Abstract
Chloromethane (CH3Cl) is a widely studied volatile halocarbon involved in the destruction of ozone in the stratosphere. Nevertheless, its global budget still remains debated. Stable isotope analysis is a powerful tool to constrain fluxes of chloromethane between various environmental compartments which involve a multiplicity of sources and sinks, and both biotic and abiotic processes. In this study, we measured hydrogen and carbon isotope fractionation of the remaining untransformed chloromethane following its degradation by methylotrophic bacterial strains Methylobacterium extorquens CM4 and Hyphomicrobium sp. MC1, which belong to different genera but both use the cmu pathway, the only pathway for bacterial degradation of chloromethane characterized so far. Hydrogen isotope fractionation for degradation of chloromethane was determined for the first time, and yielded enrichment factors (ε) of −29‰ and −27‰ for strains CM4 and MC1, respectively. In agreement with previous studies, enrichment in 13C of untransformed CH3Cl was also observed, and similar isotope enrichment factors (ε) of −41‰ and −38‰ were obtained for degradation of chloromethane by strains CM4 and MC1, respectively. These combined hydrogen and carbon isotopic data for bacterial degradation of chloromethane will contribute to refine models of the global atmospheric budget of chloromethane.
Highlights
Chloromethane (CH3Cl) is the most abundant volatile halocarbon in the atmosphere and is responsible for about 16% of chlorine-catalyzed ozone destruction in the stratosphere (Montzka et al 2011)
Independent incubation experiments were performed for determination of hydrogen and carbon isotope fractionation
Very similar values of kinetic isotope effects aC = 1.041 and aC = 1.038 were obtained for strains CM4 and MC1, respectively (Table 1). Both chloromethane-degrading bacterial strains selected for this study, Methylobacterium extorquens CM4 and Hyphomicrobium sp. strain MC1, as well as the two previously investigated Aminobacter strains (Miller et al 2001), grow with chloromethane using the well-characterized methyltransferase-dependent cmu pathway (Studer et al 2002; Nadalig et al 2011)
Summary
Chloromethane (CH3Cl) is the most abundant volatile halocarbon in the atmosphere and is responsible for about 16% of chlorine-catalyzed ozone destruction in the stratosphere (Montzka et al 2011). Major sources of CH3Cl are natural, and include tropical forests, grasslands, salt marshes, peatlands, biomass burning, and oceans. Most atmospheric CH3Cl is released from terrestrial vegetation (Yoshida et al 2004; Keppler et al 2005), with a fraction suggested to be formed in dead and senescent leaf tissue (Hamilton et al 2003). Especially the phyllosphere (i.e., the aboveground part of vegetation), represent the major biotic source of atmospheric chloromethane (Saito and Yokouchi 2008).
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