Scientists with the U.S. Geological Survey (USGS) have found a type of bacterium in the soil that literally eats methyl bromide and thrives on it.

Ronald S. Oremland leads this research for USGS in Menlo Park, California. His work was reported by colleague Tracy L. Connell at the San Diego 1997 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions, November 3–5, 1997.

"The possibility that soil bacteria directly oxidize methyl bromide, thereby contributing to its destruction, is generally overlooked," said Oremland, a USGS geochemist. "This is because both methyl bromide and chloropicrin, which is often used with methyl bromide, are potent biocides that are presumed to kill off, or inactivate, any soil bacteria as well as target organisms."

In lab experiments, Oremland and colleagues isolated a pure culture of a gram negative, motile rod-shaped bacterium from soils that had been fumigated with methyl bromide. They designated the bacterium as strain IMB–1.

This strain of bacterium is closely related to Rhizobium, the species of common bacteria that converts nitrogen to a form useful in the soil.

"We found that this new strain not only thrives on methyl bromide, but it can also grow by oxidizing methyl iodide, methyl chloride, methylated amines, acetate, and glucose," Oremland said. "In lab tests when grown on substrates of all of these compounds, IMB–1 retained its ability to break down methyl bromide, even after being carried through two consecutive transfers."

According to Oremland, these results mean that this bacterium could be used to keep methyl bromide from escaping from the soil during field fumigations.

"Our tests showed that adding cells grown from methyl bromide or glucose to soil dramatically increased the rate at which methyl bromide dissipates," he said.

Fumigation levels of methyl bromide were consumed within 1 day in soils where methyl bromide-grown cells had been added and within 2 days for soil that had been treated with glucose-grown cells. It took 7 days for methyl bromide to disappear from unamended live soil.

"We also found that adding low levels of methyl iodide to the soil increased the dissipation rate of methyl bromide. Apparently this treatment increased the numbers of bacteria like IMB–l, thereby decreasing the time that methyl bromide stayed in the soil," Oremland said. Incorporating trimethyl-amine, a methylated amine, in the soil seemed to speed up this activity.

Oremland and colleagues plan field tests to see if these measures will eliminate methyl bromide emissions from the soil and still maintain the efficacy of the chemical as an agricultural fumigant.

At least one industry group thinks the new strain of bacteria might drastically cut the amount of methyl bromide that escapes into the air.

"We signed a cooperative research and development agreement with the U.S. Geological Survey to further test this strain of bacterium," said Tom Duafala. He is director of research and development of TriCal, a company located in Hollister, California, that specializes in custom application of soil fumigants.

Duafala said his company will provide experimental plots to test the following treatments or protocols:

• Lower the amount of chloropicrin from 33 percent to 2 percent, which would mean that chloropicrin would serve as a warning agent rather than as a fumigant.
• Add trimethylamine or a similar bacterial substrate like formate, ammonium formate, methanol, or methylated amines to the soil.
• Amend the soil with traces of methyl iodide and/or trimethyl- amine prior to fumigation with methyl bromide.
• Innoculate the soil with cultures of the new strain of bacterium before applying methyl bromide.
• Use a combination of the above measures.

"We hope to find a protocol or treatment that will provide a level of pest control conducive to the yield quantity and quality that growers normally get, while eliminating, or at least significantly decreasing, methyl bromide emissions," Duafala said. "Dr. Oremland and colleagues at the U.S. Geological Survey are working on ways to mass produce the bacteria for soil application. This project looks very promising."