The Secret Behind Biogenic Methane Discovered at Stanford

Stanford University researchers have taken a quantum leap in discovering how methanogens acquire electrons from solid surfaces, how hydrogen producing enzymes are important in Methanogenesis, how certain enzymes take up electrons and many other interesting facts.

Methane, the main component (87%) of natural gas, a fossil fuel, which can be found both underground and under the sea floor, is accumulated due to methanogenesis, a form of anaerobic respiration used by single-celled microorganisms called methanogens. Methanogenesis, a chemical reaction (CO₂ + 8 H⁺ + 8 e⁻ → CH₄ (methane) + 2 H₂O) of carbon dioxide reduction with hydrogen as a reducing agent, occurs in low oxygen conditions and produces methane.

Most of the naturally occurring methane is biogenic from methanogens, including human flatulence (as methanogens present in our intestines produce methane as they aid our digestion). They are the key agents in the remineralization of organic carbon. Methanogens also play an indispensable role in biogas production and anaerobic sewage treatments.

But, combustion of this fossil fuel emits carbon dioxide which can cause climate change. At this point, Methanogens offer a promising solution. Feeding the carbon dioxide (emitted into the atmosphere due to the burning of fossil fuels) to methanogens along with a source of electrons creates a cycle of “renewable methane” as long as methanogens exist.

Researchers are trying to develop large bioreactors where billions of methanogens crank out methane around the clock. These microbial colonies would be fed carbon dioxide from the atmosphere and clean electricity from electrodes, notes Stanford News.

Though the methanogens are supplied with CO₂ and electrons, the question, how those electrons get into the methanogen cell has been a mystery to scientists. Jörg Deutzmann, the lead author of the study, said: “Right now the main bottleneck in this process is figuring out how to get more electrons from the electrode into the microbial cell,”. “To do that, you first have to know how electron uptake works in methanogens. Then you can engineer and enhance the electron-transfer rate and increase methane production.”

For the study, the team used Methanococcus maripaludis, a species of methanogen, which were grown in flasks fitted with a graphite electrode that acted as the source of electrons. During the experiment, researchers observed a build-up of hydrogen gas along with methane and are wondering if those molecules of hydrogen were the ones that were shuttling electrons to the methanogens, as occurs in nature.

To find out, scientists repeated the experiment after switching normal microbes with a genetically engineered strain of Methanococcus maripaludis, which after mutation, could no longer produce the enzyme hydrogenase that the microbes need to make hydrogen.

Alfred Spormann, professor and co-author of the study said, “When hydrogenase was absent from the culture, methane production plummeted 10-fold,”. “This was a strong indication that hydrogen-producing enzymes are significantly involved in electron uptake,”.

They found that the methane output reduced significantly. Hydrogenase and other enzymes were found to take up electrons directly from the electrode surface in the absence of methanogen cells, but, as widely assumed, the microbial cell itself is not involved in the transfer.

Deutzmann, said: “It turns out that all kinds of enzymes are just floating around in the culture medium. These enzymes can attach to the electrode surface and produce small molecules, like hydrogen, which then feed the electrons to the microbes. Now that we know that certain enzymes take up electrons, we can engineer them to work better and search for other enzymes that do it even faster.”

Spormann envisioned “The overall goal is to create large bioreactors where microbes convert atmospheric carbon dioxide and clean electricity from solar, wind or nuclear power into renewable fuels and other valuable chemicals. Now that we understand how methanogens take up electricity, we can re-engineer conventional electrodes to deliver more electrons to more microbes at a faster rate.”

The findings of the research could help in designing electrodes for microbial ‘factories’ that produce methane gas and renewable fuels sustainably. The research also provides new insights on microbially influenced corrosion, a biological process that threatens the long-term stability of structures made of iron and steel. “Biocorrosion is a significant global problem,” Spormann said. “The yearly economic loss caused by this process is estimated to be in the $1 billion range”.

Image (c): Stanford News