Methanol consumption drives the bacterial chloromethane sink in a forest soil.

Halogenated volatile organic compounds (VOCs) emitted by terrestrial ecosystems, such as chloromethane (CHCl), have pronounced effects on troposphere and stratosphere chemistry and climate. The magnitude of the global CHCl sink is uncertain since it involves a largely uncharacterized microbial sink. CHCl represents a growth substrate for some specialized methylotrophs, while methanol (CHOH), formed in much larger amounts in terrestrial environments, may be more widely used by such microorganisms. Direct measurements of CHCl degradation rates in two field campaigns and in microcosms allowed the identification of top soil horizons (i.e., organic plus mineral A horizon) as the major biotic sink in a deciduous forest. Metabolically active members of Alphaproteobacteria and Actinobacteria were identified by taxonomic and functional gene biomarkers following stable isotope labeling (SIP) of microcosms with CHCl and CHOH, added alone or together as the [C]-isotopologue. Well-studied reference CHCl degraders, such as Methylobacterium extorquens CM4, were not involved in the sink activity of the studied soil. Nonetheless, only sequences of the cmuA chloromethane dehalogenase gene highly similar to those of known strains were detected, suggesting the relevance of horizontal gene transfer for CHCl degradation in forest soil. Further, CHCl consumption rate increased in the presence of CHOH. Members of Alphaproteobacteria and Actinobacteria were also C-labeled upon [C]-CHOH amendment. These findings suggest that key bacterial CHCl degraders in forest soil benefit from CHOH as an alternative substrate. For soil CHCl-utilizing methylotrophs, utilization of several one-carbon compounds may represent a competitive advantage over heterotrophs that cannot utilize one-carbon compounds.