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?
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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.
http://seajellyexhibit.blogspot.ru/2010/08/solving-ocean-methane-paradox.html