Aroma Chemistry - Smell of the Sea

The summer holidays are here, which means there’ll soon be crowds flocking to the coast to spend the day at the beach. The supposed benefits of ‘fresh sea air’ are commonly extolled, but its origins might not be what you think: it’s the chemical compounds produced by algae and seaweed that contribute towards its characteristic smell.

Seaweed is one of the more obvious sources of malodorous compounds. It’s commonly seen washed up on the fringes of the sea, and as it decomposes, it can produce gases that contribute to the ‘sea smell’. The principle gas produced is hydrogen sulfide, which is generated via the bacterial breakdown of organic compounds in the seaweed. Hydrogen sulfide has an odour commonly described as akin to rotting eggs, and is actually a toxic gas in high concentrations.

However, before you start sprinting in terror from seaweed on all future vacations, it’s worth pointing out that, at low concentrations, hydrogen sulfide is harmless. In fact, it’s naturally produced in the body (it’s also a big contributor to the odour of flatulence), and since our bodies are capable of breaking it down, it can be tolerated at low levels pretty much indefinitely. There have been cases where large quantities of seaweed can build up on beaches and produce hazardous levels of the gas – this occurred in France several years ago, when a number of animal carcasses were found as a consequence.

Seaweed isn’t the only plant that has a hand in the smell of the sea, however. Perhaps the most important contributor is algae. Algae contain a compound called dimethylsulfoniopropiante (DMSP for short) in their cells. The precise role of this compound still isn’t exactly known, but amongst other things, it’s thought to regulate the volume of the cells, and the fluid levels. This compound can be broken down, both by enzymes in the algae, and by bacteria. When this occurs, dimethylsulfide (DMS) is one of the compounds that can be produced.

DMS is another compound with a disagreeable odour at high concentrations – often compared to that of cabbage. Birds are actually attracted to the smell, as plankton in the sea also produced the gas, and this can lead them to fish. Huge quantities of DMS are produced in the ocean, with a billion tonnes being a rough estimate. DMS and hydrogen sulfide aren’t the sole contributors to the sea smell though – chemical derivatives of DMS can also have a hand.

DMS also has another role in our atmosphere. Although only around 10% of the compound produced in the ocean is released into the air, when it is, it can be broken down, undergoing chemical reactions which lead to the formation of aerosols. Aerosols are tiny particles of solids or liquids suspended in a gas. In the atmosphere, water vapour can condense around these particles – and this leads to cloud formation. Again, DMS and its derivatives are not the only culprits here, with dust, soot, and other small compounds also taking part in the genesis of clouds.

Finally, is sea air good for you? On that question, the jury seems to be out. Most of the studies that conclude that it is seem to rely on self-evaluations of the health of people living near the sea compared to those living further from it, with actual proven health benefits seeming thin on the ground. That said, as you lay back and savour the scent of the decay products of algae and seaweed, you can see why some people might find the idea that it’s good for them an encouraging thought!

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