Thermolytic degradation of methylmethionine and implications for its role in DMS and MeCl formation in hypersaline environments

Environmental context Methyl chloride and dimethyl sulfide are important atmospheric trace gases, but their biogeochemical contributions to the atmosphere are not fully understood. The amino acid derivative methyl methionine has been hypothesised to be a precursor of these two atmospheric gases, especially in drying salt-lake environments. We found methyl chloride and dimethyl sulfide in salt crystals and soil samples of hypersaline lakes, suggesting that a thermal decay of methyl methionine could be one of the formation mechanisms responsible. Abstract Volatile organic halocarbons (VOXs) and volatile organosulfur compounds (VOSCs) play an important role in the chemical processes of the lower atmosphere. However, biogeochemical release mechanisms from terrestrial environments are complex and the current knowledge of the origin and fluxes of these compounds is incomplete. This study presents data from worldwide sampling campaigns to hypersaline salt lakes to investigate terrestrial sources for atmospheric VOXs and VOSCs. The hypothesis was tested if methionine or methylmethionine could potentially serve as a precursor for methyl chloride and dimethyl sulfide formation in salt-lake environments. Next to methyl chloride, emissions from hypersaline soil samples incubated in headspace vials showed an array of VOSCs including dimethylsulfide and dimethyldisulfide. Additionally, methyl chloride and dimethyl sulfide were released from fluid inclusions of halite crystals after grinding and purge-and-trap headspace gas chromatography–mass spectrometry analysis. An abiotic mechanism for their formation is conceivable owing to the fast response of emission on heating freeze-dried samples at 40°C. Furthermore, the compounds trapped in fluid inclusions of halite crystals correspond to those compounds originally formed in the immediately subjacent soils. Based on the thermolytic degradation of methylmethionine, the activation energies for methyl chloride and dimethyl sulfide are calculated from their Arrhenius plots. Additionally, structurally related substances were analysed and a degradation mechanism is postulated. Results indicate that thermolytic processes could play an important role in salt-lake environments on desiccation