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How an itty-bitty ocean-dwelling bacteria helps regulate our climate

In researching some of the smallest, simplest cells known to science, researchers have made an accidental discovery, putting these bacteria at the center of a cycle that helps regulate our planet's climate.

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    A rosette, a scientific tool that can take multiple water samples and measurements at once, is lowered into ocean water near Bermuda to sample SAR11 Alphaproteobacteria (order Pelagibacterales).
    Ben Temperton
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Earth's climate could be shaped by some of the smallest known free-living bacteria, new research suggests.

While studying the bacterial group Pelagibacterales, the most abundant organism at the ocean's surface and one of the most abundant across the entire globe, scientists found that they are involved in an integral process that helps regulate our climate – the production of dimethylsulfide (DMS).

This revelation will likely be instrumental in developing more accurate climate models, but it may also stimulate a fresh bout of scientific interest in the simple bacteria, which have so far seen only low levels of study, yet could turn out to hold crucial roles in various systems.

"Science often involves discovering the unexpected," says coauthor Stephen Giovannoni, of Oregon State University, in a telephone interview with The Christian Science Monitor. "We didn't expect to observe the formation of this gas, DMS. There's no gene for that."

In the course of their work, published Monday in Nature Microbiology, the researchers observed two gases being produced by the bacteria: DMS and methanethiol.

Methanethiol is a colorless gas with a distinctive scent. It is added to otherwise odorless natural gas to warn us in case of leaks. 

DMS is involved in cloud formation, forming a critical link in a negative feedback loop known as the CLAW hypothesis: Sunlight stimulates the production of phytoplankton, which in turn produce dimethylsulfoniopropionate (DMSP). Other microbes break this down to form DMS, which boosts the formation of clouds, reducing the amount of sunlight that reaches the ocean.

"Here's what I think is important," Dr. Giovannoni tells the Monitor. "We're studying the most abundant organism in the ocean surface and we've discovered an unusual mechanism that produces DMS.... Ultimately the question is how are compounds made by this organism affecting the climate?"

The generally recognized role of this bacteria is to oxidize organic matter back into carbon dioxide. As much as 5 to 20 percent of all carbon dioxide fixed on Earth in a given day may be produced by Pelagibacterales.

But the production of this DMS creates something other than a greenhouse gas. Scientists already knew there were bacteria that cleaved DMSP into DMS, but the significance lies in just how abundant this particular organism is.

Moreover, the mechanism it employs shows no latency: When the levels of DMSP reach a certain threshold, production of DMS shifts into gear smoothly, immediately.

Understanding this holds the promise of enhancing climate models' accuracy.

"What's fascinating is the elegance and simplicity of DMS production in the Pelagibacterales," said Ben Temperton, of the University of Exeter, England, in a press release. "These organisms don't have the genetic regulatory mechanisms found in most bacteria. Having evolved in nutrient-limited oceans, they have some of the smallest genomes of all free-living organisms, because small genomes take fewer resources to replicate."

Yet, if Pelagibacterales are such simple organisms, and so abundant, why has this pathway remained hidden until now, awaiting accidental discovery?

The answer, ironic though it may appear, lies in their very simplicity, evolved over as much as a billion years into "streamlined cells," honing their role into something small and specific, and discarding unnecessary genes along the way.

"You've seen these all-terrain vehicles," explains Giovannoni by way of illustration. "They can go through anything because they're heavily laden with equipment. Then look at a race car – light and simple."

"In our research, we tend to focus on these all-terrain vehicles because they're juggernauts and nothing stops them."

But as scientists gradually find more reliable methods of studying these simple creatures, they may turn out to be a key that unlocks our understanding of a multitude of systems.

"I believe they're going to turn out to be very important ecologically," says Giovannoni. "They may turn out to be important in lots of natural processes."

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