In the Arctic, the main variable determining future climate change is buried in the ground and invisible.
Microbes in the soil layers just above the frozen permafrost metabolize the carbon, turning it into carbon dioxide and methane, a much more potent greenhouse gas. As these soils warm, more carbon is released, potentially starting a warming feedback loop sometimes called “methane bomb.“Now new research into microbes living in Arctic soils shows that such a vicious cycle may not be inevitable.
“It's possible that these systems, for various reasons, aren't actually producing the methane that we think they're capable of producing,” he said. Jessica Buser-Youngmicrobiologist at the University of Alaska Anchorage who was not involved in the study.
Microbes and methane
Since 2010, a consortium of scientists from Europe has been collecting samples of permafrost in the Arctic, digging into the topsoil and subsurface, as well as the permanently frozen soil below. Collecting these samples is difficult in the vast, remote and frozen northern reaches of the world, but the team collected samples from across Canada, Greenland and Siberia.
In the new paper, the researchers conducted a genomic microbiome analysis 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, which include both bacteria and archaea, which either secrete or ingest methanea greenhouse gas that can be 30 times more potent than carbon dioxide.
When the researchers looked at the data, the first surprise was the lack of diversity among both methane-producing microbes, or methanogens, and methane-consuming microbes, or methanotrophs, said study co-author. Tim Urichmicrobiologist at the University of Greifswald in Germany.
Among methanotrophs there is only one genus, Methylobacterspecimens at each location were dominant. These bacteria are found throughout the Arctic, often living in soil layers just above their methanogenic cousins, consuming the methane that bubbles up below. It is not yet known why this single lineage has been so successful, Urich said.
The analysis “really requires a more detailed study of members of this particular clade to understand the ecophysiology and their response to changing soil conditions,” Urich said.
It is possible to defuse a methane bomb.
Urich and his co-authors also looked at places where permafrost had thawed, comparing wet and dry places. The site with waterlogged soil contained more methanogenic microbes that thrived in oxygen-deprived conditions. In arid areas, on the contrary, methanotrophic microbes won, especially those that have the unique ability to absorb methane from the air and convert it into less active carbon dioxide. The researchers noted that although these facultative methanotrophs have the ability to metabolize atmospheric methane, in practice they do not necessarily do so.
“It really depends on the hydrologic fate of those soils.”
Tim Urich, University of Greifswald
Either way, Urich says the resulting warmer, drier Arctic could be a boon for a changing climate.
“It really depends on the hydrological fate of these soils,” he said.
If the Arctic ends up on the drier end of the spectrum, its soils could become a net sink for methane (though not a big one) as microbes begin sucking the gas from the air. The mechanism described by Urich and colleagues is also not the only potential methane negative feedback loop. IN recent article V ASU achievementsBuser-Young and her co-authors found that microbes in Alaska's Copper River Delta that use iron for their metabolism have begun to outcompete those that produce methane, potentially reducing methane emissions.
“We think this could potentially happen anywhere there are glaciers in the world,” Buser-Young said.
Studies like Urich's make clear that while thawing Arctic permafrost is a clear sign of climate change, its contribution to warming is less clear, he said. Christian garlicbiogeochemist at the University of Hamburg who was not involved in the study.
“We've had so many articles about this methane bomb“,” he said. “I think that was an oversimplification or an overestimation of methane emissions.”
The future of methane is still uncertain
Researchers continue to be hampered by a lack of data about the changing Arctic.
High on Urich's list of potentially valuable data sets is research on the ecophysiology of methane-related microbes that he and his colleagues have discovered in Arctic soils. Such studies will provide more data on how microbial metabolism changes in response to rising temperatures and changes in oxygen levels, among other things.
Urich also cautioned that his study did not measure levels of methane released or absorbed from Arctic soils, leaving unanswered the question of the microbes' actual environmental impact.
Knoblauch reiterated the need for more data, noting that we still can't say with certainty whether the future Arctic will be wetter or drier and therefore what methane emissions will look like.
“We have a lot of models and simulations, but we don’t have a lot of data on the ground,” he said. “I think the big questions are really how quickly the material will degrade, how much it will melt and to what extent it will degrade. [what] the time it decomposes and is then released, and how the system will be affected by changes in vegetation.”
This article was originally published on Eos.org. Read original article.
