HR: 0830h
AN: B31E-0362    [PDF]
TI: Degradation of Methyl Bromide and Methyl Chloride in Soil Microcosms: Use of Stable C Isotope Fractionation and Stable Isotope Probing to Identify Reactions and the Responsible Microorganisms
AU: * Miller, L G
EM: lgmiller@usgs.gov
AF: U.S. Geological Survey, Mail Stop 465 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Warner, K L
EM: karenchipmunk@hotmail.com
AF: University of Warwick, Department of Biological Sciences, Coventry, CV4 7AL United Kingdom
AU: Baesman, S M
EM: sbaesman@usgs.gov
AF: U.S. Geological Survey, Mail Stop 465 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Oremland, R S
EM: roremlan@usgs.gov
AF: U.S. Geological Survey, Mail Stop 465 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: McDonald, I R
EM: imcdonald@bio.warwick.ac.uk
AF: University of Warwick, Department of Biological Sciences, Coventry, CV4 7AL United Kingdom
AU: Radajewski, S
EM: Stefan.Radajewski@warwick.ac.uk
AF: University of Warwick, Department of Biological Sciences, Coventry, CV4 7AL United Kingdom
AU: Murrell, J C
EM: cmurrell@bio.warwick.ac.uk
AF: University of Warwick, Department of Biological Sciences, Coventry, CV4 7AL United Kingdom
AB: Methyl bromide (MeBr) and methyl chloride (MeCl) are important atmospheric trace gases that contribute directly to stratospheric ozone depletion. These compounds have natural and anthropogenic sources and sinks in both aquatic and terrestrial environments. Soils comprise the largest known sink for MeBr on the Earth's surface and are also a large sink for MeCl. However, the processes that influence the flux of these compounds from air to soil or soil to air are poorly understood at present. Bacteria in soil microcosm experiments oxidized both MeCl and MeBr, the former compound more rapidly than the latter. MeBr was also removed by chemical reactions while MeCl was not. Chemical degradation of MeBr accounted for more than half of its total loss. We applied new techniques to determine if different bacteria were responsible for degrading MeBr and MeCl. Stable isotope probing revealed that different populations of soil bacteria assimilated added $^{13}$C-labeled MeBr and MeCl. Soil bacterial oxidation of MeBr and MeCl was characterized by different kinetic isotope effects (KIEs). The KIE for MeBr oxidation by bacteria was 22 $\pm$ 5 \permil and the KIE for MeCl oxidation was 56 $\pm$ 3 \permil, suggesting that different bacteria were responsible for degrading each compound. The identity of the active MeBr and MeCl degrading bacteria in soil was determined by analysis of 16S rDNA sequences amplified from $^{13}$C-DNA fractions. The diverse population of active bacteria was reflected by the range of sequences found for the cmuA gene, which codes for the enzyme that catalyzes the initial step in the oxidation of MeBr and MeCl. The diversity and number of different bacteria actively degrading MeBr and MeCl in the soil and the number of bacteria identified that contain the enzyme capable of degrading methyl halides were in contrast to the limited number of methyl halide degrading bacteria that have been isolated thus far from soil and aquatic environments; thus suggesting that the extant degraders represent only a subset of the natural diversity of methyl halide degrading bacteria.
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 1615 Biogeochemical processes (4805)
DE: 4840 Microbiology
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting