Scientists have long been trying to account for deep seafloor methane emissions – why is it that the gas leaking from methane seeps doesn’t end up in the atmosphere? Now recent findings published in Proceedings of the National Academy of Sciences shed light on the existence of methane-consuming microbial communities that live near methane seeps and keep methane from entering the atmosphere.
The study was led by Jeffrey J. Marlow, a former postdoctoral researcher in Organismic and Evolutionary Biology at Harvard University, who, along with his team, collected microbes from seven geologically diverse seeps on the ocean floor. One type of microbe found on carbonate rocks exhibited the highest rate of methane consumption seen to date.
"The microbes in these carbonate rocks are acting like a methane biofilter consuming it all before it leaves the ocean," noted senior author Peter Girguis, who is a professor of Organismic and Evolutionary Biology at Harvard. "We measured the rate at which the microbes from the carbonates eat methane compared to microbes in sediment. We discovered the microbes living in the carbonates consume methane 50 times faster than microbes in the sediment. We often see that some sediment microbes from methane-rich mud volcanoes, for example, may be five to ten times faster at eating methane, but 50 times faster is a whole new thing. Moreover, these rates are among the highest, if not the highest, we've measured anywhere."
The team found these microbe communities in a carbonate chimney reef off the coast of southern California at the deepsea site Point Dume. It was here, at the seafloor observatory, that Girguis and Marlow collaborated to look at methane oxidation rates.
"These chimneys exist because some methane in fluid flowing out from the subsurface is transformed by the microbes into bicarbonate, which can then precipitate out of the seawater as carbonate rock," said Marlow. "We're still trying to figure out where that fluid -- and its methane -- is coming from."
In the meantime, the researchers say it is interesting that some of the microbial communities are observed near pyrite. Could the presence of this electrically conductive mineral explain microbes’ high metabolic rates for methane consumption?
"These very high rates are facilitated by these carbonates which provide a framework for the microbes to grow," commented Girguis. "The system resembles a marketplace where carbonates allow a bunch of microbes to aggregate in one place and grow and exchange -- in this case, exchange electrons -- which allows for more methane consumption."
"When microbes work together they're either exchanging building blocks like carbon or nitrogen, or they're exchanging energy,” adds Marlow. “And one kind of way to do that is through electrons, like an energy currency. The pyrite interspersed throughout these carbonate rocks could help that electron exchange happen more swiftly and broadly."
The researchers plan to continue investigating to understand how all these factors come together to allow these Olympic-level methane consumers to thrive. "Next we plan to disentangle how each of these different parts of the carbonates -- the structure, electrical conductivity, fluid flow, and dense microbial community -- make this possible. As of now, we don't know the exact contribution of each," concludes Girguis.