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Increasing temperatures could significantly alter the dynamics in an ongoing biological struggle that has persisted for thousands of years beneath the Earth’s surface. In the wetland soils, microorganisms are engaged in a competition to generate and decompose the potent greenhouse gas methane. A new study conducted by researchers at the Smithsonian, published on April 23, highlights that rising global temperatures may jeopardize the mechanisms through which wetlands manage methane emissions.
Methane is implicated in about 19% of global warming, as reported by the National Oceanic and Atmospheric Administration. While wetlands are recognized for their ability to absorb carbon dioxide (CO2), the more prevalent greenhouse gas, they also serve as the greatest natural source of methane. As countries strive to reduce methane emissions arising from human activities, understanding the natural methane outputs of wetlands and the potential for future increases becomes vital.
“If wetlands are significant sources of methane emissions, and this is overlooked, our goals for carbon reduction aimed at combating climate change may become unachievable,” commented lead author Jaehyun Lee. Lee, who is now affiliated with the Korea Institute of Science and Technology, conducted the research during his time as a postdoctoral fellow at the Smithsonian Environmental Research Center.
The Battle of Microbes
Within the wetland soils, two distinct types of microbes vie for dominance. Some of these microbes produce methane, a greenhouse gas with a warming potential 45 times greater than that of CO2, while others consume methane, converting it into less harmful CO2 in the presence of oxygen. This transformation represents one of nature’s most effective strategies for regulating greenhouse gas emissions.
The research featured in the journal Science Advances delved into a unique category of microbes referred to as anaerobic microbes. These organisms thrive in environments devoid of free oxygen, conditions typical of inundated wetlands. Historically, anaerobic microbes were considered minor players in the methane conflict. Initially, it was thought that, due to a lack of available oxygen, they could not efficiently break down methane. Subsequent studies revealed that these microbes could indeed utilize oxygen from nearby sulfate molecules, though they were still presumed to be less impactful compared to their aerobic counterparts.
“It was believed that anaerobic methane consumption was too slow to make a notable difference,” Lee explained.
However, Lee highlighted that most methane is formed in oxygen-limited environments where anaerobic microbes dominate. Their capacity to moderate methane production was greater than previously acknowledged. In a wetland studied by the Smithsonian, anaerobic microbes were found to eliminate up to 12% of the methane produced—significantly more than scientists had estimated. In sulfate-rich and saline environments, their ability could reach as high as 70% degradation of methane generated in oxygen-poor soils.
Yet, the scenario changed with increased temperatures.
Simulated Climate Conditions
The team behind the recent study created a simulation of a hotter future through an experiment in a wetland at the Smithsonian Environmental Research Center in Maryland, termed “SMARTX” (Salt Marsh Accretion Response to Temperature eXperiment). The research involved increasing the temperature by 5.1 degrees Celsius in select sections of the wetland using arrays of infrared lamps and buried cables. They also elevated CO2 levels to create a more realistic climate model.
“A warmer climate will always be accompanied by higher CO2 levels… SMARTX aims to replicate that future with both aboveground and belowground warming,” elaborated Genevieve Noyce, a coauthor and senior scientist at SERC.
During the study, there was a notable surge in methane emissions linked to higher temperatures alone. Interestingly, the enhanced warmth did not diminish the activity of methane-consuming microbes; instead, it prompted them to extract even greater amounts of methane. However, their methane-producing competitors became increasingly active as well, leading to a situation where methane removal could not keep pace with production.
The extent of increased methane emissions varied based on vegetation types. In areas dominated by dense sedges, emissions surged almost fourfold, while regions with smaller grasses experienced a more modest increase of 1.5 times.
Notably, the added CO2 appeared to mitigate some of the surge, though it did not completely counteract it. In sedge-dominated plots subjected to both heightened temperatures and CO2, methane emissions only doubled compared to normal levels, rather than exhibiting a nearly fourfold increase. Researchers believe this is attributed to CO2 promoting the growth of deeper root systems, which in turn introduce more oxygen into the soil, enhancing the availability of sulfate compounds for microbial use.
“While warming greatly accelerates methane emissions, the presence of elevated CO2 slightly tempers this effect,” Noyce noted.
This phenomenon appears consistent across various microbial communities within the wetland ecosystem. Previous findings from 2021 indicated a similar pattern: as temperatures rise, both aerobic and anaerobic microbes struggle to keep methane levels in check relative to their methane-producing counterparts.
Despite the challenges posed by methane emissions, conserving wetlands remains crucial for climate change mitigation. These ecosystems serve as essential buffers against hurricanes and extreme weather events, and they are proficient at sequestering carbon in various forms. Remarkably, a single acre of coastal wetland can trap more carbon dioxide than an acre of tropical rainforest.
“The significance of protecting and restoring coastal wetlands for climate benefits is profound, especially regarding the multitude of ecosystem services they offer to humanity,” stated Pat Megonigal, the senior author and associate director of research at SERC.
However, for effective future planning, policymakers require accurate estimates of how much methane wetlands will release in the coming decades. Ultimately, according to Lee, addressing climate change necessitates a focus not just on rising temperatures, but also on how these changes may influence delicate microbial interactions that govern greenhouse gas emissions.
“We must consider how climate change will impact these intricate microbial processes, including both methane production and oxidation,” he emphasized.
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