Apparently, it's due to something akin to bacterial fart.
The smell comes from a molecule called dimethyl sulfide (DMS) which is derived from a compound produced by phytoplankton, single-celled organisms found in the sea. Bacteria feed on the compound and convert it into DMS.
Sea birds are not the only animals highly sensitive and attracted to the smell of DMS, as it indicates a plankton bloom, and therefore of fish feeding on the bloom. But also humans! Most people notice it at 0.02 parts per million. Among others, DMS is added to processed foods to give a savoury note: small amounts can impart the flavour of cabbages, tomatoes, butter and cream – even lemons or roast chicken.
DMS is also an important chemical found in many natural processes, such as cloud formation.
"One thing I have learnt along the way is that it is microbes that drive this planet – everything else on earth is mere decoration." said Prof Andy Johnston who studies DMS.
The science behind that fresh seaside smell
A tiny molecule lurks behind the evocative smell of the seaside.
Sanjida O'Connell, The Telegraph 18 Aug 09;
Think of the tangy smell of the sea, so evocative of summer holidays, the scream of seagulls and sand between your toes. Where does it come from? Ozone? Fresh sea air? Actually, the truth is slightly less tantalising: it's a gas released by bacteria.
Two years ago Andy Johnston, a professor of biology at the University of East Anglia, identified that the smell of the sea came from a molecule called dimethyl sulfide (DMS). Now, he has managed to crack the entire biochemical pathway by which the scent is produced. DMS turns out to be an important chemical found in many natural processes, such as cloud formation. Birds love the smell and will flock towards tiny concentrations. It's even added to processed foods to give a savoury note: small amounts can impart the flavour of cabbages, tomatoes, butter and cream – even lemons or roast chicken, according to Prof Johnston.
DMS is derived from a compound called dimethylsulfoniopropionate (DMSP), which is produced by phytoplankton, single-celled organisms found in the sea. DMSP is incredibly abundant – around a billion tonnes are formed every year. Bacteria feed on it and convert it into DMS. This is why birds are attracted to the smell: it indicates a plankton bloom, and therefore the presence of fish feeding on the marine plants.
Prof Johnston used genetics to try to understand how DMS is produced. He took some mud from Stiffkey Marsh in Norfolk. He isolated the bacteria that feed on DMSP from the mud, extracted the relevant genes and inserted them into E. coli, a bacterium used in laboratories.
"I knew we'd isolated the right genes," he says, "because the incubator smelled like a beach. When the concentration rose too high, it smelled like rotting cabbage." Thanks to its distinctive smell (most people notice it at 0.02 parts per million), DMS is often added to otherwise odour-free gases so that leaks can be detected.
Prof Johnston's latest finding is that bacteria use three different mechanisms to make this gas. He likens it to taking Route 66 from Chicago to Los Angeles by Cadillac or by train: "You're making the same trip, but the methods used are poles apart." His research will be published shortly in the journal Environmental Microbiology.
In addition, Prof Johnston has found that other organisms can produce DMS, too. "Unlike animals which only have sex with members of their own species, bacteria are less restrained. They can have sex with almost anything that moves – they don't give a damn who they transfer genes to," he says. "We found that genes to break down DMSP had been transferred from bacteria to fungi. That's passing genes across animal kingdoms – it's the equivalent of a mouse having sex with a sycamore tree."
Prof Johnston is now working in collaboration with the J Craig Venter Institute in America, whose scientists are sampling millions of genes from marine bacteria. The Venter Global Ocean Sampling project shows where genes are found in the ocean and in what quantities. "I can see that genes for one mechanism of creating DMS are abundant in the Galapagos Islands but not in other areas, but the genes for a different mechanism are widespread in the Sargasso and the Pacific, all from the comfort of my own lab," explains Prof Johnston.
It is not known why some bacteria species are prevalent in particular areas, nor why there appear to be hotspots around coral reefs. Equally mysterious is the fact that we still do not know why DMSP, from which the seaside gas is produced, even exists: it could be a way of protecting seaweed against the sun's ultraviolet light, or the saltiness of the sea.
DMS gas has important commercial applications. The fungus to which it has transferred is a type of Aspergillus, used in soy sauce, sake and tofu production, and one of the major components that gives truffles their earthy odour. Meanwhile, the US-based Gaylord Chemical Corporation, the largest manufacturer of DMS, produces it for use in petroleum refining and in the hydrocarbon industry to synthesise ethylene, a chemical that has a wide variety of uses from creating plastic bags to hastening the ripening of fruit.
"This is just the start," says Prof Johnston, "We need to know more. But one thing I have learnt along the way is that it is microbes that drive this planet – everything else on earth is mere decoration."