Though they occupy a small fraction of Earth’s surface, freshwater wetlands are the largest natural source of methane emitted into the atmosphere. New research identifies an unexpected process that acts as a key gatekeeper in regulating methane emissions from these freshwater environments.
The study results are published this week in the journal Nature Communications by biologist Samantha Joye of the University of Georgia and colleagues.
The researchers report that high rates of anaerobic (no oxygen) methane oxidation in freshwater wetlands substantially reduce atmospheric emissions of methane.
The process of anaerobic methane oxidation was once considered insignificant in freshwater wetlands, but scientists now think very differently about its importance.
“Some microorganisms actually eat methane, and recent decades have seen an explosion in our understanding of the way they do this,” says Matt Kane, program director in the National Science Foundation’s Division of Environmental Biology, which funded the research. “These researchers demonstrate that if it were not for an unusual group of methane-eating microbes that live in freshwater wetlands, far more methane would be released into the atmosphere.”
Although anaerobic methane oxidation in freshwater has been gathering scientific attention, the environmental relevance of this process was unknown until recently, Joye says.
“This paper reports a previously unrecognized sink for methane in freshwater sediments, soils and peats: microbially-mediated anaerobic oxidation of methane,” she says. “The fundamental importance of this process in freshwater wetlands underscores the critical role that anaerobic oxidation of methane plays on Earth, even in freshwater habitats.”
Without this process, Joye says, methane emissions from freshwater wetlands could be 30 to 50 percent greater.
Comparison of wetlands
The researchers investigated the anaerobic oxidation process in freshwater wetlands in three regions: the freshwater peat soils of the Florida Everglades; a coastal organic-rich wetland in Acadia National Park, Maine; and a tidal freshwater wetland in coastal Georgia.
All three sites were sampled over multiple seasons.
The anaerobic oxidation of methane was coupled to some extent with sulfate reduction. Rising sea levels, for example, would result in increased sulfate, which could fuel greater rates of anaerobic oxidation.
Similarly, with saltwater intrusion into coastal freshwater wetlands, increasing sulfate inhibits microbial methane formation, or methanogenesis.
So while freshwater wetlands are known to be significant methane sources, their low sulfate concentrations previously led most researchers to conclude that anaerobic oxidation of methane was not important in these regions.
The new findings show that if not for the anaerobic methane oxidation process, freshwater environments would account for an even greater portion of the global methane budget.
“The process of anaerobic oxidation of methane in freshwater wetlands appears to be different than what we know about this process in marine sediments,” Joye says. “There could be unique biochemistry at work.”
Adds Katherine Segarra, an oceanographer at the U.S. Department of the Interior’s Bureau of Ocean Energy Management and co-author of the paper: “This study furthers the understanding of the global methane budget, and may have ramifications for the development of future greenhouse gas models.”