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Abstract

Natural processes continuously produce organic and inorganic volatile compounds with one or two carbon atoms (C1 and C2), for instance, methane, ethane, methanol, formaldehyde, methyl chloride and carbon dioxide from the whole range of organic matter. They act as greenhouse gases, control the oxidation capacity of the atmosphere, destroy tropospheric and stratospheric ozone and play an important role in atmospheric chemistry and physics, thus influencing the global carbon cycle. Their origin is typically ascribed to complex enzymatic and metabolic processes and the combustion of organic matter. Methane emissions were, in general, attributed to methanogenic archaea, which grow under anoxic conditions. High emissions of methanol into the atmosphere originate from plant growth. Lignin, as part of the plants, is degraded by fungi, which is a well-described process that leads, for instance, to the release of methanol. Also, the demethylation of lignin under elevated pressure and temperature is utilised in industrial processes to generate sustainable resources like methanol and bioaromatics. This study presents compelling evidence for the oxic and abiotic formation of C1 and C2 compounds. These were generated from environmentally important organic substrates with sulfur-, nitrogen-, phosphorus-, and oxygen-bonded methyl groups. This was proven with the us of precursor compounds and extensive isotopically labelling studies in laboratory incubation experiments. Then, naturally occurring macro molecules like lignin were incubated, and for a direct link to nature, soil samples were incubated. The cleavage of the methyl group occurs through a highly reactive iron-oxo species which produces methyl radicals. The iron-oxo species is generated through the Fenton reaction in which iron reacts with hydrogen peroxide. Methyl radicals from methyl group-containing compounds serve as crucial intermediates in these reactions as they act as precursors of the C1 and C2 compounds. The product distribution of C1 and C2 compounds is influenced by the binding of the methyl group to different heteroatoms, ascorbic acid concentrations, and the specific iron species involved. An exchange of the iron species with other transition metals leads to identical C1 and C2 compounds with varying conversion rates. The use of isotopically labelled compounds determines the origin of carbon, hydrogen, and oxygen in the C1 and C2 compounds, identifying the methyl group, hydrogen peroxide and dioxygen as precursors depending on the heteroatom. A special case is the demethoxylation of lignin monomeric units and other aromatic methoxy compounds where the whole methoxy group is cleaved off and leads to the formation of methanol and, under the oxic conditions, additionally to formaldehyde. Extensive isotopic studies confirmed this newly described process. With a series of sterilised soil samples with different organic carbon and methoxy contents, this process was transferred to natural environments, resulting in the observation of significant amounts of methanol and formaldehyde with the methoxy group as a precursor and, to a lesser extent, methane and ethane formation. The incubation experiments of wet-dry cycles with soil samples have demonstrated their ability to produce methanol and formaldehyde continuously with decreasing amounts. All environmentally significant processes described here represent a substantial abiotic source of ubiquitously distributed C1 and C2 compounds. The specific case of demethoxylation is particularly important in the pedosphere due to the high levels of lignin in organic matter; this process is expected to provide an energy source for various microorganisms. The novel demethoxylation mechanisms and the expanded demethylation mechanism demonstrate the abiotic production of C1 and C2 compounds that affect the chemical and physical properties of natural environments and the global carbon cycle, thereby highlighting the significance of these processes.