Hungry, hungry germs in tree bark gobble up methane


“When I saw this article, I just said, ‘Holy shit, this is really interesting,’ says Jeffrey White, professor emeritus at the O’Neill School of Public and Environmental Affairs at the University of London. ‘Indiana. White, who was not involved in the study, has studied the methane cycle for more than 30 years and says it gracefully responded to a hunch the researchers had – but were unable to pinpoint – that the activity of the methanotrope occurs in the bark of trees. He describes this work as “deeply important”.

Methanotrophs are everywhere and have been around for as long as atmospheric oxygen has existed on Earth, so White is convinced this is not an isolated case: he has noticed similar behavior in birch trees in Minnesota.

Wetlands bring more methane to the atmosphere than any other natural source. But without methanotrophs, they would go out about 50 to 90 percent After. These microbes convert methane into carbon dioxide just like combustion does. The process is, almost literally, a slow burn. But it prevents most wetland methane from reaching the sky, making the soil a source and sink. Much less is known about the methane parties that take place inside trees.

Jeffrey wanted more clarity. A few years ago his attention turned to paper bark. “It’s such a unique tree with amazing layers of bark,” says Jeffrey. These layers are wet, dark, and known to contain methane. (Jeffrey sometimes calls it “treethane.”) “We just thought this might be a great place for methanotrophs,” he continues. He therefore sought to prove that the gas-eating microbes were hiding there. Jeffrey designed a series of experiences that would meet their appetites. First, he cut the bark of trees in three wetland sites and sealed those strips in glass bottles containing methane. Then he waited. For a week, he measured the drop in methane levels in the bottles. In some samples, more than half has disappeared. In control vials containing either sterilized bark or nothing at all, methane levels remained flat.

Photography: Luke Jeffrey

Jeffrey’s team also knew that methanotrophs have difficult palates. A carbon atom in methane can exist as two stable isotopes: the classic carbon-12 or the heavier carbon-13 which lags an extra neutron. Carbon-13 bonds are more difficult to break, so methanotrophs prefer to nibble on the lighter isotope. Jeffrey’s team found that the relative levels of carbon-13-methane in the bottles increased over time. Something in the bark was alive and selectively eating, like a child leaving the yellow Starbursts in the bag after choosing the roses.

Encouraged by these traces of activity, they sent bark across town to microbiologists at Monash University, who performed microbial analysis of all the species that lived in the bark. The verdict: The paper bark samples contained a unique, lively population of bacteria not found in the surrounding soil or swamps, most of which belong to the methane-starved genus. Methylomonas.

But all of these results happened in a lab, and Jeffrey’s team needed to see how real living trees behave, especially how quickly they leak methane. They waded through a humid New South Wales forest, gently attached sealed chambers and spectrometers alongside the paper bark, and measured how much trees were emitting per second.

Then Jeffrey injected a gas called difluoromethane into the chamber. Difluoromethane is a sneaky treat for methanotrophs – it temporarily suppresses their appetite. “It actually prevents them from consuming methane,” says Jeffrey. After allowing the gas to diffuse for an hour, Jeffrey flushed it out and reconsidered the emissions. Because the microbes stopped eating, the methane levels spiked. On average, the team calculated that the microbes had removed 36% of the methane that would otherwise enter the atmosphere.



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