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Mineral cryosols in the High Arctic consume Methane from the atmosphere


 

Bastion Ridge on Axel Heiberg Island, Nunavut, Canada. (Photo by Brandon T. Stackhouse, GEO Ph.D. student)


Bastion Ridge on Axel Heiberg Island, Nunavut, Canada. (Photo by Brandon T. Stackhouse, GEO Ph.D. student)

 

In a recent investigation led by Dr. Maggie C.Y. Lau, scientists in the Department of Geosciences, Princeton University, McGill University and the University of Tennessee‐Knoxville have identified a surprising sink for atmospheric methane (CH4): the mineral soils of the Canadian high Arctic.  Previous work had suggested that with global warming the Arctic would become a significant source of CH4, a potent greenhouse gas, but this study found just the opposite would be true.
 
The research team investigated samples of mineral soils and 1 meter long soil cores that they collected near a remote research station on Axel Heiberg Island (AHI), the third largest uninhabited island in the world in the Canadian high Arctic. When these samples were incubated in air in a cold room back at Princeton University, they consistently removed the CH4 from the air regardless of their moisture status. When the samples were incubated at 10°C they exhibited even greater CH4 consumption rates than had been observed at 4°C in the cold room.  The team complemented these laboratory experiments with in situ measurements of land‐atmosphere CH4 exchange at the field site on AHI during spring thaw and mid‐summer.  These field measurements all showed atmospheric CH4 uptake at rates consistent with the laboratory observations, further supporting the idea that the mineral soils on AHI act like sponges for atmospheric CH4 during the summer time when other regions of the Arctic are emitting CH4.
 
To discover why these soils were CH4 consumers, Lau and co‐workers probed the soils using the high throughput technologies of metagenomics, metatranscriptomics and metaproteomics.  They discovered that the marker gene for atmospheric CH4‐oxidizing bacteria was present in the upper layers of the soil and that it was actively being expressed.  Atmospheric CH‐oxidizing bacteria are a specialized group of microorganisms that are capable of oxidizing CH4 at extremely low concentrations but they have never been isolated in the lab.  They are presumably related to an alpha‐proteobacterial group and are called Upland Soil Cluster Alpha (USCα).  This marker gene is found in soils around the world and the microorganisms are responsible for the biological modulation of atmospheric CH4.  Notably, this was the first report of the USCα bacteria being metabolically active this far north in any Arctic terrestrial ecosystems.
 
Lau’s team then combined the results from their experiments with global warming scenarios to predict the atmospheric‐CH4 oxidizing capacity of the AHI mineral soil in the future.  The result indicated that the mineral soils would continue to carry out their role as CH4 sinks with warming.  Earlier this year, a Denmark research group also identified another Arctic CH4 sink in the tundra stretching across northeastern Greenland.  They determined that this CH4 sink greatly exceeded the summer time CH4 release by the more vegetated areas and far less numerous fens.

Whether the Arctic acts as a net source or a net sink of CH4 is a fundamental question of climate science.  In contrast to Lau et al., previous studies have focused on the Arctic’s capacity to act as a CH4 source.  Global warming may undermine the stability of permafrost‐affected soils in the Northern Hemisphere, and these soils contain ~45% of the global soil carbon.  When permafrost thaws, wetlands are formed.  Because typical wetland soils are water‐saturated and rich in organic carbon, they create a favorable environment for methanogens, the microorganisms that produce CH4.  According to this conventional paradigm, the Arctic under global warming may act as a source of CH4 to the atmosphere. However, the carbon‐poor mineral soils studied by Lau et al. occupy five times the spatial area of carbon‐rich soils in the Arctic. Given that these mineral soils have now been shown to have the capacity to consume atmospheric CH4, large parts of the Arctic may actually act as CH4 sinks in the 21st century.

Lau et al. continue to investigate the physiological requirements of these USCα bacteria for CH4‐oxidation and growth and to incorporate empirical, microbial observations for CH4 flux modeling.  In addition, the research team also characterizes the CO2 and trace gas fluxes, pore water geochemistry, and carbon turnover by these mineral soils.

These recent findings illustrate that many mysteries remain in the remote Polar regions of our planet where global warming is having the greatest impact.  Investigations of these mysteries will likely yield further surprises that will affect the future of climate much closer to our home.

Related article:

Lau, M.C.Y., B.T. Stackhouse, A.C. Layton, A. Chauhan, T. A. Vishnivetskaya, K. Chourey,
J. Ronholm, N.C.S. Mykytczuk, P.C. Bennett, G. Lamarche-Gagnon, N. Burton, W.H. Pollard,
C.R. Omelon, D.M. Medvigy, R.L. Hettich, S.M. Pfiffner, L.G. Whyte, and T.C. Onstott (2015) An active atmospheric methane sink in high Arctic mineral cryosols, ISME, 14 April DOI:10.1038/ismej.2015.13.

 

McGill Arctic Research Station during early-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada.


McGill Arctic Research Station during early-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada. Under the snow are polygonal terrains where the top tens cm of soil experiences seasonal freeze-and-thaw cycles and the permafrost underneath remains frozen for at least two consecutive years. (Photo by Brandon T. Stackhouse, GEO Ph.D. student)

 

McGill Arctic Research Station during late-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada.


McGill Arctic Research Station during late-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada. (Photo by Nadia Mykytczuk, Laurentian University)

 

McGill Arctic Research Station during mid-summer at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada.


McGill Arctic Research Station during mid-summer at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada. (Photo by Brandon T. Stackhouse, GEO Ph.D. student)