Hydrogen and carbon isotope fractionation during degradation of chloromethane by methylotrophic bacteria

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.

Chloromethane Degradation in Soils: A Combined Microbial and Two‐Dimensional Stable Isotope Approach

“…1B) and the stable isotopic signatures remained unchanged for 24 h at 20°C and 10 ppmv CH 3 Cl, in contrast with the native soil, for which a CH 3 Cl degradation rate and a stable carbon isotope pattern typical of a biotic process were found. The ε values for CH 3 Cl degradation of all three native soils tested are in agreement with reported values for bacterial pure cultures that degraded CH 3 Cl (Nadalig et al, 2013). Although soil sterilization by autoclaving inactivates biotic processes, other structural modifications of soil components may also occur (Berns et al, 2008).…”

“…Stable hydrogen and carbon isotope ratios of CH 3 Cl were measured by an in‐house cryogenic preconcentration unit coupled to a Hewlett‐Packard 6890 gas chromatograph (Agilent Technologies) and an IRMS (Isoprime), as described by Nadalig et al (2013). For stable hydrogen isotope analysis, a ceramic tube reactor without chromium pellets at 1450°C was used for high‐temperature conversion.…”

“…The combination of compound-specific isotope analysis and CH 3 Cl flux measurement has the potential to better constrain the atmospheric CH 3 Cl budget (Keppler et al, 2005;Saito and Yokouchi, 2008). Several studies have determined the stable carbon isotope signatures (d 13 C values) for CH 3 Cl in the atmosphere (Thompson et al, 2002); of its sources in forest soils (Redeker and Kalin, 2012), plants Keppler et al, 2004), fungi , and from biomass burning (Czapiewski et al, 2002); and of its sinks, including methylotrophic CH 3 Cl-degrading bacteria (Miller et al, 2001;Nadalig et al, 2013Nadalig et al, , 2014. Bacterial degradation of CH 3 Cl has been demonstrated in soils (Miller et al, 2004), but modeling studies (Keppler et al, 2005) have suggested that the microbial soil sink for the global atmospheric CH 3 Cl budget might be larger than originally estimated (Montzka and Fraser, 2003).…”

Fluorescence-Based Bacterial Bioreporter for Specific Detection of Methyl Halide Emissions in the Environment

“…This is due to large uncertainties in the sources described above but also in the sinks of these compounds, which include oxidation by hydroxyl radicals, loss to the stratosphere and to polar ocean waters, uptake by soils, and bacterial degradation (6,17). Current efforts to constrain the biogeochemical cycles of methyl halides involve analytical approaches such as gas chromatography-mass spectroscopy (GC-MS) including stable isotope techniques for carbon and hydrogen elements (17)(18)(19). These methods are time-consuming and labor-intensive, and this may constitute a drawback for screening potential sources of methyl halides in the environment.…”

Response to referee#2

“…However, current estimates of the CH 3 Cl global budget and the apportionment between sources and sinks are still highly uncertain. Known natural sources of CH 3 Cl include tropical plants (Yokouchi et al, 2002(Yokouchi et al, , 2007Umezawa et al, 2015), wood-rotting fungi (Harper, 1985), oceans (Moore et al, 1996;Kolusu et al, 2017), plants of salt marshes (Rhew et al, 2000(Rhew et al, , 2003, aerated and flooded soil (Redeker et al, 2000;Keppler et al, 2000), senescent leaves and leaf litter Derendorp et al, 2011) and wildfires. Anthropogenic CH 3 Cl release to the atmosphere comes from the combustion of coal and biomass with minor emissions from cattle (Williams et al, 1999) and humans (Keppler et al, 2017).…”