Some species of bacteria produce methane

Nitrogen-fixing bacteria contain a previously unrecognised pathway for producing methane, researchers have discovered. The ability appears to be a by-product of an enzyme reaction that brings about another gas transformation.

Nitrogen-fixing bacteria are critically important for life on Earth, because they convert atmospheric nitrogen into a form that can be exploited by plants and animals.

However, microbiologists Caroline Harwood and Yanning Zheng of the University of Washington School of Medicine, and published in the journal Nature Microbiology, have uncovered other, more complex chemical feats performed by some of the microbes.

About 10% of nitrogen-fixing species manufacture an enzyme called “iron-only nitrogenase”. The primary function of the enzyme is to convert nitrogen gas into ammonia. Harwood and Zheng, however, discovered that the same enzyme also converts carbon dioxide into methane.

Although iron-only nitrogenase was identified in the mid-Twentieth Century, until now its methane-producing capacity had gone unnoticed.

“It’s been a neglected enzyme,” Zheng says.

Microbes, primarily archaea, are responsible for producing and consuming around a billion tonnes of methane at all. However, nitrogen-fixing bacteria have until now not been seen as involved in the cycle.

“Methane is potent greenhouse gas,” says Harwood. “That is why it is important to account for all of its sources.”

The scientists made their discovery while working with a bacterial species known as Rhodopseudomonas palustris.

To make sure it was not a property unique to the species, they then tested three other nitrogen-fixing varieties and found the same result. They then checked DNA data and found the genes that produce iron-only nitrogenase in several – indicating that they too are unacknowledged sources of methane.

“There is now recent evidence that iron-only nitrogenase is active in microbes more often and in more conditions than we had previously thought,” Zheng says.