Biodegradation

views updated

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.