Microbes in Tree Bark Cycle Greenhouse Gases

The cracked, hard bark of woody plants carries more than just lichen, insects, and years of dust. The seemingly barren layer teems with trillions of microbes, including bacteria, algae, and fungi.¹

Tree bark across the planet spans a surface area roughly similar to that of all terrestrial land, placing these bark-associated microbes all across the globe. Despite this, scientists do not fully understand the metabolism of these microbes and the roles they play in the ecosystem.

Now, researchers led by Monash University microbiologists Pok Man Leung and Chris Greening and Southern Cross University biogeochemist Luke Jeffrey used metagenomics to characterize the bark microbiota of eight common Australian tree species.² They found that bark microbes can cycle the greenhouse gases methane, hydrogen, and carbon monoxide. The results, published in Science, suggest that bark microbiota likely contribute to the climate benefits of trees by regulating gas cycling.

Leung, Jeffrey, Greening, and their team first measured gas concentrations within the bark of one tree species. They observed increased abundance of hydrogen, carbon monoxide, and methane in the stem compared to the atmosphere. They also examined volatile organic compounds (VOCs) in the bark and observed high concentrations of methanol, acetaldehyde, butene, and other compounds commonly produced by microbial communities.

Consistent with this, quantitative polymerase chain reaction using DNA isolated from tree trunks revealed the presence of more than six trillion bacteria per square meter of bark. Sampling seven additional common tree species indicated similar abundance of microbes in the trunk.

To profile the composition and metabolic functions of microbes in this community, the researchers carried out metagenomic sequencing. Metagenomic analyses indicated that the microbes encoded several enzymes that would help them metabolize the high concentrations of gases and VOCs in tree bark.

They noticed that bark microbes exhibited a wide variety of enzymes to break down hydrogen gas. This suggested that the microbes could metabolize gases under variable environmental conditions. This could include periods of oxygen abundance—such as during the day—as well as oxygen scarcity—such as such as in water-saturated conditions or at night.

To test this hypothesis, the researchers placed strips of tree bark into bottles and flushed them with air either containing or lacking oxygen. They analyzed the flux of gases in each case and observed that bark microbes consumed hydrogen, carbon monoxide, and methane under oxic conditions. During anoxic conditions, the bark microbes produced these gases, indicating that tree trunks are biogeochemical hotspots for climate-active gas cycling.

“Collectively, these results suggest that trees and their microbiota contribute to regulating global atmospheric cycles and should be considered in biogeochemical models, forest management, and conservation efforts,” the authors wrote in the paper. They further noted that these properties should be considered in forest management to optimize plantation of tree species that could remove greenhouse gases under local conditions.

“Future work should extend these findings by examining the role of trees as sources and sinks of trace gases beyond the few species and narrow geographic location of this study,” wrote Vincent Gauci, a University of Birmingham biogeochemist who was not associated with the study, in a related perspective article.³