Warming temperatures may cause methane emissions from wetlands to rise — by helping methane-producing bacteria thrive. Higher temperatures favor the activity of wetland soil microbes that produce the potent greenhouse gas, at the expense of other microbes that can consume it, researchers report April 23 in Science Advances.
The scientists, led by microbiologist Jaehyun Lee of the Korea Institute of Science and Technology in Seoul, conducted a summer field study in coastal wetlands near the Chesapeake Bay, analyzing soil conditions in a set of marshy plots with differing environmental conditions. The findings may offer clues to a puzzling and worrisome spike in wetland emissions of methane over the last decade.
From above, the coastal wetlands off the Chesapeake Bay are peaceful, stalks of marsh grasses and sedges waving gently in the wind.
But below the surface, microbes in the mud are engaged in a fierce, albeit tiny, chemical tug-of-war for food. Some of these microbes produce methane; others consume it. Which microbes thrive can determine how much of the greenhouse gas escapes the soil to make the planet’s atmosphere hotter.
The water-inundated soils of coastal or inland wetlands are oxygen-poor, and in these conditions, methane-producing microbes can thrive, munching on organic carbon in the soils to generate the gas. Alongside them, other populations of microbes snag some of that methane, oxidizing the gas back into carbon dioxide before it wafts into the atmosphere.
That balance between methane production and consumption can keep emissions of the gas from wetlands in check. But rising temperatures, and rising CO2 concentrations, may be tipping the scales, shifting the biogeochemistry of wetlands and altering the relative microbial activity, says study coauthor Genevieve Noyce, a biogeochemist with the Smithsonian Environmental Research Center, or SERC, in Edgewater, Md.
“The microbes are always there, but they’re only active when they have the substrate [or fuel source] available to them,” Noyce says.
In the brackish Chesapeake Bay marshes, one of the primary substrates available to the microbes is sulfate, a molecule in seawater that periodically flushes in with the tide. So which microbes are more active depends on who gets to the sulfate first.
To test how that competition might change with future warming, the team cordoned off a series of 18 plots within the research center’s brackish wetlands. Each 2-meter square was given different environmental parameters, including vegetation type, temperature and ambient CO2 concentration.
Two main types of native plants are rooted in the muddy soil of these tidal flats: smooth-bladed salt marsh grasses and triangular-stemmed sedges. These two plants use different photosynthetic pathways, which respond differently to changing atmospheric CO2 concentrations.
To fully assess the possible conditions, one set of plots contained the grasses, and the other contained sedges. Heat lamps aimed at different plots adjusted the air temperature over the land, with the warmest plots always about 5 degrees Celsius hotter than the control plots; belowground, warming cables also kept the soil at the desired temperature. In several enclosed plots, the team piped in additional CO2 to simulate likely future Earth conditions.
Analyses from the soils of the warmest plots, with no CO2, confirmed that under warming conditions alone, the methane-producing bacteria were able to snag sulfate faster, leaving less for the methane consumers. To the team’s surprise, the added CO2 actually counteracted the warming trend somewhat, Noyce says, by encouraging the conversion of hydrogen sulfide back to sulfate, offering a bit more food for the methane consumers.
Right now, coastal marshes are the largest natural source of methane to the atmosphere. But, all things considered, wetlands are still a carbon sink overall: The thick soils sequester large amounts of carbon. And coastal wetlands can also act as shields, buffering coastal communities against the impacts of rising sea levels and powerful storm surge from cyclones.
But recent research has identified a worrisome trend: an uptick in wetlands’ emissions of methane over the last decade, with strong spikes in 2013 and again in 2020. “It’s clear that many of our current models of wetlands seem to be underestimating the emissions,” says Euan Nisbet, a geochemist at Royal Holloway, University of London in Egham, who was not involved in the new study. “We don’t have a good understanding of how [soils’ methane uptake] will vary with climate change.”
These findings offer a valuable clue, by highlighting the role that sulfate plays in those emissions, information that researchers can use to better estimate sources and sinks of methane in the future, Nisbet says.
Identifying what helps methane-consuming bacteria thrive could also offer clues to how to reduce those emissions.
The study fills in one piece of the puzzle, Noyce says. But “you can’t actually predict what’s going to happen until you understand all the little pieces.”