The surfaces of the tree trunks are home to miniature ecosystemsLéo Ramos Chaves / Pesquisa FAPESP
Bacteria that inhabit tree bark appear capable of absorbing one of the most significant greenhouse gases, methane (CH4), according to an article published in Nature at the end of July. This is an important finding because over the past decade, measurements of gases that contribute to global warming have indicated that the Amazon rainforest could be contributing to the problem, rather than being part of the solution. The discovery of these new participants suggests a more complex situation than it seems, in addition to presenting potential new weapons in the battle against the global damage aggravated by human activity.
The results were based on samples collected in the Amazon since 2013 by an international group led by Brazilian biologist Alex Enrich Prast at the Federal University of São Paulo (UNIFESP), in partnership with a group led by British biologist Vincent Gauci, from the University of Birmingham in the UK. “We used buckets to measure methane fluxes in the rainforest, while others monitored them with airplanes,” says Prast. In addition to his own work, he is referencing a study led by Luciana Gatti, a chemist from the Brazilian National Institute for Space Research (INPE), during which air samples were collected by airplane from different regions of the Amazon, detecting a higher volume of emissions than expected between 2011 and 2013 (see Pesquisa FAPESP issues 217, 219, and 287).
– Injecting particles into the atmosphere could temporarily reduce global warming
– The world has been around 1.5 °C warmer for over a year
– Global warming increased droughts and heat by 40% during June fires in the Pantanal
The fieldwork carried out by the researchers provided an explanation: the methane formed in the oxygen-depleted soil of flooded areas is processed by bacteria on the roots of trees, which act as chimneys that release the harmful gas into the atmosphere. The combined efforts of the two groups found that the trees in these floodplains emitted the same amount of methane as all the oceans on Earth, which they described in a 2017 article in Nature, raising an alarm about the rainforest’s role in global warming.
Undeterred by the worrying results, Prast, Gauci, and other scientists continued lugging their equipment in and out of the forest, soon realizing that the trees often do the opposite of the previous conclusions: they absorb more than they emit, acting as methane sinks. This happens in the floodplains themselves, when they are not flooded and have oxygen in the soil, but also—and mainly—in areas of rainforest that do not flood.
The reason remained unclear. To figure it out, the biologists attached gas monitoring devices onto trees at different heights, which showed that the trunks absorb CH4. To be precise, it was the microbiota on the tree trunks, which are therefore classified as methanotrophic, meaning they consume methane. “We saw that the level of absorption is greatest at the highest part of the trunk,” adds Prast. Absorption also occurs in the floodplains, but it does not impact the total emissions balance during the wet season due to the methane produced in the oxygenless soil.
When trees on floodplains are underwater, they channel methane from the soil into the airLéo Ramos Chaves / Pesquisa FAPESP
The researchers also collected samples of the wood from different heights, from which they then extracted DNA. “We identified bacteria that oxidize methane in the microbiota of the trunk.” They also found that there are differences, for example, in the microscopic communities of smoother or rougher barks. It will therefore be important to characterize the composition in different plant species—something that has not yet been done due to the difficulty of identifying all the trees amid the diversity of the South American rainforest.
The measurements were taken at the Lago do Cuniã Sustainable-Use Reserve in Rondônia, on the banks of the Madeira River, and approximately 130 kilometers (km) northeast of Porto Velho. The calculations indicate that carbon absorption by trunk surfaces in mature forests is equivalent to 15% of the average total carbon absorption by plant biomass in the Amazon, which is a significant amount. Prast adds that although it is not yet possible to know the exact amount, the trees absorbed more methane than the soil, the microbiota of which was until now believed to play the greatest role in this gas cycle. He also points out that the methane flux in leaves—which are also home to an entire microscopic ecosystem—is not high.
The study included similar analyses of Gigante Forest at the Barro Colorado Island Research Station in Panama, Wytham Woods in the United Kingdom, and the Skogaryd hemiboreal coniferous forest in Sweden. A comparison of these ecosystems showed a clear gradient associated with temperature. Tree trunks absorb more methane in warmer climates—the Amazon and Gigante—than in the British and particularly the Swedish forests. “This difference is probably related to the microbiota’s ability to survive at different temperatures,” suggests Prast.
