Tree bark microbiome has important overlooked role in climate

The bark of a single tree can harbor billions of bacteria, and these microbes could play an important but overlooked role in controlling greenhouse gases in Earth’s atmosphere.

The total area of ​​tree bark on the planet is estimated to be about 143 million square kilometers, almost as much as the total land area of ​​the planet. This surface constitutes a huge microbial habitat known as the caulosphere, but the microbes that live there have received little attention from scientists.

“In some ways it’s so obvious, but we’ve always neglected the bark of trees,” says Bob Leung of Monash University in Melbourne, Australia. “We’ve never thought about microbes on the bark of trees, but it makes sense, because bacteria are everywhere, and if we can find microbes in the soil, on the leaves of trees, there will most likely be microbes on the bark.”

Leung and his colleagues began by studying a wetland species commonly called paperbark (Melaleuca quinquenervia). They found that there were more than 6 trillion bacteria living in or on each square meter of bark, comparable to the volumes found in soil.

Genetic analysis of 114 of these bacteria showed that they mostly came from three bacterial families – Acidobacteriaceae, Mycobacteriaceae and Acetobacteraceae – but that all species were completely unknown to science.

Surprisingly, these microbes have one thing in common: they can use hydrogen, carbon monoxide and methane as fuel to survive. Hydrogen (H₂) is not in itself a greenhouse gas, but through its reactions with other molecules it can increase the warming effect of methane in the atmosphere.

The researchers then examined the bark of seven other Australian tree species from a range of habitats, including casuarinas, gums and banksias, measuring, both in the field and in the laboratory, whether the bark of the different species absorbed or emitted greenhouse gases.

They found that all the barks consumed hydrogen, carbon monoxide and methane under aerobic conditions when oxygen was available. But when trees are submerged in water and oxygen is limited, such as in swamps, microbes in the bark begin to produce the same gases.

The team estimates that the total amount of hydrogen absorbed by bark microbes globally is between 0.6 and 1.6 billion kilograms each year, representing up to 2% of the total atmospheric hydrogen removed.

This is the first time scientists have attempted to assess the contribution of tree bark to atmospheric hydrogen, says team member Luke Jeffrey from Southern Cross University in Lismore, Australia.

“Uncovering the hidden role of trees that do more than just capture carbon dioxide in their wood is very important,” says Jeffrey. “They are actively recycling other greenhouse gases. This is exciting, because H₂ affects the lifespan of methane in our atmosphere, so H₂ Eating bark can help reduce our growing methane problem.

However, the global situation is very uncertain, as the team only sampled eight tree species from eastern Australia. “Much work remains to be done now on various forest types, tree species, microbial communities and site conditions,” says Jeffrey.

Brett Summerell of the Sydney Botanic Gardens says the study highlights how little we know about the composition, diversity, abundance and role of microorganisms in the bark. “How this can vary across a wider range of tree species, particularly in drier climates such as savannahs and forests, is interesting,” says Summerell.

It will also be important to understand the interactions between fungi and bacteria present in the bark, he adds.