Eau de seashore

An English scientist has pinpointed the process behind that unmistakable smell of the beach

February 02, 2007|By Erika Gebel | Erika Gebel,Special to the Sun

On a popular episode of Seinfeld, Kramer says he wants to make a perfume that re-creates the smell of the beach.

Now, British scientists say they have effectively bottled that unmistakable fragrance - in a test tube filled with bacteria.

"It smells, that's for sure," scientist Andrew Johnston said of his laboratory at the University of East Anglia in Norwich, about 115 miles northeast of London. Johnston and colleagues wrote about their study in today's edition of the journal Science.

The rank but pleasantly memorable odor of the seashore comes from a gaseous bacterial excretion known as dimethyl sulfide, or DMS, that pervades the salt marshes near the shoreline.

Johnston and his colleagues set out to determine exactly what genes enable the bacteria to make DMS. They found that before the bacteria can produce the gas, they need to eat a healthy portion of dimethylsulfoniopropionate, or DMSP, a molecule produced by tiny ocean plants - such as algae and phytoplankton - that congregate around the nutrient-rich seashore.

DMSP prevents salt from infiltrating sea plants and causing them to burst. But if predators or viruses stress out the little plants, they will leak DMSP into the surrounding water.

Before the precious nutrient descends too far into the murky depths, hungry bacteria come along and scarf up the DMSP.

"DMSP is the single most abundant compound in the ocean," said William Whitman, professor of microbiology at the University of Georgia. He and his colleagues recently discovered a metabolic pathway for DMSP that doesn't lead to DMS production.

"Johnston's study is a major breakthrough," Whitman said. "Both DMSP metabolic pathways are extremely important."

The DMS gas is really just a smelly side effect of metabolizing DMSP.

Until Johnston's study, however, no one knew how marine bacteria produced the trademark seashore smell - or whether they produced it at all.

DMS is well known - mostly to environmental scientists who study how its release into the atmosphere affects cloud formation, climate and wildlife.

"Some of the ecological thinkers ... studied DMS without asking where does it come from, at least at a molecular level," Johnston said. "So I was pretty surprised that no one had done it before."

The first step involved slogging through muddy salt marshes outside Norwich on a bacteria hunt. In the lab, researchers looked through their catch to see whether any could live on DMSP alone.

They found one: Marinomonas. After chopping the Marinomonas genome into manageable little pieces of DNA, the researchers took each bit and stuffed it into a strain of bacteria (E. coli) that normally can't consume DMSP.

After adding DMSP to these hybrid E. coli, the scientists and their noses got to work, sniffing test tube by test tube. "After about 20 tubes, you really need to take a smell break," said Johnston "but you can screen 400 to 500 in a day."

When the researchers found a test tube that smelled like the ride home after a day at the beach, they knew those E. coli must contain the chunk of DNA with Marinomonas' DMSP-digesting gene.

Once the identity of the gene was known, Johnston scanned the bacterial kingdom to see what other species could make DMS. That's when he got a surprise. Two of the species he turned up lived on land, far from the sea plants that make the DMSP food supply required for that seashore smell.

How did these land-faring bacteria get this gene in the first place and why do they make DMS if they can't eat DMSP?

The answer to the first question was easy. Bacteria are promiscuous; they have inter-species affairs. "It's like an elephant having sex with an oak tree," Johnston said, "transferring just three to four genes at a time."

Usually, bacteria hang on to these genes only if they confer some evolutionary advantage. Johnston guessed it may help these terrestrial bacteria communicate with one another.

Scientists know that certain birds use the smell of DMS to hone in on food near the seashore, or in ocean pockets where marine life thrives. Also, "DMS attracts tiny crustaceans. They swim like crazy toward it," Johnston said.

Then there's the so-called "Gaia" hypothesis, named after the Greek words for grandmother and Earth. This is the notion that the Earth and its creatures behave like a single organism; when one part of the system gets out of whack, another part puts it back in line.

According to this theory, under warm conditions, sea plants happily multiply, resulting in an increase in DMSP concentrations and thus atmospheric DMS. DMS, in turn, triggers the formation of clouds, which reflect solar radiation back into outer space. The surface temperature gets cooler, and the original sea plant population drops. The balance is maintained.

So can scientists use this knowledge to counteract global warming? "DMS is an anti-greenhouse gas," Williams said. "The easy answer is no, but the profound answer is maybe it is possible to stimulate the DMS producing metabolic pathway."

Johnston is more doubtful. "If we shifted the balance to more DMS production, the temperature would get lower. But then there would be more acid rain and toxic sulfur in the atmosphere, too. I'd put that at about 823 on the list of things to do about global warming," he said.

So what can he do with DMS? Johnston said he's not a Seinfeld fan, but he's intrigued by Kramer's perfume idea.

"I don't think I'd make any money. But I had considered putting it on eBay," he said.

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