Thursday, August 19, 2010

Unraveling the Ocean Methane Paradox

ResearchBlogging.org Mention methane production, and cows or oil companies usually come to mind. But much of the methane in the atmosphere (1-4%) actually escapes from the oceans, some of it produced by microbes known as methanogens (like Methanosarcina acetivorans, above). Some methanogens live in anaerobic – oxygen-free – sediments on the seafloor. Others make their homes in anaerobic fish intestines, the guts of some plankton, or fish and plankton fecal matter.

Methanogens live in anaerobic environments. However, measurements of oceanic methane consistently show a high concentration of methane in shallow, oxygenated waters. Thus, we have the “ocean methane paradox:” How are large amounts of methane being produced in an environment with plentiful oxygen?

In the image below, the connected, closed circles represent methane concentrations at different water depths at a seawater sampling site in the Pacific Ocean. Note the spike in concentration around 150 meters, representing the high methane concentrations leading to the ocean methane paradox:

Explaining the paradox is important for scientists' understanding of how the oceans contribute to global climate change. Methane is a powerful greenhouse gas, and high surface concentrations of marine methane result in more of it entering the atmosphere. The high methane concentrations giving rise to the ocean methane paradox are too widespread to be due to non-living sources of marine methane – like geochemical sources partly responsible for natural methane seeps in the Gulf of Mexico and offshore Santa Barbara, CA. So, in working out the paradox, scientists have focused on methanogens.

In 1994, David Karl of University of Hawaii, Honolulu, and Bronte Tilbrook of Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO), measured the flow of methane out of sinking “particulate matter.” This matter included some plankton, plankton fecal material (see image below), some fish fecal material, and marine snow – small pieces of dead organic matter, dust, and other particles that constantly sink through the ocean.

Karl and Tilbrook found that the amount of methane released by the sinking materials is enough to account for the elevated methane levels leading to the ocean methane paradox. They hypothesized that methanogens produce methane in the guts of some plankton and, for a brief period of time, in the anaerobic “microenvironments” of plankton feces. The methane is then released into the ocean from the droppings. Karl and Tilbrook’s results were supported by both previous and subsequent studies that found methanogens living in plankton and fish fecal pellets, as well as other particulate matter.

Karl and Tilbrook’s study - and related research - seemed to provide a straightforward solution to the ocean methane paradox: marine creatures, their feces, and other particles provide anaerobic microenvironments in which methanogens produce methane, which is then released into the ocean.

But it turns out the solution may not be so simple. A 2008 paper by Karl and colleagues provided evidence for the possibility of aerobic (in the presence of oxygen) methane production by marine microbes.

Using seawater samples, the scientists determined that some marine bacteria can use the compound methylphosphonate (MPn, see image below) as a source of phosphorous – an element necessary for synthesis of many important biological compounds. As MPn is broken down, methane is produced as a byproduct. Karl and his colleagues point out that the results obtained in his 1994 paper with Tilbrook could actually be explained by MPn breakdown by free-living microbes or those residing in sinking particles or the guts of animals.

The hypothesis that aerobic methane production could contribute to elevated ocean methane concentrations was further supported by a 2010 paper by Ellen Damm of the Alfred Wegener Institute for Polar and Marine Research and her colleagues. They compared aerobic methane production in two different ocean regions and found an association between increased methane production and a low seawater nitrate to phosphate ratio. Since such a ratio occurs during certain seasonal ecological shifts, aerobic marine methane production by bacteria could be a seasonal occurrence.

While scientists have made good progress in resolving the ocean methane paradox, the mystery still stands. Are both aerobic and anaerobic microbes contributing to the high shallow marine methane concentration? What are the mechanisms and locations of methane production for the organisms? Are there seasons and geographic regions with more methane production by the creatures responsible for the paradox? For now, I'll leave these questions to the scientists and move on to this post's suggested musical accompaniment, a beautiful, ocean-evoking track called Waves by Chihei Hatakeyama:



References:

Damm, E., Helmke, E., Thoms, S., Schauer, U., Nöthig, E., Bakker, K., & Kiene, R. (2010). Methane production in aerobic oligotrophic surface water in the central Arctic OceanBiogeosciences, 7 (3), 1099-1108 DOI: 10.5194/bg-7-1099-2010

Karl, D., & Tilbrook, B. (1994). Production and transport of methane in oceanic particulate organic matter Nature, 368 (6473), 732-734 DOI: 10.1038/368732a0

Karl, D., Beversdorf, L., Björkman, K., Church, M., Martinez, A., & Delong, E. (2008). Aerobic production of methane in the sea Nature Geoscience, 1 (7), 473-478 DOI: 10.1038/ngeo234

Reeburgh, W. (2007). Oceanic Methane Biogeochemistry Chemical Reviews, 107 (2), 486-513 DOI: 10.1021/cr050362v

Image Sources:

Top image: Genome News Network
Methane Maximum plot: Reeburgh, 2007 (see above reference)
Zooplankton fecal material: Sam Wilson, Scottish Association for Marine Science
Methylphosphonate: PubChem

5 comments:

  1. Thanks for posting about methane. More research nees to be done on methane, as well as hydrocarbons to get a better model of global warming. In particular we need to understand what kinds of events trigger large methane releases, to safeguard against mankind accidently triggering a large methane release.

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  2. Hey Sarah, great post! It's very well-written and informative. I didn't even know this paradox existed, so thanks!

    Question: do you know the biological origin of MPn?

    (Nice tunes, too. Did this guy invent chillwave years ago?)

    Best,
    Hannah

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  3. Thanks, Hannah! Looks like we both had marine poop on our minds last Friday. :)

    According to Karl (2008), MPn is the simplest of one of two major classes of organic phosphorous-containing compounds in the ocean - the phosphonates. (The other class is the phosphate esters, found in AMP and DNA, which have a C-O-P bond instead of the phosphonate C-O bond.) Phosphonates are known to be a significant component of marine snow, but, as of 2009, their precise biological sources were still being worked out (http://www.nature.com/ngeo/journal/v2/n10/abs/ngeo639.html). It has been speculated that major sources of phosphonates are cell membrane phospholipids, especially from marine bacteria. The 2009 paper is the first to report on significant phosphonate production in a cyanobacterium. Though the authors cite another study that found phosphonates to be present in lipids, proteins, and antibiotics, they report that phospholipids are not a main source of phosphonates in their cyanobacterium, so there's definitely more work to be done.

    This is already way too long a comment, but I should also mention that the Damm paper actually looked at methane production as a byproduct of breakdown of the compound DMSP, which acted as a carbon source (not a phosphorous source). So, if aerobic methane production is significant for the paradox, there may be more than one pathway...

    Glad you liked the music!

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  4. Yogi's comment comes close to what I was thinking.
    We know that temperature increases lead to more plankton, right? More plankton = more feces = more methane => positive feedback loop. What is the effect of temperature on microbe-generated methane? If it's the same, we're pretty much screwed. If it goes in the opposite direction, then will the reduction of microbe-generated methane be enough to counter the plankton-generated methane?

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  5. Yogi and Xristoforos, I agree that it's in our best interests to understand how climate change affects ocean methane production. Yogi, are you thinking of the clathrate gun hypothesis? Xristoforos, interesting questions...I think a lot will have to be factored into answering that last one, in particular. For example, scientists are also working hard to understand the roles of marine methanotrophs, microbes that consume methane.

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