Hungry microbes on the bark of trees, hungry to swallow methane
“When I saw this article, I said,‘ Holy shit, this is very interesting, ’” says Jeffrey White, professor emeritus at O’Neill School of Public and Environmental 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 Minnesota birches.
Wetlands provide more methane to the atmosphere than any other natural source. But without methanotrophs, they would release approximately 50-90 percent more. These microbes convert methane to carbon dioxide in the same way that combustion does. The process is almost literally slow burning. 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-rich. (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 methane levels in the bottles as they 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 carbon atom in methane can be present in two stable isotopes: the classic carbon 12 or the heavier carbon 13 that surrounds an extra neutron. Carbon 13 bonds are more difficult to break, so methanotrophs prefer to snack on a light isotope. Jeffrey’s team has seen relative levels of carbon-13-methane in bottles increase over time. Something on the surface was alive and eating selectively, like a child who left the yellow Starburst in the bag after picking the pink ones in the bag.
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. Judgment: The Paperbark samples contained a noisy population of bacteria not found in the surrounding soil or swamps, most of which fall into the hungry methane genus. Methylomonas.
But all of these results were created in a lab, and Jeffrey’s team had to see how straight, real trees behave, exactly the speed at which they emit methane. They walked through the wetlands of New South Wales, gently 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, it 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 group estimated that microbes removed 36% of the methane they emit into the atmosphere.