Even immature forests with smaller trees have a large surface area capable of harboring bacteria. Understanding the role it plays reinforces the importance of reforestation to mitigate greenhouse gas emissions. The Nature article estimated a benefit corresponding to 7% absorption in temperate forests and 12% in tropical forests, which would represent an extra 10% on top of the advantage that had already been calculated for expanding forests.
Devices attached to trees at various heights measure gas exchangesNathalia Bulcão Soares / UFRJ
Jean Ometto, an agronomist from INPE, is pleased that forest restoration could have a substantial additional climate benefit. “Reducing anthropogenic methane concentrations in the atmosphere, due to the dynamics and lifetime of the gas, is of enormous importance to achieving the goals of the Paris Agreement,” says Ometto, who did not take part in the study, referring to the international treaty signed in 2015. Methane has a short atmospheric lifespan of about 10 years, while CO2 remains for more than a century. Even so, CH4 has a greater capacity to heat up the Earth due to the way its molecular structure reacts with solar radiation.
Ometto also warns of the need to better understand how gas fluxes occur within forests. The researcher, who specializes in greenhouse gases, explains that the methane circulating near the trunks primarily comes from forest fires, but also from the biotic activity of communities of anaerobic microorganisms living in the ecosystems of the trunks and the soil. Identifying this cycle together with the gases from the atmosphere is no simple task.
In recent years, Prast and his colleagues have taken periodic measurements in different regions of the Amazon to better understand the role of the forest, since tree biomass varies greatly depending on location. To reach any comprehensive conclusions, however, more research groups need to get involved. “The Amazon is the size of the whole of Europe, plus some more,” points out the UFRJ biologist. He enjoys comparing the difficulty of getting to Cuniã and setting up camp there (an easily accessible place by Amazon standards) with working in Skogaryd, Sweden, which can be reached via a short journey by road. “And they go back home to sleep when they’re done.”
He highlights that the knowledge of tree trunk microbiota came from something negative: the methane emitted by the forest, placing it in the role of a villain. “This new field of science would not have advanced if we had not paid attention to this problem.”
“The consumption of methane by tree bark microbiota significantly alters the gas balance,” says Brazilian agricultural engineer Júlia Gontijo, a postdoctoral researcher at the University of California, Davis, with a group led by Brazilian agricultural engineer Jorge Rodrigues. Gontijo recently published an article in the journal Environmental Microbiome in which she analyzed the methanotrophic capacity of the soil microbiome in forests on floodplains and dry land in the Amazon near Santarém, Pará, as part of her doctorate at the Center for Nuclear Energy in Agriculture at the University of São Paulo (Cena-USP). She incubated soil samples from the study areas and simulated the wet and dry seasons and temperature increases from climate change projections. Although the forest floor on dry land normally acts as a methane sink, Gontijo observed that the level of absorption decreases as temperatures rise. In the floodplain soil, there were no significant changes in microbial behavior. “These microorganisms naturally deal with major fluctuations in the environment, such as flooding, and appear more capable of handling these changes,” she notes.
She is enthusiastic about the possibility of sequencing the genomes of the microbiota in tree trunks and understanding, in depth, which organisms are present and how their composition varies depending on the environment. “Methanotrophs are my favorite because they could help us in the future.” She is currently studying genetic material and metabolic indicators in Amazonian soil samples to investigate microbial action. “The composition of the microbiota does not reveal everything, because a microorganism could be present, but dormant,” she explains. Later, she also intends to infer the activity of these organisms by sequencing their RNA.
The story above was published with the title “Methane consumers” in issue 343 of September/2024.
Project
US-Biota Dimensions – Sao Paulo: Collaborative research: Integrating dimensions of microbial biodiversity across areas of land-use change in tropical forests (n° 14/50320-4); Grant Mechanism Thematic Project, Biota Program; Agreement NSF Dimensions of Biodiversity; Principal Investigator Tsai Siu Mui (USP); Investment R$4,199,250.78.
Scientific articles
GAUCI, V. et al. Global atmospheric methane uptake by upland tree woody surfaces. Nature. Online. July 24, 2024.
GONTIJO, J. B. et al. Methane-cycling microbial communities from Amazon floodplains and upland forests respond differently to simulated climate change scenarios. Environmental Microbiome. Vol. 19, 48. July 17, 2024.
PANGALA, S. R. et al. Large emissions from floodplain trees close the Amazon methane budget. Nature. Vol. 552, pp. 230–4. Dec. 4, 2017.
