Hidden tree bark microbes munch on important climate gases

Trees are known for capturing carbon dioxide as they grow. But they also soak up other gases implicated in climate change through microbes in their bark.

The tree bark microbes feast on hydrogen, methane and carbon monoxide, researchers report January 8 in Science. Methane is a greenhouse gas 28 times more potent than carbon dioxide over a 100-year period. Carbon monoxide — which is lethal to humans — and hydrogen enhance global warming indirectly, by helping methane persist longer in the atmosphere.

Getting rid of these gases “is a hidden benefit of trees that we previously didn’t realize was happening,” says Luke Jeffrey, a biogeochemist at Southern Cross University in Lismore, Australia.

An estimated 41 million square kilometers of tree bark exist worldwide — about equal to the combined area of North and South America, and about six trillion microbes inhabit every square meter of tree bark, Jeffrey and his colleagues estimate.

This newly discovered bark microbiome was “hidden in plain sight,” says Jonathan Gewirtzman, a forest ecologist at Yale University, who was not part of the project. It “highlights this as an environment that we know so little about.”

These discoveries about the hidden tree bark biome stem from years of research into the sources of methane, which is responsible for about 30 percent of human-caused warming. This gas bubbles up from oxygen-starved microbes living in the waterlogged sediments of lakes and wetlands.

When scientists measured methane percolating up from the flooded lowlands of the Amazon, the amount coming out was only about half what it should be, based on measurements from space. Then in 2017, another team of scientists realized that only half of region’s methane was coming out of the ground. The other half — amounting to 15 or 20 million metric tons per year — was seeping out of Amazonian tree trunks.

People thought the trees were acting as passive chimneys — gushing out soil methane that came in through their roots. But in 2021, Jeffrey and his colleagues discovered a wrinkle.

Working with broad-leaf paper bark trees (Melaleuca quinquenervia) in Australia, the team found that the amount of methane coming out of tree bark was about 35 percent less than what enters from below. They concluded that microbes in the bark were eating it — oxidizing it for energy – as it seeped out.

“That could be a really huge ecosystem service that these microbes are providing” by removing a major greenhouse gas, says Pok Man Leung, an ecophysiologist at Monash University in Clayton, Australia. He and Chris Greening, a microbiologist also at Monash, helped identify the microbes living in the bark of those trees.

In the latest study, Jeffrey, Leung, Greening and colleagues profiled the collective genomes of thousands of microbial species living in paper bark trees and seven other common tree species in Australia. The researchers found that microbes that oxidize hydrogen gas for energy were even more common than the methane-eaters. Microbes that oxidize carbon monoxide were also abundant.

Experiments in live trees showed that bark microbes don’t just eat these gases as they diffuse up through the trees; they also suck in methane, hydrogen, and carbon monoxide from the surrounding air. These gases exist in the atmosphere at only trace levels, ranging from 2 parts per million to 40 parts per billion. But multiplied across the entire world, tree microbes are consuming vast amounts of them – an estimated 25 to 50 million tons of methane alone, according to a 2024 study.

By removing these other climate gases, tree bark microbes enhance the already significant benefits trees provide by absorbing CO₂, Leung says.

Forest restoration remains an important strategy for combating climate change, and this new knowledge could make it more effective. The eight tree species examined in this study had differing mixes of microbes in their bark, eating different amounts of trace gases. This insight could help scientists select the tree species best suited to blunt climate change.

“You’re not just thinking about the tree you’re planting, but also the microbes within the tree,” Greening says. “You can ideally get rid of three or four climate-active gases for the price of one.”