Arctic Oxidation Culprits

Austin, Texas
May 26, 2019


Photo of the Day:

Cory collecting data
Dr. Rose Cory collects a stream sample near Toolik Field Station in the Alaskan Arctic. Image Source: Dr. Rose Cory (University of Michigan).


Permafrost houses around 1.6 billion tons of carbon that could potentially be released as dissolved organic carbon (DOC) as the climate warms.1 As noted in my previous post on the permafrost positive feedback loop, this unlocked organic material is being converted in carbon dioxide and serving to intensify global warming, especially in Arctic regions. In this post, I’d like to focus on the specific mechanisms by which DOC is oxidized to carbon dioxide.

What Is Oxidation?

You might remember the mnemonic from your high school chemistry class: LEO the lion says GER. LEO stands for Lose Electrons Oxidation; GER stands for Gain Electrons Reduction. In a chemical reaction, carbon is oxidized when it loses electron density, and it is reduced when it gains electron density.

Organic chemists denote oxidation reactions as those in which carbon gains one or more bonds to a more electronegative element (typically oxygen in organic reactions). In doing so, carbon is losing electron density. Reductions are accordingly defined as reactions in which carbon gains one or more bonds to a less electronegative element (typically hydrogen in organic reactions).

As you can see in the below diagram, when carbon is completely oxidized, the end product is carbon dioxide (in which all four of carbon's bonds are to oxygen).

Oxidation and reduction
Organic oxidation and reduction pathways.

Bacterial Decomposition and Respiration

One of the possible mechanisms by which DOC is oxidized to carbon dioxide in Arctic watersheds is bacterial decomposition and respiration. In decomposition, the complex organic molecules present in DOC (lignins, aromatics, etc.) are broken down by bacterial enzymes.2 This process yields many nutrients, one of which is glucose, the fuel of cellular respiration. In aerobic cellular respiration, glucose (C6H12O6) and oxygen are converted into carbon dioxide, water, and energy.

Overall Reaction for Aerobic Respiration:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

This overall reaction can be broken down into three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation. In glycolysis, glucose (6 carbons) is broken down into two molecules of pyruvate (3 carbons each). In the Krebs cycle, each pyruvate is oxidized into three molecules of carbon dioxide. Oxidative phosphorylation is the terminal metabolic pathway in aerobic respiration, in which byproducts of the Krebs cycle (NADH and succinate) are oxidized to generate energy.

Cellular respiration
Overview of the three stages of cellular respiration. Image Source: Hana Zavadska (CK-12 Foundation)

Photooxidation

Photooxidation from sunlight exposure represents a second pathway by which DOC is oxidized to carbon dioxide. As mentioned in my previous post, many of the constituents of DOC have a high degree of electron delocalization. Accordingly, DOC is able to absorb a wide breadth of energy from sunlight, both in the ultraviolet and visible range. When a component of DOC absorbs energy from the sun, electrons in the molecule are promoted to higher energy molecular orbitals, and the molecule adopts an excited state. At this point, there are two potential outcomes:

  1. It is broken down directly into simpler constituents.3
  2. It reacts with water, oxygen, and iron in the water to yield peroxides, superoxide, other reactive oxygen species, and oxygen radicals. Here, a critical reaction is the photo-Fenton reaction, in which iron reacts with hydrogen peroxide to yield oxygen radicals. Reactive oxygen species are in turn responsible for partially or completely oxidizing DOC, even DOC not originally susceptible to direct photochemical degradation.3

Photochemical degradation of DOC
Graphical summary of the direct and indirect pathways for photooxidation of DOC. Image Source: Dr. Rose Cory.

The Gist of It All

So, two general processes are responsible for oxidation of DOC in Arctic watersheds: bacterial degradation/respiration and photooxidation from sunlight exposure. Below is a great summary graphic.

Oxidation pathways
Overview of the fates of DOC in Arctic watersheds. Image Source: Dr. Rose Cory (University of Michigan).


