Optimism from Oxidation: Archaea could lessen the amount of methane released from melting submarine permafrost
By: R. Grace Owens-Kurtz
Edited by: Katherine Hill and Tyler Peterson
Many of us may not realize that the Arctic and the oceans are storing megatons of greenhouse gases that are a ticking time bomb. As climate change progresses, the melting of permafrost in the Arctic and Antarctic has become a major concern. In these regions, soil has been frozen for hundreds to thousands of years as permafrost. As the average yearly temperatures at the poles increase, this soil is beginning to thaw, releasing materials that were frozen in time. One specific effect may be the release of carbon that was previously trapped in permafrost, which would only increase temperatures further in a vicious cycle of positive feedback.
A large pool of carbon is in gases like carbon dioxide and methane, which can be trapped between the particles in permafrost. Carbon dioxide is uniformly distributed in permafrost sediments, while methane forms distinct layers. This stratification of methane results from the way it is formed. Certain types of archaea, single-celled organisms similar to bacteria that compose the third domain of life, produce methane and the layers in which it was frozen are a remnant of the growth layers of these archaea (1). Permafrost therefore stores vast quantities of methane, representing the products of ancient life, but now this icebound gas could exacerbate the problems facing our planet’s atmosphere.
Besides terrestrial permafrost, the Arctic Ocean harbors submarine permafrost that has been frozen for thousands of years. Now, warmer currents above this submerged, frozen soil are thawing it and releasing its stored carbon. If the permafrost were not thawing in the warming water, its carbon contents would be stored for centuries more, but now it is adding to the already increasing atmospheric concentrations of carbon dioxide and methane. Concerns are being raised that, as permafrost melts, up to 1300 metric gigatons of organic carbon could be released and potentially converted to the same gases that are already changing the way our planet functions (2). Methane and carbon dioxide are both greenhouse gases, meaning that they play a role in keeping solar energy in the Earth’s atmosphere. This means that when the amount of carbon dioxide and methane in the atmosphere increase, it becomes more difficult for the Earth to shed heat into outer space. The atmospheric concentration of methane and carbon dioxide has been increasing well beyond the typical variation patterns, which disrupts global climate patterns and ultimately results in the overall warming of the planet.
Unfortunately, the effects of climate change end up becoming the causes of further climate change in a positive feedback loop. One example is that as more sea ice melts, the darker exposed water absorbs more heat than the white ice cover, increasing the rate at which sea ice melts. The case of thawing permafrost similarly exacerbates the threat of climate change. As the globe warms, permafrost melts and releases gases that further add to the greenhouse effect, which speeds up the rate of permafrost melting. This runaway feedback worsens the problem and can seem like an unsolvable challenge.
Despite this ominous forecast, there remains hope and it stems from the diversity of microbial metabolism. Recently, a group of scientists detected evidence that archaea within the permafrost sediments can use released methane as their own energy source. In the process, they convert methane into a less harmful product: CO2 (2). While both gases are involved in the greenhouse effect, methane poses a greater threat because it traps heat twenty times more than carbon dioxide. Occurring in the deep ocean without readily available oxygen, this specialized metabolism of methane is called anaerobic oxidation of methane (AOM). It is already known that a wide variety of microbial processes contribute substantially to global carbon cycles, including the production of methane. The transformation of methane into less damaging CO2 by archaea in anaerobic environments like the deep sea has not been previously documented and it could drastically reduce the amount of the potent greenhouse gas that is released from submarine permafrost.
A team of researchers at the German Research Centre for Geosciences identified the species of archaea living within the permafrost and determined which were producing the enzymes necessary to perform AOM. Their research included sequencing of the 16S rRNA gene, a common method to identify species of bacteria and archaea. This gene mutates at a relatively constant rate, so distinct species have unique sequences and sequencing this single gene allows quick identification of all the species present in a sample. Additionally, the researchers examined how the composition of the permafrost changed as the researchers moved deeper into the seafloor. Comparing the changes in chemical makeup with the 16S sequencing results, they found that as the number of unique sources of chemical energy decreased, there were more archaea that could take energy from methane through AOM (2).
This result is not surprising, as AOM is an inefficient method for archaea to produce energy. Therefore, it seems that when there are not many better options available, AOM-capable archaea activate the system and can produce energy when other species are dying off from starvation. In the chemical reactions to oxidize methane, archaea require electron acceptors to take the place of oxygen. Sulfate, iron, or nitrate could all serve this role. The researchers found that with the combination of location and metabolism, these archaea could not fit under any existing classifications. Noticing that there were two similar, but still distinct, groupings of species, they called them the Deep Submarine Permafrost Euryarchaeotal Groups I and II (2). Besides discovering new species, the most impressive finding was that these archaea oxidized 72–86% of the methane in these deep ocean sediments (2). These never-before-identified archaea thus could contribute substantially to global cycles by reducing the amount of methane release by thawing permafrost.
The identification of these AOM archaea demonstrates the versatility of microorganisms and reminds us all that their contributions to global cycles cannot be ignored. Melting permafrost still poses a major threat to the balance of global systems, but new models to predict increases in greenhouse gases — and the results that might affect humans — can now include a few more organisms helping us out. While AOM archaea cannot put methane back into ocean permafrost, their conversion of methane into carbon dioxide could lessen the extent of the damage. The discovery doesn’t provide a solution, and in fact still involves the release of a greenhouse gas: CO2. Nonetheless, preventing the release of that much methane is no small feat for these microscopic organisms. In the face of seemingly insurmountable global challenges, it’s nice to know we might have a little help.
[1] Rivkina, E., Shcherbakova, V., Laurinavichius, K., Petrovskaya, L., Krivushin, K., Kraev, G., Pecheritsina, S., and Gilichinsky, D. (2007). Biogeochemistry of methane and methanogenic archaea in permafrost. FEMS Microb Ecol 61(1), 1–15.
[2] Winkel, M., Mitzscherling, J., Overduin, P.P., Horn, F., Winterfeld, M., Rijkers, R., Grigoriev, M.N., Knoblauch, C., Mangelsdorf, K., Wagner, D., and Liebner, S. (2018). Anaerobic methanotrophic communities thrive in deep submarine permafrost. Scientific Reports 8, 1291. http://www.nature.com.ezp1.lib.umn.edu/articles/s41598-018-19505-9
