Inundate and Chill: The Fate of Microbes in Submarine Permafrost

Citation: Mitzscherling, J., M. Winkel, M. Winterfeld, F. Horn, S. Yang, M. N. Grigoriev, D. Wagner, P. P. Overduin, and S. Liebner (2017), The development of permafrost bacterial communities under submarine conditions, J. Geophys. Res. Biogeosci., 122, 1689–1704, doi:10.1002/2017JG003859.

 

Why is permafrost  important?

At the heart of climate science is understanding what is happening to carbon across the planet. As scientists, we are interested in figuring out where the carbon is coming from, where it is going, and how these things could change in the future. One place that is really important to look at as we try to answer these questions is soils. Soils can be both carbon sinks, as when organic matter gets preserved and taken up into plants, and sources of carbon, as when microbes eat up the organic matter and release greenhouse gases such as carbon dioxide and methane. In permafrost (soil that is frozen year-round) most organic matter gets stored, making it a critical carbon sink. However, if the permafrost starts to thaw, that can change. When permafrost starts to melt, the microbes that were previously chilling out (literally) in the permafrost start to eat up more of the organic matter, releasing more greenhouse gases into the atmosphere.

Crumbling permafrost along the Beaufort Coast (Credit: US Geological Survey)

While there is a lot of terrestrial permafrost throughout the Arctic, there is also permafrost at the bottom of the ocean. This permafrost started out as terrestrial permafrost thousands of years ago during the last ice age when there were more glaciers and lower sea levels. When those glaciers started to melt, sea levels rose and covered up that permafrost inundating it with seawater. When this occurs, it drastically changes the conditions that the permafrost is experiencing. Suddenly there is liquid water, a lot more salt, and it is also warmer than it was on land. In a recent paper, Julia Mitzscherling and colleagues set out to find out what happens to microbes in the permafrost when this occurs and what it could mean for the carbon cycle as a whole.

 

How do we study microbes in permafrost?

In order to understand what happens to the microbes after permafrost is covered by seawater, scientists collected samples of submarine permafrost from two different locations in the Arctic. At one site, the permafrost was inundated 2500 years ago. At the second site, it was inundated 540 years ago. For each sample, they measured the chemistry of water in the sediment, including pH, salinity, and ion concentration, which can collectively indicate to what degree the ocean water has influenced the permafrost. The scientists also conducted various assessments of changes to the microbial communities within the permafrost. They measured how many microbial cells were present in the permafrost and also used genetic analysis to determine the types of microbes. Because the two samples were inundated at different times, these data also provide a sense of how the impacts of inundation play out over an extended time period.

 

What happens to permafrost at the bottom of the ocean?

In the younger permafrost, the microbes were less abundant and diverse. This suggests that the influx of seawater and changing physical conditions initially cause a disruption to the microbial environment. Comparatively, in the older permafrost,  microbes were more abundant and diverse, they had also stratified into groups of different types of microbes. These groups inhabited distinct depths corresponding to the degree to which the sea water had infiltrated and melted the permafrost. In other words, the mircobes that were more tolerant of seawater conditions inhabited permafrost that showed signs of seawater intrusion. This indicates that although inundation can initially upset the microbes with changes in chemistry, over time the warmer temperatures that come with inundation can support microbial growth. Furthermore, the researchers found that all the microbial species in the permafrost were of terrestrial origin. In other words, terrestrial microbes survived inundation and resisted takeover by marine microbes.

These findings are important because in addition to providing a glimpse into Earth’s past they also give as idea of what could happen in the future. A significant amount of the planet’s coastlines are covered in permafrost, and with the threat of rising sea levels it is likely that inundation of permafrost by sea water will continue to be an important environmental process. The more we understand about how microbial populations in permafrost are affected by sea water inundation, the more we will know about its impacts on microbial greenhouse gas release from permafrost.

Occurrence of permafrost in the Arctic (Credit: US Geological Survey)
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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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