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Marine microbes more effective in reducing methane than expected

A new study from Svalbard gas hydrate mounds shows that microbes are unexpectedly effective in consuming methane from marine sediments before the gas leaks into the ocean causing undesirable consequences for the environment.

Sediment sample
The white substance in the sediments is gas hydrate that hosts a large quantity of methane. The photo was taken during the expedition in 2016(?) where samples were collected. The diameter of the sediment core in the middle of the photo is about 10 cm. Photo: Wei-Li Hong

Microbes play an important role in suppressing release of methane from marine sediments and thus prevent consequences such as ocean acidification (if methane is converted to CO2 in the ocean) or global warming (if methane leaves the ocean).

Anaerobic oxidation of methane, or AOM, is a consortium of two microbes consuming methane as their sources of carbon and energy. AOM is one of the most important sinks for methane in global marine sediments. It has been expected that the microbial consortium that operates AOM only slowly responses to the changes in methane supply and thus is very inefficient in preventing methane from leaving the marine sediments. Now, a new study published in Nature Communications shows that this might not be the case.

“Emissions of methane from the Arctic Ocean seafloor is a well-documented phenomenon, but few studies investigate how the natural environments cope and mitigate the release of methane,” says Wei-Li Hong, assistant professor of geochemistry at Stockholm University and the corresponding author of the article.

 

Sediment sampling at Svalbard

Fjärrstyrd autonom farkost (ROV) tog bottenprover vid expeditionen 2016
The remotely operated vehicle (ROV) that was used during the 2016 cruise for sampling.
Photo: Wei-Li Hong

The sampling was conducted in 2016 outside of Svalbard when Wei-Li Hong was a post-doctoral researcher from UiT, The Arctic University of Norway. They collected sediment samples from a water depth of 380 meters with the assistance from a remotely operated vehicle to precisely determine the sampling location.

“We show that the response of methane-consuming microbial communities is largely synchronized to the increase of methane escaping rates from the seafloor of an Arctic site where a large quantity of methane is stored as gas hydrate in the sediments,” says Scott Klasek the lead author of this article who conducted the analyses when he was a graduate student from the Oregon State University (OSU) in USA.

“In other words, when a greater amount of methane is about to leave the seafloor, the microbes are able to keep up the rate and consume much of the methane before it leaks to the ocean and influencing the carbon cycle in that environment,” says Rick Colwell, professor of oceanography at OSU.

 

Faster reaction than researchers anticipated

The AOM microbial consortium reacts faster than the researchers anticipated from this Arctic site at Storfjordrenna near Svalbard. “There is still methane leaving the sediments of this site today; nonetheless, without these fast-responding microbes, there will be a lot more methane entering the ocean. In this sense, these microbes have help mitigate the warming if such methane or its oxidation products reach the atmosphere over thousands of years,” says Marta Torres, professor of oceanography at OSU.

Whether the leakage of methane from the investigated site is intensified by global warming is unclear but unlikely. Previous studies from the same location suggest little effect from the current Arctic Ocean warming in increasing methane emission*.
 

Wei-Li Hong
Wei-Li Hong

“All evidence suggests that the release of methane from this site has been continuous for hundreds, if not thousands, of years. Regardless of the driver, methane emission from this site is certain, and still happening as we speak, and the microbes are able to cope with it as we have shown in the study,” says Wei-Li Hong.
 
According to Wei-Li Hong there are two potential future directions for research in this area. “First, will we have similar observations from other locations in the Arctic Ocean? How representative is our observation from southern Svalbard to be applicable to the larger Arctic Ocean is still an unknown. Second, how do we resolve the conflicting conclusions between theoretical investigation from other studies, saying AOM is a slow process, and our field observations, showing it is actually much faster.”

Article in Nature Communications. DOI: 10.1038/s41467-021-26549-5
Read more om Wei-Li Hong´s research.

*See Hong, WL., Torres, M., Carroll, J. et al. Seepage from an arctic shallow marine gas hydrate reservoir is insensitive to momentary ocean warming. Nat Commun 8, 15745 (2017). https://doi.org/10.1038/ncomms15745