From: Allison Winter, ENN
Published July 8, 2014 08:07 AM

SAR11 and Methane

With the focus on reducing carbon emissions, we often forget about methane — another greenhouse gas that is way more powerful as an atmospheric pollutant than carbon dioxide.

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Methane emissions can come from industry, agriculture, and waste management activities, but can also be emitted from a number of natural sources.

One newly discovered natural source: SAR11.

SAR11 defines a lineage of bacteria and is the smallest free-living cell known. It dominates life in the oceans, thrives where most other cells would die, and plays a huge role in the cycling of carbon on Earth. These bacteria are so dominant that their combined weight exceeds that of all the fish in the world's oceans, scientists say.

Researchers at Oregon State University have now demonstrated the ability of some SAR11 strains to generate methane as a byproduct of breaking down a compound for its phosphorus.

"Anaerobic methane biogenesis was the only process known to produce methane in the oceans and that requires environments with very low levels of oxygen," said Angelicque "Angel" White, a researcher in OSU's College of Earth, Ocean, and Atmospheric Sciences and co-author on the study. "In the vast central gyres of the Pacific and Atlantic oceans, the surface waters have lots of oxygen from mixing with the atmosphere — and yet they also have lots of methane, hence the term 'marine methane paradox.'

"We've now learned that certain strains of SAR11, when starved for phosphorus, turn to a compound known as methylphosphonic acid," White added. "The organisms produce enzymes that can break this compound apart, freeing up phosphorus that can be used for growth — and leaving methane behind."

The discovery is an important piece of the puzzle in understanding the Earth's methane cycle, scientists say.

Steve Giovannoni, a professor of microbiology at OSU and his team found that some SAR11 strains produced methane byproduct, and some didn't. 

White explains, "Just as some humans have a different capacity for breaking down compounds for nutrition than others, so do these organisms. The bottom line is that this shows phosphate-starved bacterioplankton have the capability of producing methane and doing so in oxygen-rich waters."

"Their ability to cleave off methane is an interesting finding because it provides a partial explanation for why methane is so abundant in the high-oxygen waters of the mid-ocean regions," Giovannoni added. "Just how much they contribute to the methane budget still needs to be determined."

Read more at Oregon State University.

Methane image via Shutterstock.

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