Soil Microbes Could Help Clean the Environment

Philip C. Kearney, research leader, Pesticide Degradation Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service.

Pesticides, nitrites, and soluble salts are all potential groundwater pollutants if not properly managed. Two major sources of groundwater pollution result from the extensive use of these materials.

The first source results from the downward movement of pesticides out of the root zone and through the vadose zone (the area below the root zone and above the water table), eventually moving into ground water. This problem results from the normal use of agricultural pesticides.

A second major source of groundwater pollution results from disposal of pesticide wastewater in unlined soil pits or from any form of ground disposal in an uncontained area.

One of our first needs, therefore, is to identify the contributions that each source makes toward the groundwater problems and take appropriate technological or regulatory actions to alleviate it.

In 1986, trace amounts of 23 different pesticides were detected in ground-water samples from 16 different States. This number probably will increase as further monitoring studies are begun and completed.

Micro-organisms in Cleanup

Soil micro-organisms help to prevent extensive movement of the intact pesticide out of the root zone. But microbes are not infallible; some compounds are not readily degraded because of environmental factors or lack of appropriate genes or enzymes. If carefully manipulated, selected, and engineered, however, micro-organisms could play a major role in solving many waste disposal problems.

Over the past four decades, we have made significant progress in identifying the soil micro-organisms responsible for metabolizing many of the organic pesticides, the microbial enzymes responsible for separating specific bonds in the molecule, and the products resulting from these metabolic reactions. Moreover, with the advent of modern molecular biology, we are beginning to understand the role of DNA in directing the synthesis of specific degradative enzymes.

A particular type of DNA, termed a plasmid, has been associated with the metabolism of several specific pesticides. These circular DNA elements are much smaller than the cell's chromosome and carry specialized genetic information for specific biodegradation genes that may not be present on the chromosome genes. Plasmids can replicate independently within the cell but cannot exist outside it. They also can move from cell to cell in certain microbial communities and transfer these genes to other micro-organisms.

Plasmids have been isolated and characterized for the initial metabolic reactions associated with the herbicides 2,4-D and MCPA, the herbicides dalapon and TCA, and the insecticides parathion, diazinon, and coumaphos. New discoveries of pesticide-degrading genes will increase rapidly over the next two decades as we learn more about DNA in the diverse soil microbial flora.

The ability to manipulate these genes and move them into micro-organisms more readily adaptable to the conditions that often make waste disposal difficult holds considerable promise for solving some potential pollution problems.

A Unique Cleanup Situation

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service conducts a tick control program along the U.S.-Mexican border. Cattle are dipped in large vats containing about 3,000 gallons of the tick control compound, coumaphos. About 42 of these vats are refilled with formulated coumaphos annually, so a large amount of pesticide wastewater is generated that must be degraded. Normal soil micro-organisms degrade coumaphos too slowly to be useful. Some classical chemical methods of pesticide disposal also did not provide a simple solution to the problem. But inoculating a very active Flavobacterium sp., known to degrade parathion rapidly, directly into the cattle dip solution resulted in a complete loss of coumaphos in several hours. When a field trial was conducted at Laredo, Texas, headquarters for the Tick Eradication Program, 680 gallons of used coumaphos solution were destroyed within 48 hours. The enzyme responsible is parathion hydrolase.

Future Research

Considerable attention is now being focused on the opd gene in the Flavobacterium sp. that is responsible for Producing parathion hydrolase. Working independently, scientists at the Agricultural Research Service and the University of Texas have isolated and described the gene responsible for the synthesis of parathion hydrolase. What is even more fascinating is that although the two groups worked with different micro-organisms of diverse origin, the gene appears to be the same.

Microbiologist studies bacteria that Degrade pesticides at the ARS Pesticide Degradation Lab. Researchers have found a way to make it easier for soil microbes to detoxify hazardous wastes.

The gene also has been transferred successfully to other bacteria by using cloning vector plasmids as shuttles for moving the gene. The possibility exists, therefore, to move this valuable gene into micro-organisms better adapted to the dip vat environment.

In a larger context, it may be advantageous to move this and other pesticide degradative genes into a variety of soil micro-organisms to clean up existing problems and prevent future pollution of our natural resources.