Arctic Fact of the Day:

Recall that it is the high degree of electron delocalization in many components of DOC that allows these compounds to absorb light in the visible spectrum, yielding colorful Arctic streams. When DOC is broken down by sunlight, networks of electron delocalization are broken, and the water becomes lighter and lighter. This is called photobleaching.1

Photobleaching
Arctic streamwater with high DOC content is photobleached over time when exposed to sunlight. Image Source: Dr. Rose Cory (University of Michigan).


Arctic Question of the Day:

Which is the more important factor controlling oxidation of DOC in Arctic watersheds: bacterial degradation/respiration or photooxidation from sunlight?

(Comment below!)


  1. Taterka, Bruce and Rose Cory. “Thawing Permafrost Lessons and Lab Manual: A Multi-Lesson Resource.” PolarTREC, 20 Sept. 2015, www.polartrec.com/resources/lesson/thawing-permafrost-lessons-and-lab-manual-a-multi-lesson-resource. ↩︎ ↩︎

  2. Kowalski, Kathiann. “Recycling the Dead.” Science News for Students, 6 Aug. 2016, www.sciencenewsforstudents.org/article/recycling-dead. ↩︎

  3. Kaplan, L.A., and R.M. Cory. “Dissolved Organic Matter in Stream Ecosystems: Forms, Functions, and Fluxes of Watershed Tea.” Stream Ecosystems in a Changing Environment, 1st ed., Academic Press, 2016, pp. 241–320. ↩︎ ↩︎

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Rose (not verified)

Another great post David!! You could also ask your audience if they are familiar with photo-bleaching in their every day lives ... everyone is, they may not realize it (hint: sun-brewed tea, and what happens to car paint or colored clothing if left out in the sun..?)

David Walker

Yes! Many different possibilities for the classroom laboratory. I plan to focus a future post entirely on photobleaching, will definitely focus more on this question.

Daniel Ferra (not verified)

We are already Locked in to 10C. Temp Rise Even If We Stopped Emitting Now Methane has a 10 year lag time an Carbon has 30–50 Year Lag time in Reaching its Full Molecule Potential in Holding Heat Mass Methane has 130 Times More Heat Mass in the Molecule in First Ten years Then 86 Times More Heat Mass per Molecule over 20 years Then 34 Times More Heat Mass per Molecule over next 80 Years Than Carbon Four Geological Formations Spewing and Venting Methane Now In The Arctic and one Venting Along The Washington Oregon Coast Perma Frost Melting Methane Hydrates Mantle Methane From Isostatic Rebounding of Greenlands North American Plate "Pingoes — Started in 2005 Finger Size Blow Holes, Now 300 Foot Wide Methane Blow Holes Increasing in Number and Size, in Siberia, Canada, an the Euro-Asian Plate Pingoes and Under Water Pingoes “Pingos preceded blow-outs Researchers have now examined satellite images of northern Siberian from a few years back and looked at the area where the explosions occurred. They found that the year before the huge crater appeared, there were large pingos in the same place. Pingos are found in the arctic and are usually raised hills, like a giant pimple, with a core of ice. In this case, however, the pingos must have been filled with gas in the form of ice, bound up with water in gas hydrates and permafrost. Russian scientists have now mapped 7,000 gas-filled pingos that are poking through the thawing permafrost, visible in satellite images that illustrate how the pingos form and grow, published in The Siberian Times.” Siw Ellen Jakobsen Methane Burp Or Pingoe Popping Pimple and at some point, like Natalia Shakhov, Guy Mcpherson, and Kevin Hester point out, we are going to experience a Methane blow out in the Arctic, from Mantle Methane, Perma-Frost, Methane Hydrates, Pingoes. The Methane has been telling us, it is going to blow since 2005 by Maria Shakhov, what was a finger size blow whole in 2005 is now a 200–300 ft. wide blow hole called Pingoes 2015 And their increasing in number and size in Siberia, North American Plate, and on the Euro-Asian Plate