Microbial transhalogenation: A complicating factor in determination of atmospheric chloro- and bromomethane budgets

The sources and sinks of the ozone-depleting halocarbons, chloromethane (CH3Cl) and bromomethane (CH3Br), have been the subject of recent controversy. Considerable uncertainty surrounds the relative contributions of oceanic and terrestrial sources of CH3Cl and natural versus anthropogenic fluxes of CH3Br. Halogen stable isotope ratios in atmospheric halomethanes could provide a valuable tool in estimating relative magnitudes of sources, particularly those of CH3Cl. However, the reliability of such techniques is critically dependent on the conservative nature of the halogens within these atmospheric halomethanes. Here we demonstrate that intact cells of the soil bacterium strain CC495 under anaerobic or microaerophilic conditions rapidly exchange Cl-37(-) with organically bound chlorine in CH3Cl. Since Cl- occurs ubiquitously and such bacteria appear to be widespread, any chlorine isotope fractionation during biological or abiotic CH3Cl production may therefore not be apparent in atmospheric CH3Cl. Cells of strain CC495 also catalyzed transhalogenation of CH3Br to CH3Cl, suggesting that this transformation may represent a significant sink for atmospheric CH3Br in soil.