Scientists Unravel the Ocean Methane Mystery
Methane is a potent greenhouse gas, 20 times more effective at trapping heat than CO2. Four percent of the methane in our atmosphere comes from the oceans. Until recently, scientists have been unable to explain where that methane was coming from, but a combination of research in the last four years has finally uncovered the most likely scenario.
I bet readers of the Microbelog won’t be surprised to hear that methane is created biologically by microorganisms. In fact, methane produced by microbes trapped in arctic/subarctic permafrost is actually a major concern for climate scientists, because as temperatures continue to rise, that permafrost will thaw, releasing more methane into the atmosphere. This creates a nasty feedback loop expected to cause climate change to accelerate.
Electron microscopy image of traditional methanogens, courtesy of Maryland Astrobiology Consortium, NASA, and STScI.
Microbial methane production has traditionally been thought to be the exclusive purview of a specialized group of Archaea termed methanogens. These organisms are highly oxygen sensitive, and only produce methane in places where all the oxygen has already been consumed, like in lake and ocean sediments, or inside the guts of animals, including humans.
However, such traditional methanogenesis wasn’t enough to explain the large amount of methane coming out of the oceans, mostly because methane generated in ocean sediments has to travel a long way to get to the surface, and it usually gets eaten by other microorganisms termed methanotrophs (methane eaters) prior to being released into the atmosphere. Nevertheless, methane can be observed in high concentrations in surface waters worldwide, and this puzzle has been dubbed the “oceanic methane paradox.”
|Image courtesy Pastoral Greenhouse Gas Research Consortium.|
The first half of the marine methane puzzle was solved in 2009 by researchers at University of Hawaii and MIT. They discovered that microorganisms, frequently starved for phosphorous, would metabolize a phosphorous-containing compound called methylphosphonate, and in the process release a methane molecule. This was the first time that methanogenesis had been discovered occurring in water where oxygen was present, and importantly, happened in surface waters where the methane could make it to the atmosphere before being consumed by methanotrophs.
What remained a mystery was where all the methylphosphonate was coming from. In order for this “aerobic methanogenesis” to be able to explain 4% of the total methane in the atmosphere, there would have to be a huge amount of methylphosphonate in the ocean, but no one had observed that. In August, researchers from the University of Illinois working on soil microbes reported that microorganisms called Thaumarchaea create a large amount of methylphosphonate with a previously unknown set of genes. Now, it turns out that these Thaumarchaea are also one of the most abundant groups of microorganisms in the oceans, and surveys demonstrated these newly discovered genes in abundance throughout the world’s oceans, not only in Thaumarchaea, but also in other microbes that dominate the water column. The reason why methylphosphonate had not been previously observed was because it was bound to the microbial cells that make it, not freely dissolved in the water. When these cells die, they have the potential to release that methylphosphonate which can be consumed by organisms that do aerobic methanogenesis, creating, as the authors describe, “a plausible explanation for the methane paradox.”
References and further reading:
Metcalf WW, Griffin BM, Cicchillo RM, Gao J, Janga SC, et al. (2012) Synthesis of methylphosphonic acid by marine microbes: a source for methane in the aerobic ocean. Science 337: 1104–1107. doi:10.1126/science.1219875.
Karl DM, Beversdorf L, Björkman KM, Church MJ, Martinez A, et al. (2008) Aerobic production of methane in the sea. Nature Geoscience 1: 473–478. doi:10.1038/ngeo234.
Karner MB, DeLong EF, Karl DM (2001) Archaeal dominance in the mesopelagic zone of the Pacific Ocean. Nature 409: 507–510. doi:10.1038/35054051.
Kiene, R. P. in Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides, and Halomethanes (eds Rogers, J. E. & Whitman, W. B.) 111–146 (ASM, Washington DC, 1991).