A five-year study conducted by Australian scientists has identified that microbial communities residing on tree bark actively consume atmospheric gases, including hydrogen, carbon monoxide, and methane. This discovery expands the understanding of trees' role in influencing climate and air quality beyond their known ability to absorb carbon dioxide. Researchers from Southern Cross and Monash universities, whose findings were published in the journal Science, suggest that this microbial activity, occurring on an estimated global bark surface area equivalent to all continents combined, could contribute to climate change mitigation and air purification efforts.
Research Uncovers Microbes' Role
The research, led by Dr. Bob Leung from Monash University's Biomedicine Discovery Institute (BDI) and involving Luke Jeffrey from Southern Cross University and Professor Chris Greening of BDI, focused on microbial communities on tree bark, referred to as the 'caulosphere' or 'barkosphere'. Scientists had largely overlooked the presence and functions of these microbial cells on tree bark for decades, previously considering bark biologically inert regarding climate influence.
The study examined eight common Australian tree species across various sites in eastern Australia, including freshwater wetlands, upland forests, and mangroves. Researchers employed metagenomic sequencing to map the genes in bark samples, revealing that bark microbial communities differed from those in nearby soil and water and possessed genes for gas metabolism. Real-time measurements of gas movement and experiments with sealed bark strips in bottles (microcosms) confirmed the active consumption of atmospheric gases by these microbial communities. An estimated 6 trillion microbial cells per square meter were found on species like wetland paperbarks.
Gas Consumption and Environmental Impact
The microbes engage in 'aerotrophy,' a process of consuming various atmospheric gases.
-
Hydrogen (H): A significant finding was the consistent removal of hydrogen from the air by bark microbes across all examined tree species, forest types, and stem heights. Three bacterial families—Acidobacteriaceae, Mycobacteriaceae, and Acetobacteraceae—were identified as predominant and capable of consuming hydrogen. Global estimates for hydrogen consumption vary. Some projections indicate annual consumption between 0.6 and 1.6 billion kilograms, representing approximately 2% of the total global hydrogen output. Other calculations suggest that tree-bark microbes could remove up to 55 million tonnes of hydrogen from the atmosphere annually, a process estimated to indirectly offset up to 15% of human-induced annual methane emissions by influencing methane's atmospheric lifespan.
-
Carbon Monoxide (CO): Abundant microbial enzymes capable of oxidizing toxic carbon monoxide into carbon dioxide were identified. This process suggests that bark contributes to air purification, particularly in urban environments, by helping to eliminate pollutants.
-
Methane: Methanotrophs, bacteria that consume methane, were present in most wetland trees. While experiments showed methane consumption when bark was exposed to elevated methane levels (similar to those found inside tree stems), minimal methane uptake was observed at normal atmospheric levels.
The research notes that the removal of hydrogen and carbon monoxide by bark microbes can free up the hydroxyl radical, a natural atmospheric cleanser, to break down methane, potentially mitigating atmospheric warming. The climate benefits from these microbial processes are noted to vary based on bark moisture and time of day. Internal stem gases and volatile organic compounds (VOCs) released by trees, such as methanol, may also serve as nutrient sources for these microbial communities, allowing them to sustain themselves even when external air provides limited metabolic fuel. Different bark textures were observed to support distinct microbial populations.
Global Scale and Future Implications
With an estimated 3 trillion trees globally and a total bark surface area spanning approximately 143 million square kilometers (or 55 million square miles), the scale of this microbial activity is considered substantial. The study suggests that even small changes in bark chemistry per square foot could significantly influence global gas budgets.
Researchers indicate that this knowledge could inform future reforestation, conservation, carbon accounting strategies, and potentially influence climate change mitigation efforts. Professor Chris Greening suggested that trees containing the most active gas-consuming microbes could be prioritized in reforestation and urban greening initiatives. Urban planning efforts might consider trunk biology alongside other factors like canopy shade and pollution.
Further studies are deemed necessary to confirm these measurements and to ascertain if this pattern of microbial activity is consistent across all tree types and regions worldwide. More comprehensive sampling outside Australia, combined with careful monitoring of factors such as oxygen levels, moisture, and seasonal variations, will be required to improve climate predictions. Biogeochemical models, which currently often treat tree stems as simple gas pathways, may need to incorporate microbial metabolism within bark to more accurately reflect the types and quantities of gases exiting tree trunks.