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“When I saw this article, I said,‘ Holy shit, this is very interesting, ’” says Jeffrey White, an emeritus professor at the O’Neill School of Public and Environment Affairs at Indiana University. White, who did not participate in the study, has been studying methane cycling for more than 30 years and elegantly addressed the hobby that researchers have had, but have not been able to nail down, as methanotrophic activity occurs on tree bark. The work is called “absolutely important”.
Methanotrophs are everywhere and have been around as long as atmospheric oxygen exists on Earth, so White is confident that this is not an isolated case: he has observed similar behavior in birch trees in Minnesota.
Wetlands emit more methane into the atmosphere than any other natural source. But without methanotrophs, they would release approximately 50 to 90 percent more. These microbes convert methane to carbon dioxide, just as combustion does. The process is almost literally a slow burn. But most of the methane in wetlands prevents it from reaching the sky, turning the soil into a source and sink. Much less is known about the methane festivities that take place in the trees.
Jeffrey wanted more clarity. A few years ago, attention was turned to traces of paper. “It’s a unique tree with amazing layers of bark,” says Jeffrey. These layers are known to be wet, dark, and methane. (Jeffrey sometimes calls him “treetano”). “We thought it might be the perfect place for methanotrophs,” he continues. So he set out to prove that the food-eating microbes were hidden in it. Jeffrey designed a series of experiments that would respond to their hunger. First, he split the bark of trees in three wetlands and sealed the strips inside glass bottles containing methane. Then he waited. Over the course of a week, he measured the level of methane in the bottles as it fell. In some samples, more than half disappeared. They had sterilized skin or nothing in control bottles, methane levels remained on paper.
Jeffrey’s team also knew that methanotrophs have sharp palates. A single carbon atom in methane can exist as two stable isotopes: the classic carbon 12 or the heavier carbon 13 that surrounds an extra neutron. The carbon 13 bonds are broken, so methanotrophs would like to make a snack with a light isotope. Jeffrey’s team found that the relative levels of carbon-13-methane in the bottles increased over time. Something on the surface was alive and eating selectively, like a baby leaving yellow Starbursts in a bag after picking roses.
Encouraged by these traces of activity, the skin was sent across the village to microbiologists at Monash University, who performed a microbial analysis of all the species that lived on the skin. Ruling: Paperbark samples contained a noisy population of bacteria not found in the surrounding soil or swamps, most of which fall into the methane-hungry genus. Methylomonas.
But all of these results were created in a lab, and Jeffrey’s team had to see how real, living trees behave, specifically the speed at which methane flows. They walked through the wetlands of New South Wales, slowly glued the chamber and spectrometers to the sides of the paper covers, and measured how many seconds the trees spilled per second.
Jeffrey then injected a gas called difluoromethane into the chamber. Difluoromethane is an abuse of methanotrophs; inhibits temporary hunger. “It actually lets them consume methane,” Jeffrey says. After letting the gas disperse for an hour, Jeffrey cleaned it up and re-examined the emissions. As the microbes stopped eating, the methane level jumped. On average, the team estimated that microbes removed 36% of the methane they emit into the atmosphere.
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