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Chloromethane (CH3Cl) is the most abundant long-lived chlorinated organic compound in the atmosphere and contributes significantly to natural stratospheric ozone depletion. Salt marsh ecosystems including halophyte plants are a known source of atmospheric CH3Cl but estimates of their total global source strength are highly uncertain and knowledge of the major production and consumption processes in the atmosphere-halophyte-soil system is yet incomplete. In this study we investigated the halophyte plant, Salicornia europaea, and soil samples from a coastal salt marsh site in Sardinia/Italy for their potential to emit and consume CH3Cl and using flux measurements, stable isotope techniques and Arrhenius plots differentiated between biotic and abiotic processes. Our laboratory approach clearly shows that at least 6 different production and consumption processes are active in controlling atmospheric CH3Cl fluxes of a salt marsh ecosystem. CH3Cl release by dried plant and soil material was substantially higher than that from the fresh material at temperatures ranging from 20 to 70 °C. Results of Arrhenius plots helped to distinguish between biotic and abiotic formation processes in plants and soils. Biotic CH3Cl consumption rates were highest at 30 °C for plants and 50 °C for soils, and microbial uptake was higher in soils with higher organic matter content. Stable isotope techniques helped to distinguish between formation and degradation processes and also provided a deeper insight into potential methyl moiety donor compounds, such as S-adenosyl-l-methionine, S-methylmethionine and pectin, that might be involved in the abiotic and biotic CH3Cl production processes. Our results clearly indicate that cycling of CH3Cl in salt marsh ecosystems is a result of several biotic and abiotic processes occurring simultaneously in the atmosphere-plant-soil system. Important precursor compounds for biotic and abiotic CH3Cl formation might be methionine derivatives and pectin. All formation and degradation processes are temperature dependent and thus environmental changes might affect the strength of each source and sink within salt marsh ecosystems and thus considerably alter total fluxes of CH3Cl from salt marsh ecosystems to the atmosphere.

German Science Foundation (DFG KE 884/10-1 ; ANR-14-CE35-0005-01) ; French National Research Agency (DFG KE 884/10-1 ; ANR-14-CE35-0005-01); International audience; Chloromethane (CH3Cl, methyl chloride) is the most abundant volatile halocarbon in the atmosphere and involved in stratospheric ozone depletion. The global CH3Cl budget, and especially the CH3Cl sink from microbial degradation in soil, still involves large uncertainties. These may potentially be resolved by a combination of stable isotope analysis and bacterial diversity studies. We determined the stable isotope fractionation of CH3Cl hydrogen and carbon and investigated bacterial diversity during CH3Cl degradation in three soils with different properties (forest, grassland, and agricultural soils) and at different temperatures and headspace mixing ratios of CH3Cl. The extent of chloromethane degradation decreased in the order forest > grassland > agricultural soil. Rates ranged from 0.7 to 2.5 μg g−1 dry wt. d−1 for forest soil, from 0.1 to 0.9 μg g−1 dry wt. d−1 for grassland soil, and from 0.1 to 0.4 μg g−1 dry wt. d−1 for agricultural soil and increased with increasing temperature and CH3Cl supplementation. The measured mean stable hydrogen enrichment factor of CH3Cl of −50 ± 13‰ was unaffected by temperature, mixing ratio, or soil type. In contrast, the stable carbon enrichment factor depended on CH3Cl degradation rates and ranged from −38 to −11‰. Bacterial community composition correlated with soil properties was independent from CH3Cl degradation or isotope enrichment. Nevertheless, increased abundance after CH3Cl incubation was observed in 21 bacterial operational taxonomical units (OTUs at the 97% 16S RNA sequence identity level). This suggests that some of these bacterial taxa, although not previously associated with CH3Cl degradation, may play a role in the microbial CH3Cl sink in soil.

Chloromethane (CH3Cl), currently the most abundant chlorinated organic compound in the atmosphere at around ~ 550 parts per trillion by volume (pptv), is considered responsible for approximately 16% of halogen-catalyzed stratospheric ozone destruction. Although emissions of CH3Cl are known to occur from animals such as cattle, formation and release of CH3Cl from humans has not yet been reported. In this study a pre-concentration unit coupled with a gas chromatograph directly linked to a mass spectrometer was used to precisely measure concentrations of CH3Cl at the pptv level in exhaled breath from 31 human subjects with ages ranging from 3 to 87 years. We provide analytical evidence that all subjects exhaled CH3Cl in the range of 2.5 to 33 parts per billion by volume, levels which significantly exceed those of inhaled air by a factor of up to 60. If the mean of these emissions was typical for the world's population, then the global source of atmospheric CH3Cl from humans would be around 0.66 Gg yr− 1 (0.33 to 1.48 Gg yr− 1), which is less than 0.03% of the total annual global atmospheric source strength. The observed endogenous formation of a chlorinated methyl group in humans might be of interest to biochemists and medical scientists as CH3Cl is also known to be a potent methylating agent and thus, could be an important target compound in future medical research diagnostic programs.