Arctic 'methane bomb' may not explode as permafrost thaws, new study suggests

In the Arctic, a major variable for future climate change lives in the ground, invisible.

Microbes in the layers of soil just above the frozen permafrost metabolize carbon, turning it into carbon dioxide and methane, a far more potent greenhouse gas. As these soils warm, more carbon is being unlocked, potentially setting in motion a warming feedback loop sometimes nicknamed the "methane bomb." Now, new research on the microbial denizens of Arctic soils indicates that such a vicious cycle may not be inevitable.

By cataloging the kinds of microbes found in permafrost soils from around the Arctic, as well as in recently thawed permafrost itself, a group of researchers delivered a clearer picture of microbial diversity in Arctic soils, as well as how those microbial communities change as their environment warms up. One key finding in their paper, recently published in Communications Earth and Environment, is that under certain conditions there could be more methane-eating microbes than methane-making microbes in the Arctic, meaning the soil could actually end up being a carbon sink.

"It could be that these systems for a variety of reasons are not actually producing the methane we believe that they're capable of producing," said Jessica Buser-Young, a microbiologist at the University of Alaska Anchorage not affiliated with the research.

The microbes and the methane

Since 2010, a consortium of scientists from Europe has been gathering permafrost samples in the Arctic, digging through topsoil and subsoil and into the permanently frozen ground below. Gathering these samples is difficult in the vast, remote, and frozen northern reaches of the world, but the group retrieved samples from across Canada, Greenland, and Siberia.

In the new paper, the researchers conducted genomic analyses of the microbiome of eight pan-Arctic permafrost and soil samples as well as samples of both intact and degraded permafrost near Fairbanks, Alaska. They focused specifically on microbes, comprising both bacteria and archaea, that either release or consume methane, a greenhouse gas that can be 30 times more potent than carbon dioxide.

When the researchers looked at the data, the first surprise came from the lack of diversity among both methane-producing microbes, or methanogens, and methane-consuming microbes, or methanotrophs, said study coauthor Tim Urich, a microbiologist at the University of Greifswald in Germany.

Among methanotrophs, a single genus, Methylobacter, dominated samples at every location. These bacteria are found across the Arctic, often living in soil layers just above their methanogen counterparts, consuming the methane that bubbles up from below. Why this single genus has been so successful isn't yet known, Urich said.

The analysis "really calls for studying representatives of this specific clade in more detail to understand the ecophysiology and their response to changing conditions in the soil," Urich said.

Possibly defusing the methane bomb

Urich and his coauthors also looked at sites where permafrost had thawed, comparing wet and dry locations. The site with sodden soils held more methanogenic microbes, which thrived in the oxygen-deprived conditions. At dry sites, by contrast, methanotrophic microbes won out, especially a variety with the unique ability to take methane from the air and turn it into less potent carbon dioxide. While these facultative methanotrophs have the ability to metabolize atmospheric methane, researchers noted, they don't necessarily do it in practice.

Regardless, Urich said, the upshot is that a warmer, drier Arctic may be a boon for the changing climate.

"It really depends on the hydrologic fate of these soils," he said.

If the Arctic ends up on the dry end of the spectrum, its soils could become a net sink for methane (though not a large one) as microbes begin sucking gas from the air. The mechanism described by Urich and his colleagues is not the only potential negative methane feedback loop, either. In a recent paper in AGU Advances, Buser-Young and her coauthors found that microbes in Alaska's Copper River Delta that use iron for their metabolism have begun outcompeting those that produce methane, potentially reducing methane emissions.

"We believe that this could be happening potentially everywhere there's glaciers in the world," Buser-Young said.

What studies like Urich's are making clear is that while thawing Arctic permafrost is an obvious sign of climate change, its contribution to warming is less apparent, said Christian Knoblauch, a biogeochemist at the University of Hamburg who was not involved with the research.

"We had so many papers about this methane bomb," he said. "I think this was an oversimplification or an overestimation of methane release."

Future of methane still uncertain

Researchers are still hampered by a paucity of data about the changing Arctic.

High on Urich's list of potentially valuable datasets are studies on the ecophysiology of the methane-associated microbes he and his colleagues found in Arctic soils. Such studies would provide more data on how microbe metabolism changes in response to warming temperatures and varying levels of oxygen, among other things.

Urich also cautioned that his research did not measure levels of methane release or uptake from Arctic soils, leaving unanswered the question of the microbes' actual impact on the environment.

Knoblauch reiterated the need for more data, noting that we still cannot say with certainty whether the future Arctic will be more wet or more dry and therefore what methane release will look like.

"We have a lot of models, and there are a lot of simulations, but we do not have so much data on the ground," he said. "I think the big questions are really how fast is the material decomposed, how much will thaw and in [what] time it is decomposed and then released, and how the system will be affected by changing vegetation."

This article was originally published on Eos.org. Read the original article.