Abstract
Chloromethane is a halogenated volatile organic compound, produced in large quantities by terrestrial vegetation. After its release to the troposphere and transport to the stratosphere, its photolysis contributes to the degradation of stratospheric ozone. A better knowledge of chloromethane sources (production) and sinks (degradation) is a prerequisite to estimate its atmospheric budget in the context of global warming. The degradation of chloromethane by methylotrophic communities in terrestrial environments is a major underestimated chloromethane sink. Methylotrophs isolated from soils, marine environments and more recently from the phyllosphere have been grown under laboratory conditions using chloromethane as the sole carbon source. In addition to anaerobes that degrade chloromethane, the majority of cultivated strains were isolated in aerobiosis for their ability to use chloromethane as sole carbon and energy source. Among those, the Proteobacterium Methylobacterium (recently reclassified as Methylorubrum) harbours the only characterisized 'chloromethane utilization' (cmu) pathway, so far. This pathway is not representative of chloromethane-utilizing populations in the environment as cmu genes are rare in metagenomes. Recently, combined 'omics' biological approaches with chloromethane carbon and hydrogen stable isotope fractionation measurements in microcosms, indicated that microorganisms in soils and the phyllosphere (plant aerial parts) represent major sinks of chloromethane in contrast to more recently recognized microbe-inhabited environments, such as clouds. Cultivated chloromethane-degraders lacking the cmu genes display a singular isotope fractionation signature of chloromethane. Moreover, 13CH3Cl labelling of active methylotrophic communities by stable isotope probing in soils identify taxa that differ from the taxa known for chloromethane degradation. These observations suggest that new biomarkers for detecting active microbial chloromethane-utilizers in the environment are needed to assess the contribution of microorganisms to the global chloromethane cycle.
Highlights
Chloromethane and stratospheric ozone depletionChloromethane is the most abundant organohalogen in the Earth atmosphere
Methylotrophs isolated from soils, marine environments and more recently from the phyllosphere have been grown under laboratory conditions using chloromethane as the sole carbon source
13CH3Cl labelling of active methylotrophic communities by stable isotope probing in soils identify taxa that differ from those known for chloromethane degradation
Summary
Chloromethane (methyl chloride, CH3Cl) is the most abundant organohalogen in the Earth atmosphere. Photolytic degradation of chloromethane releases a halogen radical, which catalyses the destruction of ozone. Chloromethane contributes to depletion of the stratospheric ozone layer (altitude of approximately 20 to 30 km), which constitutes the Earth’s natural protective shield that absorbs the solar UVC and partially UVB radiation dangerous for living organisms. Chloromethane has a stratospheric lifetime of about one year, much shorter than most other chlorofluorocarbons (CFCs), solvents and halons banned by the international agreement of 1987 known as the Montreal Protocol on substances that deplete the ozone layer (see web resource section). Chloromethane is responsible alone for approximately 16% of stratospheric chlorine-catalysed ozone destruction (Carpenter et al, 2014). A detailed understanding of its sources and sinks will be essential to predict changes in atmospheric chloromethane fluxes in the context of global climate change
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