and animals that live in these environments. Humans can also be affected by drinking water or eating crops contaminated by these toxic substances.

Common, everyday substances that are biodegradable include food wastes, tree leaves, and grass clippings. Many communities now encourage people to compost these materials and use them as humus (decayed organic material in soil) for gardening. Because plant and animal materials are biodegradable, this is one way to for towns and cities to reduce solid waste.

The development of detergents in the 1950s and the problems their surfactants caused (wetting agents that allow water to dissolve greasy dirt) raised the issue of the biodegradability of these chemicals. It was found that bacteria in sewage systems degraded some surfactants very slowly. This resulted in the chemicals being released into lakes and streams not fully decomposed and forming suds in the water. Environmental concerns led to the development of new detergents that are more easily biodegradable.

In efforts to control the use of nonbiodegradable materials, governments and industries have taken various measures. For example, the plastic rings that bind six-packs of soda and beer pose a danger to wildlife, who can becoming entangled in them; these rings must now be biodegradable by law in Oregon and Alaska. Italy has banned all nonbiodegradable plastics. Certain manufacturers have responded to the issue by experimenting with biodegradable packaging of food. Many garbage bags and disposable diapers are now being made using degradable plastics, with the goal of reducing litter, pollution, and danger to wildlife.

[See also Composting; Recycling; Waste management ]

bi·o·de·grad·a·ble / ˌbīōdiˈgrādəbəl/ • adj. (of a substance or object) capable of being decomposed by bacteria or other living organisms.DERIVATIVES: bi·o·de·grad·a·bil·i·ty / -ˌgrādəˈbilitē/ n.