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Appl. Environ. Microbiol., Nov 1997, 4346-4354, Vol 63, No. 11
LG Miller, TL Connell, JR Guidetti and RS Oremland
The oxidation of [(sup14)C]methyl bromide ([(sup14)C]MeBr) to
(sup14)CO(inf2) was measured in field experiments with soils collected from
two strawberry plots fumigated with mixtures of MeBr and chloropicrin
(CCl(inf3)NO(inf2)). Although these fumigants are considered potent
biocides, we found that the highest rates of MeBr oxidation occurred 1 to 2
days after injection when the fields were tarped, rather than before or
several days after injection. No oxidation of MeBr occurred in heat-killed
soils, indicating that microbes were the causative agents of the oxidation.
Degradation of MeBr by chemical and/or biological processes accounted for
20 to 50% of the loss of MeBr during fumigation, with evasion to the
atmosphere inferred to comprise the remainder. In laboratory incubations,
complete removal of [(sup14)C]MeBr occurred within a few days, with 47 to
67% of the added MeBr oxidized to (sup14)CO(inf2) and the remainder of
counts associated with the solid phase. Chloropicrin inhibited the
oxidation of MeBr, implying that use of this substance constrains the
extent of microbial degradation of MeBr during fumigation. Oxidation was by
direct bacterial attack of MeBr and not of methanol, a product of the
chemical hydrolysis of MeBr. Neither nitrifying nor methane-oxidizing
bacteria were sufficiently active in these soils to account for the
observed oxidation of MeBr, nor could the microbial degradation of MeBr be
linked to cooxidation with exogenously supplied electron donors. However,
repeated addition of MeBr to live soils resulted in higher rates of its
removal, suggesting that soil bacteria used MeBr as an electron donor for
growth. To support this interpretation, we isolated a gram-negative,
aerobic bacterium from these soils which grew with MeBr as a sole source of
carbon and energy.
Copyright © 1997, American Society for Microbiology
Bacterial Oxidation of Methyl Bromide in Fumigated Agricultural Soils
U.S. Geological Survey, Menlo Park, California 94025
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