Something new under the sun
AMHERST — For 40 years, scientists thought they understood how certain bacteria work together to anaerobically digest vegetative matter to produce methane gas, important in bioenergy and the major source of greenhouse gas. But now microbiologists in Derek Lovley’s lab at the University of Massachusetts show for the first time that one of the most abundant methane-producing microorganisms on earth makes the gas in a completely unexpected way. It makes direct electrical connections with another species to produce methane.
Lovley and colleagues describe the newly discovered properties of the methane-producing bacterium Methanosaeta in the current issue of the British Royal Society of Chemistry journal, Energy and Environmental Science.
“We discovered that Methanosaeta have the ability to reduce carbon dioxide to methane,” Lovley explains. “They do this by a remarkable mechanism in which they make electrical connections with other microorganisms, something methanogens have never been known to do before.”
There are short-term practical implications. “Once you realize that there are methane producers that can directly feed on electrons, you start thinking differently about how to optimize methane production from wastes,” the microbiologist notes. “Although generating methane from wastes is one of the oldest bioenergy strategies and is practiced even in small villages in developing countries, its application on a large scale has been limited because it is slow.”
Methanosaeta species are important for a couple of reasons, Lovley and his co-authors point out. They are so active in methanogenic wetlands that they are considered the most prodigious methane producers on the planet. This is a concern because atmospheric methane is 20 times more effective at retaining heat than CO2, and as tundra soils warm due to climate change, even greater methane releases are expected. Also, methane produced in anaerobic biomass digesters is economically important as “one of the few proven, economical, large-scale bioenergy strategies” in use today, they say.
Methane-producing microbial communities have been studied for decades, Lovley notes, “but all this time we were missing a major pathway of methane production.” His group’s study of Methanosaeta started when they found that digesters converting brewery wastes to methane contained large quantities of the microorganism Geobacter. Geobacter cannot produce methane, but it does break down more complex substrates to compounds that methane-producing bacteria can use.
The UMass teams knew from previous studies that Geobacter grow electrically conductive filaments known as microbial nanowires, which 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, but analysis of the gene expression in the digester revealed that Methanosaeta were also highly expressing genes for converting carbon dioxide to methane. The researchers speculated that Geobacter were feeding Methanosaeta electrons through their nanowires to promote Methanosaeta’s methane production from CO2.
Further studies in which individual Geobacter and a Methanosaeta species were cultured together confirmed these suspicions, Lovley says. They dubbed this transfer via microbial nanowire “direct interspecies electron transfer,” or DIET.
Lovely says the discovery of DIET challenges the concept held for decades that natural methane-producing microbial communities primarily exchange electrons through the production and consumption of hydrogen gas. DIET is a much more direct, and potentially more efficient, mechanism for feeding electrons to methane-producing bacteria. “Now we need to improve predictions of how methane-producing microbial communities will respond to climate change. Microbial communities using DIET may react much differently than those that rely on hydrogen exchange,” he says.