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