Surprising discovery: Contrary to what was thought, not only certain bacteria form the potent greenhouse gas methane, but probably all living things – including humans. Because our cells also have a biochemical mechanism through which methane can be produced during normal metabolic activity, as researchers report in “Nature”. This clarifies some mysteries about biological sources of methane and also sheds new light on our own metabolism.
Methane is a potent greenhouse gas, around 40 percent of which is released from natural sources. Sources are primarily wetlands, lakes, thawing permafrost, but also agriculture. The gas is created by microorganisms that form methane from the decomposition of organic material in the absence of air – at least that’s what was thought until now.
Anaerobic microbes are not the only
Strange, however: Some time ago, scientists discovered that some cyanobacteria, plants and fungi can apparently also produce methane – without the involvement of microbes and even in the presence of oxygen . However, the mechanism by which this biogenic methane synthesis occurs remained a mystery. Although it was suspected that these organisms may have special enzymes for this, they could never be proven.
Now new analyzes reveal a completely different picture. Because Leonard Ernst from the University of Heidelberg and his colleagues have discovered how methane is produced in the cells of these organisms – and how amazingly widespread this newly discovered ability really is. “This study is a milestone in our understanding of aerobic methane formation in the environment,” says senior author Frank Keppler from the University of Heidelberg.
On iron and free radicals on methane
The start of their discovery was made by experiments with the bacterium Bacillus subtilis, on which the researchers pursued the hypothesis of enzyme-free, biochemical methane formation. According to this, a so-called Fenton reaction takes place in the cells of most organisms, in which highly reactive oxygen compounds in the form of hydroxyl radicals are formed as a result of the reaction of reduced iron with hydrogen peroxide.
The decisive factor The radicals generated in this way can split off methyl groups (-CH3) from sulphur-containing compounds, from which then in a further step Methane is formed – so the assumption. In fact, this was confirmed in the studies on Bacillus subtilis: in the presence of iron and dimethyl sulfoxide, these bacteria formed methane even in the presence of oxygen. Contrary to what was previously assumed, neither special enzymes nor other catalysts are required for this methane formation.
Methane synthesis also in human cells
“It is remarkable that this Fenton-driven methane synthesis takes place under normal environmental conditions,” state Ernst and his colleagues. “This suggests that these reactions also occur in other living cells.” The team therefore checked whether this is the case at 30 various model organisms from all areas of the life tree.
And indeed: From the intestinal germ Escherichia coli to mold, yeast, plant cells to cultures of animal and human cells – they could Researchers demonstrate the production of methane. Apparently, it always starts when cells contain enough iron, methylated sulfur and nitrogen compounds such as amino acids and oxidizing molecules.
“Universal by-product of all life”
According to the scientists, this proves that this form of methane formation can take place in almost any organism. “In other words, methane could be produced as a universal by-product of life,” write Ernst and his team. Apparently, cells always turn on this reaction pathway when their metabolic activity is high and they are under cellular stress.
“The more active the cell, the more methane is formed,” explains Ernst’s colleague Ilka Bischofs. In their experiments, the test cells always released more methane when they were subjected to stress due to increased ambient temperatures or the addition of radical-forming substances. Under such conditions, methane formation in the cell cultures increased on average by a factor of 1.2 to 3.2. The intestinal germ Escherichia coli even increased the production a good 300-fold.
“This interaction with physical and chemical stressors would also explain why a single organism can release very different amounts of methane,” says Keppler.
New view also on biogenic methane emissions
These new findings have far-reaching significance for our view of cell metabolism, but also on biogenic methane emissions. This could mean that methane release from living beings of all kinds could continue to increase under potentially stressful conditions such as warming associated with climate change. “Changes in environmental conditions on Earth will therefore have a significant impact on methane emissions from living organisms,” the team writes offer new diagnostic possibilities in medicine: “Methane fluctuations in the air breathed by humans could provide indications of the oxidative stress level or indicate immune reactions,” according to the researchers The scientists involved in the study, Chang Liu and Jingyao Zhang from Xi’an Jiaotong University in China, consider the new findings to be significant: “The biogenic, non-enzymatic methane production opens up new avenues for research in a whole range of disciplines, from medicine to geosciences to astrobiology,” they write in an accompanying comment. (Nature, 2022; doi: .188/s41586-14-04511-9)
Source: University Heidelberg, Max Planck Institute for Terrestrial Microbiology
14. March 2022
– Nadia Podbregar