08/22/2023
Methane formation in the inanimate environment
Iron and reactive oxygen species - exposed to light and heat - drive the formation of the greenhouse gas methane in aqueous environments. Methane formation is also possible under abiotic conditions, that is to say, in an ecosystem without the involvement of living beings. That has been shown by researchers from the Institute of Earth Sciences of Heidelberg University and the Max Planck Institute for Terrestrial Microbiology in Marburg.
According to the scientists, this purely chemical process might have favoured the emergence of life in early Earth history, as methane arose in the early aqueous regions. This process possibly also supplies an explanation of the phenomenon that methane is released over the surfaces of the oceans.
Back in primeval history, before the evolution of life, methane release led to a warming of the atmosphere. The methane haze prevented the planet from freezing, which was one of the prerequisites for the emergence of life. Today methane is regarded as a greenhouse gas particularly harmful to the climate.
According to Heidelberg earth scientist Prof. Dr Frank Keppler, over a period of 20 years it increases global warming roughly 80 times more strongly than the same quantity of carbon dioxide, even though methane is degraded much more quickly in the atmosphere. Research to identify the multiple sources of methane production is going on worldwide in order to better estimate and predict the consequences of human-induced climate change.
In 2022 Prof. Keppler's biogeochemistry research group discovered, in cooperation with colleagues from the Max Planck Institute for Terrestrial Microbiology, that methane is released not only through the activity of special microorganisms but purely chemically in the cells of all living beings. This mechanism is driven by reactive oxygen species that arise through the metabolic activity of cells.
In interplay with the essential element iron, such compounds, in all organisms, are involved in a process which, through various steps, leads to the formation of highly reactive metabolites and ultimately to methane formation. This process takes place without an enzymic catalyst.
In their current experiments, the scientists investigated whether this non-enzymatic process can also take place in the inanimate environment, i.e. under abiotic conditions like those prevailing in primeval history even before the emergence of life. The researchers used a chemical model system imitating the environmental conditions in early Earth history.
They were then able to show that what is called the Fenton reaction of iron and reactive oxygen species can lead to the formation of methane and also ethane in aqueous solutions, without free oxygen having to be present. The reactive oxygen species arise from water by photolytic and thermolytic processes, i.e. the influence of light and heat.
According to the scientists, it is noteworthy that methane is produced via the Fenton reaction from organic sulphur compounds as well. As early as in 2014, Prof. Keppler's team had shown that this was possible in principle under abiotic conditions. Sulphuric organic compounds are found at hydrothermal vents in the deep sea, better known as "black smokers". Until now a specific geological process whereby rocks are transformed into serpentine minerals was thought to be responsible for methane formation in the deep sea.
"The fact that methane can be formed on the ocean floor via the Fenton reaction is a finding that surprised us," says study head Dr Johannes Rebelein from the Max Planck Institute for Terrestrial Microbiology. The scientists say that this process could basically take place in all wetlands on Earth because heat and light drive it under ambient temperature and pressure conditions.
The researchers think that the present findings could supply a further piece of the puzzle to solve the "oceanic methane paradox". That refers to the light-dependent emission of methane from water bodies. By contrast with microbial methane production, this process takes place in the presence of oxygen. The origin of this methane escaping via the ocean surfaces has not yet been finally explained. The scientists also proved that iron-reducing biomolecules further promote methane formation.
"In our experiments, they enhanced the intensity of the Fenton reaction. That means that, after the emergence of life, the process probably increased in intensity because the biomolecules served both as substrates and as iron-binding activators," explains lead author Leonard Ernst, who studied at Heidelberg University and is now working for his doctorate at the Max Planck Institute in Marburg.
The findings provide an important basis for further investigations into the evolution of the Earth's atmosphere, as well as the extent to which methane formation in the inanimate environment contributes to the methane balance of the planet. Also participating, besides the team from Heidelberg University and the Max Planck Institute for Terrestrial Microbiology, were scientists from the Microcosm Earth Center in Marburg - a cooperative venture between the Max Planck Institute and the University of Marburg - and the German Cancer Research Center in Heidelberg. The research findings were published in the journal "Nature Communications".
Source: University of Heidelberg