Mortality of Seagrass Meadows May Not Kill Their Methane Release

Vegetated coastal ecosystems, including seagrass meadows, mangrove forests, and salt marshes, are some of the world’s most efficient ecosystems at converting carbon dioxide (CO₂) into biomass. Their oxygen-poor sediments slow the decay of organic matter and allow the carbon-rich material to be sequestered beneath the seafloor. Conservationists advocate using vegetated coastal ecosystems to help mitigate climate change through so-called blue carbon initiatives.

Such initiatives are complicated by the ecosystems’ methane emissions. The same low-oxygen environments that allow carbon burial also allow microbes to make methane. Because methane is a greenhouse gas 28–36 times more potent than CO₂, methane released from coastal ecosystems can offset blue carbon storage. Although the amount of emitted methane is highly variable system to system, seagrass meadows are generally carbon sinks and tend to remove more greenhouse gas than they release.

However, new research published in the Proceedings of the National Academy of Sciences of the United States of America has suggested that blue carbon storage provided by seagrass meadows might be more fragile than scientists realized. Researchers, led by microbiologist Sina Schorn of the Max Planck Institute for Marine Microbiology, measured methane production in sediment cores and slurries collected from Mediterranean seagrass meadows off the island of Elba, Italy.

Researchers studied living and dead meadows, as well as disturbed and pristine environments. Previous research already suggested that disturbed seagrass meadows could release between 25% and 40% of their buried carbon as CO₂ within 3 years. Now, Schorn and her colleagues’ research has suggested that dead seagrasses release methane in addition to CO₂.

Salty Methane Should Not Be Ignored

Seagrasses produce and excrete methylated carbon in response to changing salinity. Schorn and her colleagues found that microbes in seagrass meadow sediments produce methane from that carbon.

Methane production from methylated carbon differs from more well known pathways through which microbes make methane. These pathways are inhibited in high-salinity ecosystems because marine water is rich in sulfates. Some microbes that “breathe” sulfate compete for the same compounds from which other microbes make methane. “Breathing” sulfate yields more energy, so the sulfate-using microbes normally win the contest.

“Even though seagrass scientists have been measuring methane processes…since the ‘70s in seagrasses, it’s something that has been there but [been] discounted.”

According to James Fourqurean, a marine ecologist at Florida International University who is not involved in the research, there is a rule of thumb in the blue carbon community that methane production is low in salty environments.

But the methane-producing microbes in seagrass meadows avoid competition with sulfate-using bacteria by consuming methylated carbon instead of the contested compounds. This allows microbes to produce methane in unexpectedly salty habitats. “Even though seagrass scientists have been measuring methane processes…since the ‘70s in seagrasses, it’s something that has been there but [been] discounted,” said Fourqurean. “This [new] paper [suggests] that it should not be discounted.”

Dead Seagrass Meadows Might Continue to Release Methane

The methane released from healthy Mediterranean seagrasses offsets their carbon burial between 4% and 5% because of methane’s higher global warming potential. Schorn and her colleagues found that sections of a meadow that died more than 25 years ago produced about as much methane as healthy meadows did. Because dead seagrasses do not convert CO₂ to biomass, these findings suggest that dead seagrass meadows might become sources of atmospheric greenhouse gases when disturbed.

Human activity has caused seagrass meadows to decline globally. If human activities (such as pollution, dredging, and development) continue to threaten seagrass ecosystems, areas that once stored carbon could start releasing it back to the atmosphere and fuel further climate change.

Expanding the Study

Scientists require further study to determine how much methane is produced by disturbed seagrasses in the long run. Schorn and her colleagues’ research focused on one site at only one time of year.

The study was also limited to one seagrass species, Posidonia oceanica, which forms extensive carbon deposits that can be thousands of years old. According to Fourqurean, P. oceanica’s impressive carbon deposits are a unique characteristic. So too could be the way that ecosystems dominated by P. oceanica produce methane.

Although the current research is preliminary, it provides new insight into how seagrass meadows produce methane and aims to stimulate additional research into how human activity might harm blue carbon storage.

—Derek Smith (@djsmitty156), Science Writer

Citation: Smith, D. (2022), Mortality of seagrass meadows may not kill their methane release, Eos, 103, https://doi.org/10.1029/2022EO220172. Published on 31 March 2022.

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