The production of methane by diverse species of microorganisms is among the most important biological processes on Earth. Remarkably, the mechanism through which such a vital reaction occurs has been unknown until now. The discovery may lead to ways to enhance the speed with which the reaction occurs, when that is what we want, and possibly to reverse it.
Methanogenic microbes are both a blessing and a curse for humanity. As the source of 90 percent of the second most important greenhouse gas, they're helping to cook the planet. On the other hand, breaking down plant material into an easy-to-burn gas can be a very useful contribution to biofuel production.
Two methods have been proposed by which microbes could be converting organic carbon to methane: a nickel atom, or a free radical. For many years evidence has been presented for each, but it is only now, with a paper in Science, that biochemists feel confident they have worked out which is correct.
The problem has been that turning carbohydrates to methane is not a single reaction, but a multi-step process, with the first step happening so fast it has been a challenge to work out what is going on, made worse by extreme sensitivity to oxygen.
Dr. Thanyaporn Wongnate of the University Michigan, Ann Arbor, used the chemical equivalent of a slow motion camera to catch the reaction in progress, slowing the process down using low temperatures and a “slow substrate”. Wongnate was able to confirm the methyl radical mechanism. The alternative theory relied on methyl-nickel instead.
After satisfying themselves that the process is the same irrespective of the substrate, Wongnate and her co-authors demonstrated that both mechanisms, and a third one that has also been floated, would produce intermediate chemicals, which would be different in each case. No signs were observed of the intermediate associated with the hypothetical methyl-nickel process. The intermediate for the third possible mechanism was already present in the experiment, but showed no rise in concentration.
On the other hand, multiple observations were consistent with the methyl-free radical path.
In addition to resolving the question of the mechanism bacteria evolved to produce methane, the paper explores how the rate is affected by temperature and other variable factors, and what similarities can be seen in the reverse process when methane is oxidized.
In an accompanying perspective article Northwestern University's Dr. Amy Rosenzweig, and her postdoctoral researcher Thomas Lawton, explain that the work “ends more than two decades of controversy and sets the stage for building a consensus MCR [methyl-coenzyme M reductase] mechanism.”
However, Rosenzweig and Lawton note “several key questions remain unanswered,” including whether the oxidation of methane involves a direct reversal of the process, or takes a somewhat different path. Answers to these questions could lead to commercially viable ways to make and destroy methane synthetically. As Wongate's paper notes, methane-consuming bacteria already oxidize 100 million tonnes (110 million tons) of methane a year, without which the world would be much warmer.