Methane (CH₄) is a potent greenhouse gas, and its presence in Earth's early atmosphere is crucial to understanding the "faint young sun paradox." The paradox highlights the discrepancy between a less luminous early Sun and a relatively warm early Earth. While methanogenesis by microorganisms is a significant source of CH₄, the origin of CH₄ before life remains debated. Previous research has suggested hydrothermal vents and serpentinization as potential sources. This study investigates a novel abiotic CH₄ formation mechanism, previously observed in diverse organisms, driven by reactive oxygen species (ROS) and ferrous iron (Fe²⁺) interacting with methylated sulfur and nitrogen compounds. The researchers hypothesized that this mechanism, driven by light and heat, could have contributed significantly to CH₄ levels in the prebiotic Hadean and anoxic Archean periods. The study's importance lies in proposing a globally distributed CH₄ source, unlike previously suggested spatially restricted mechanisms, offering a potential solution to the faint young sun paradox and impacting our understanding of early Earth's atmospheric chemistry and the evolution of life.

The paper reviews existing literature on the faint young sun paradox, focusing on the role of CH₄ and other greenhouse gases in maintaining a warm early Earth. It examines previous proposals for abiotic CH₄ sources, including submarine volcanism and serpentinization, and discusses the limitations of using isotopic signals from sedimentary deposits to infer past CH₄ levels. The authors cite studies supporting high CH₄ concentrations in the early Archean based on sulfur and xenon isotope analysis. Importantly, the paper introduces their previously discovered non-enzymatic CH₄ formation mechanism, active in various organisms, involving a cascade of radical reactions driven by ROS and Fe²⁺. This mechanism serves as the foundation for the current study, which extends its application to abiotic conditions of the early Earth.

The researchers used aqueous model systems to investigate CH₄ formation under abiotic conditions. These systems consisted of a nitrogen atmosphere, a potassium phosphate-buffered solution (pH 7), Fe²⁺, and dimethyl sulfoxide (DMSO) as a methyl donor. Experiments were conducted under dark and light conditions, with varying temperatures (30–97 °C) to study the influence of heat. The effects of light were studied through broad-spectrum illumination and specific wavelengths, assessing CH₄ production rates and Fe²⁺ levels. The role of various (bio)molecules (citrate, malate, ATP, serine, glucose, pyruvate) as potential Fenton-promoting Fe²⁺ chelators was investigated, along with the competitive effect of Ca²⁺. The influence of oxygen levels and different transition metals (Cu, Co, Ce) on CH₄ formation was also examined. The study further investigated the role of light in driving an iron redox cycle, facilitating CH₄ formation by simultaneously generating ROS and reducing Fe³⁺ to Fe²⁺ via ligand-to-metal charge transfer (LMCT). Deuterium labeling experiments with *Bacillus subtilis* biomass were conducted to confirm the use of dead biomass as a substrate for CH₄ formation. Gas chromatography (GC) was extensively used to quantify CH₄, C₂H₆, CO₂, and H₂, while microsensors and endpoint measurements provided real-time and final H₂O₂ concentrations. Stable carbon (¹³C) and hydrogen (²H) isotope measurements were performed to assess isotopic fractionation during abiotic CH₄ formation. Finally, the growth of *Methylocystis hirsuta* (a methanotroph) on CH₄ produced by the model system was examined to show its potential as a carbon source for early life.

The study found consistent CH₄ formation from DMSO under dark, abiotic conditions, with rates increasing with temperature. Light enhanced CH₄ formation, attributed to water photolysis and H₂O₂ generation. Under acidic conditions, light-driven CH₄ formation was significantly enhanced by the presence of Fe³⁺, indicating light-driven ROS and Fe³⁺ formation from Fe(III)-aqua complexes. The addition of various (bio)molecules, particularly citrate, significantly enhanced heat-driven CH₄ formation by acting as Fenton-promoting Fe²⁺ chelators. Ca²⁺ competitively inhibited this effect. Light was shown to drive an iron redox cycle via LMCT, reducing Fe³⁺ to Fe²⁺ and generating organic radicals, further enhancing CH₄ production. Different transition metals (Cu, Co, Ce) also enhanced CH₄ formation, albeit less effectively than iron. Shorter wavelengths of light increased CH₄ formation rates. Various methylated sulfur and nitrogen compounds served as substrates for ROS-driven CH₄ formation. Deuterium labeling confirmed that CH₄ was formed from dead biomass, highlighting the increased CH₄ formation after the origin of life. Isotopic analysis showed less negative δ¹³C values for abiotically produced CH₄ compared to methanogenesis, suggesting distinct isotopic signatures for abiotic and biotic CH₄ production. The CH₄:C₂H₆ ratios also showed distinct differences between abiotic and biotic processes. The presence of synergistic effects between light and heat on CH4 formation is also reported.

The findings demonstrate that a globally distributed abiotic CH₄ formation mechanism, driven by light and heat via Fenton chemistry, existed under early-Earth conditions. This mechanism differs from previously proposed spatially restricted models. The observed CH₄ production under both anoxic and oxic conditions implies the potential relevance of this mechanism in contemporary CH₄ emissions. The increased availability of biomass following the origin of life further intensified this abiotic CH₄ formation by providing substrates and chelators, enhancing the efficiency of the process. The abiotically produced CH₄ could have played a significant role in maintaining a liquid hydrosphere on early Earth and potentially even influenced the evolution of methanotrophy before methanogenesis.

This study presents compelling evidence for a novel, globally distributed abiotic CH₄ formation mechanism operating under early Earth conditions. Driven by light and heat-generated ROS and Fenton chemistry, this mechanism could significantly contribute to early Earth's atmospheric CH₄ levels, offering a possible solution to the faint young sun paradox and influencing early life evolution. Future research should focus on further exploring the isotopic fractionation during abiotic CH₄ formation, quantifying the contribution of this mechanism to the global CH₄ budget throughout geological history, and investigating the significance of this pathway on other planetary bodies.

While the study extensively explored the abiotic CH₄ formation mechanism, a more detailed quantitative assessment of its contribution to the global CH₄ cycle throughout Earth's history is needed. Further research is also needed to refine our understanding of isotopic fractionation in abiotic CH₄ formation and determine the relative contributions of different factors influencing the CH₄ production rate (light, heat, substrates, chelators, transition metals). The experiments predominantly used model systems, and further in situ measurements under realistic early-Earth conditions might strengthen these findings.