Methane gas

For decades scientists only knew volcanoes, enclosed gas in the arctic ice and forest fires act as abiotic methane sources, in which the gas is formed in a non-biochemistry way. Microorganisms were considered as the only biotic source which produce methane without oxygen. In recent years, Frank Keppler and his team have found that plants, fungi and also mammals emit methane, and this, into the air. Whether this is biotic, i.e. catalysed by enzymes, or abiotic, and thus a purely chemical process, is currently being investigated by researchers. We have now established that hydrocarbon in plants is formed by the amino acid methionine.

The traces of methane in the atmosphere are small but they have a great impact on climate. After all, its greenhouse effect is 25 times greater than that of carbon dioxide. This means that other sources of methane is also relevant to the understanding of the climate.

Labelled methionine reveals the plant methane source

In order to get to the bottom of the methane source, Keppler from the Max Planck Institute for Chemistry in Mainz and the University of Heidelberg and his colleagues took two approaches. Firstly, they infiltrated tobacco plant leafs with specially labelled methionine and let the offshoots grow on a culture medium with the prepared methionine in further experiments. The amino acid in the methyl group which scientist considered could be a precursor for methane, received a particularly high amount of carbon-13. This heavy carbon isotope can be clearly differentiated from ordinary carbon-12 in appropriate analyses. “We then found the labelled carbon atom in the methane which was emitted from the plants,” says Frank Keppler.

The researchers conducted test-tube experiments, in an aqueous solution – a  realistic scenario, as most plant cells largely consist of water. They combined various methylated substances, which contain a methyl group derived from methane such as for example methionine, dimethyl sulphoxide or lecithin with iron ions, ascorbic acid and hydrogen peroxide. The latter are also responsible for the incorporation or removal of methyl groups in other molecules in plants, fungi and animals. In the experiments methane was formed from methionine and a few other sulphur containing substances which, however, are not found in plants – this happened by abiotic means, i.e. without any biochemical addition of enzymes.

“Apparently the linking of the methyl group with another sulphur atom is a prerequisite for this,” says Frank Keppler. “Methane was only produced in noteworthy quantities in such substances.” The hydrocarbon was also split off in the form of dimethyl sulphoxide. “In plants this does not play a role, however in algae this could be an important chemical precursor from which methane is emitted,” says Keppler. “This finding could be of great assistance in explaining the methane-ocean paradox.” A considerable amount of methane is emitted from the world’s oceans, although the oceans have a high oxygen content. Biologists cannot explain this as long as they accept the assumption that microorganisms only produce methane when no oxygen is available.

Methane, a coincidence or waste product?

“We have not proven yet if the abiotic mechanism which we observed in the test-tubes also really works like this in plants,” says Frank Keppler. “This is what we want to find out in future work.” By identifying methionine as the chemical precursor for greenhouse gas, they already took the first step. At the moment it is also still unclear if methane forms in plants by coincidence, as a result of the required substances coming together in their cells at this particular point in time. Or whether it is a waste product, so to say, from a reaction of which the other products are important for the metabolism of plants.

It is not yet possible to assess the quantities of methane emitted from the sources that Frank Keppler and his colleagues have found in plants, fungi and possibly also animals. Consequently, it is also not yet clear what role these methane emissions play with regard to the climate. Researchers will only be able to present reliable prognoses about this once they have found and understood all previously unknown methane sources, when they know which factors, as for example UV radiation or oxygen content in the environment, impacts on the emission of methane. This is exactly what the researchers led by Frank Keppler are working on.

Source: Dr. Susanne Benner, Max Planck Society