by Jessica Ernakovich
Greetings to those of you with inquiring minds about our environment,
I am a Ph.D. candidate at Colorado State University. I study how the microbes in permafrost (permanently frozen Arctic soil) change their activity when permafrost warms and thaws. Sounds kind of obscure, right? Well, it actually might have direct relevance to you (or maybe your children) because permafrost soils store vast amounts of carbon which could be released to the atmosphere when permafrost thaws due to warming air temperatures in the Arctic. Over the course of a few posts, I am going to explain my science starting with the big picture and I’ll then step down to the microbial scale. In this first post, I will explain the global carbon cycle and the role of permafrost in that cycle. In the next post, I will go into more detail about the role of decomposer microorganisms in the carbon cycle. Then, I will discuss what I am finding out about permafrost microorganisms. But, just as a teaser- it’s looking like permafrost microorganisms might “behave” a bit differently than other decomposer microbes. Check back to find out why!
Global carbon cycle
Carbon moves (cycles) through many reservoirs on earth including the atmosphere, plants, soils, rivers, and the ocean.
You can see from the figure that 650 gigatons (Gt) are stored in plants. A gigaton is equal to 1015 grams. For reference, a medium sports utility vehicle is around 2 tons, so the carbon in the plants globally weighs as much as 325 billion SUV’s. The carbon in plants is stored in organic molecules that the plants build during photosynthesis (the conversion of atmospheric carbon dioxide (CO2) to sugars (“net primary productivity” arrow)). Probably even more surprising to you is that the soil contains 3.5 times more carbon than plants!! Much of this carbon comes from plants as they drop leaves and as roots decay on a yearly basis, and is stored through chemical interactions with soil minerals. You can see above that the permafrost (“frozen soils”) contains 400 Gt of carbon (and newer estimates state that this number is even bigger now that we have better tools to dig deeper in the frozen soil (Tarnocai et al. 2009)). Much of the organic carbon in permafrost has been “frozen in time” since the last glaciation, when whole plants and other undecomposed materials were buried. When organic carbon in the soil is decomposed, it is released to the atmosphere as CO2 and methane (CH4) (“heterotrophic respiration” arrow). The atmosphere stores 750 Gt of carbon (IPCC 2007). The amount of carbon in the atmosphere is a major determinant of our climate, because CO2 and CH4 (among other greenhouse gasses) trap radiation and warm the earth’s surface. This warmed climate from greenhouse gasses has allowed life to become abundant and diverse on earth, but there is danger that with too many greenhouse gasses in the atmosphere, the earth will get too warm.
More on permafrost in the carbon cycle
As you can see, permafrost contains almost as much carbon as is stored in plant biomass globally or in the atmosphere and is therefore an important carbon reservoir (Tarnocai et al. 2009, Gorham 1991). Climate warming in the Arctic has caused degradation and thaw of permafrost in some locations (Nelson et al. 2002). All of that permafrost carbon has essentially been stored because it’s frozen, so the concern is that when these soils thaw, this carbon will be decomposed by microorganisms and enter the atmosphere through “heterotrophic respiration”. This would add much more CO2 and CH4 to the atmosphere and could warm the climate further, causing a “positive feedback” where more atmospheric carbon causes warming which causes the release of more permafrost carbon into the atmosphere. Understanding how the permafrost microbes “behave” under warmer conditions is important to understanding this carbon balance.
Next time, I’ll talk more about the role of decomposer microorganisms in the carbon cycle and give more details on how permafrost microbes are acting in my warming experiments.
Thanks for reading! Until next time!
For further reading on the carbon cycle & climate change: