Unexpected Properties of Methane-Producing Microbe May Impact Climate Models

Understanding how methane is produced is crucial for better predicting rising temperatures and climate change. A certain type of bacteria is a huge source of this methane. For 40 years, scientists believed that they understood how these bacteria worked together to anaerobically digest biomass to produce this greenhouse gas. Now, though, scientists have found that one of the most abundant methane-producing microorganisms on Earth makes direct electrical connections with another species to produce the gas in a completely unexpected way.

The Methanosaeta species of bacteria are important for several reasons. They are active in methanogenic wetlands and are the most prodigious methane producers on the planet. This, in particular, is a concern since atmospheric methane is 20 times more effective at retaining heat than CO2. This heat, in turn, can cause tundra soils to warm due to climate change and can cause even more methane to be released. In addition, the methane produced in anaerobic biomass digesters is economically important as one of the few proven, large-scale bioenergy strategies.

Scientists knew from previous studies that Geobacter, a microorganism that can break down more complex substrates to compounds that methane-producing bacteria can use, can grow electrically conductive filaments known as nanowires. These nanowires can transport electrons outside the cell to make electrical connections with minerals, electrodes or other cells. Methanosaeta were the dominant methane-producing microorganisms in the digesters and known to convert acetate to methane. Analysis of the gene expression, though, showed that Methanosaeta expressed genes for converting carbon dioxide to methane. This led to the conclusion that Geobacter were feeding Methanosaeta electrons through their nanowires to promote the production of methane from CO2. This transfer via microbial nanowire was dubbed "direct interspecies electron transfer," or DIET.

The discovery of DIET actually challenges the concept held for decades that natural methane-producing microbial communities primarily exchange electrons through the production and consumption of hydrogen case. DIET is much more direct and potentially more efficient.

"Now we need to improve predictions of how methane-producing microbial communities will respond to climate change," said Derek Lovely, one of the researchers, in a news release. "Microbial communities using DIET may react much differently than those that rely on hydrogen exchange."

The findings are published in the journal Energy and Environmental Science.