Forest-soil fungi emit gases that harm ozone layer

Laboratory tests have revealed for the first time that certain types of common fungi can produce ozone-destroying methyl halide gases. The origin of substantial fractions of these gases in the atmosphere has eluded scientists.

The ectomycorrhizal fungi (pink) coating these oak roots nourish the tree but also produce ozone-destroying gases. J. Lansing/University of Pennsylvania

Ectomycorrhizal fungi–literally, fungi that envelop roots–form symbiotic relationships with trees. The fungus pulls nitrogen and phosphorus from the soil and then provides some of those nutrients to its arboreal partner. In return, the microbes receive carbohydrates produced by the tree (SN: 6/16/01, p. 372: New test traces underground forest carbon). These fungi are often, by weight, the most prevalent group of microbes in the soil of temperate forests, says Kathleen K. Treseder, a biogeochemist at the University of Pennsylvania in Philadelphia.

Treseder and her colleagues studied gases produced by four types of ectomycorrhizal fungi. They found that each gram of fungi churned out methyl halides–which include methyl chloride, methyl bromide, and methyl iodide–at a daily yield of a few millionths of a gram. That doesn’t sound like much, she notes, but these fungi occur in forests worldwide and can make up as much as 15 percent of the organic matter in soil. The researchers presented their findings last week at the American Geophysical Union’s fall meeting in San Francisco.

Many of the natural sources of methyl halide gases haven’t been identified, says Kelly R. Redeker, an atmospheric scientist at the University of California, Irvine and a coauthor of the study. For example, scientists don’t know where 25 percent of the atmosphere’s methyl bromide comes from. One known source is agriculture in which the gas is applied to sterilize soil.

Methyl bromide is responsible for about 10 percent of the high-altitude ozone destruction in Earth’s stratosphere, and methyl chloride gets between 10 and 15 percent of the blame, Redeker notes.

Although human-made gases, such as chlorofluorocarbons, account for much of the stratospheric-ozone loss, production of those substances is being phased out.

Chemical reactions in the atmosphere will eventually eliminate those gases already released, so identifying the sources of natural ozone destroyers such as methyl halides is becoming more important, says Redeker. Such knowledge could enable scientists to better predict the long-term influence of those gases, which may fluctuate in response to a changing climate.

The discovery by Redeker’s team is “an intriguing first result,” says Ray F. Weiss, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “People haven’t found a lot of natural sources for these [methyl halide] gases.” Field tests that measure the methyl halides escaping from soil are needed to determine whether the fungi indeed contribute significant amounts of ozone-destroying gases, he adds.

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