What Is the Smell of the Sea?
The Gas Released When Plankton Get Eaten
Sometimes you can tell "the sea is near" with your nose before you see it. That smell is not the smell of salt. It is a gas given off when tiny, invisible plants are eaten or broken apart. And your nose can pick it up even when it is a mere speck in the air.
You have the car window open a crack, heading down a coastal road. Buildings still block the view, and you can't see the sea.
Yet a distinctive scent drifts in. It is a little fishy, a little like seaweed, and oddly nostalgic.
But if you dissolve table salt in water, you don't get that smell. So what is your nose actually picking up?
Two ideas explain the smell of the sea
Phytoplankton floating in the sea store a substance called "DMSP" inside their bodies. When it breaks down, it produces a gas called "dimethyl sulfide (DMS)". This is the star of the sea smell.
Dimethyl sulfide smells even when it is extremely dilute. People are said to notice it when just one molecule is mixed in among about 300 million air molecules. That is why you can tell before the sea comes into view.
So the smell of the sea is not the smell of "seawater itself". It is a signal rising from the scene where living things are eating and being eaten.
When plankton are "eaten", the smell is released
Phytoplankton are thought to store DMSP to keep the water in their bodies balanced in salty seawater. DMSP itself has almost no smell.
The change happens when a plankton cell breaks open. If it is nibbled by a tiny animal plankton, or destroyed by a virus or bacteria, its contents spill out. Enzymes then cut the DMSP apart, and dimethyl sulfide forms.
Dimethyl sulfide does not dissolve well in water, so it escapes from the sea surface into the air. Look at Figure 1. The gas made in the sea at lower left spreads upward like the arrows, and reaches the person's nose on the right, the seabird above, and even the clouds.
This smell tells seabirds, "There's food here"
To us it is a pleasant scent, but to seabirds it is life-saving information. Shearwaters, albatrosses and their relatives are known for having a good sense of smell, for birds.
Where zooplankton are busily eating phytoplankton, dimethyl sulfide gets stronger. Small fish and krill that feed on zooplankton gather there too. By flying toward the stronger smell, seabirds are thought to find feeding grounds in a sea that looks the same as far as the eye can see.
In recent years, though, a worrying finding has emerged. Algae grow on plastic drifting in the sea, and it can give off the same smell. This is thought to be one reason seabirds mistake plastic for food and swallow it.
Dimethyl sulfide is not found only at the sea. It is also said to be part of the scent of green laver (aonori) seaweed, and of the smell of boiled cabbage or sweetcorn. Strong, it is unpleasant. Faint, it can feel "like the sea" or "appetising". The impression of a smell can change completely with its amount.
A smell that seemed strong when you arrived at the sea stops bothering you after a few minutes. The smell has not gone away. Your nose and brain are turning down their response to a stimulus that does not change. It is a body mechanism that makes new changes easier to notice.
Summary
The smell of the sea is not the smell of salt. It is the smell of dimethyl sulfide, formed when DMSP stored by phytoplankton breaks down. Your nose notices it even when very faint, so you can tell before the sea is in view. That smell guides seabirds to feeding grounds, and is also thought to play a part in how clouds form.
The smell of the sea is news that the sea's "eating and being eaten"
has reached all the way into the air.
Smells that arise from the air are also covered in the article on the smell of rain, and living things that find their way by smell are covered in the article on salmon returning to their birth river.
- When you visit the sea, compare how strong the smell is when the wind blows from the sea and when it blows from the land. The wind from the sea carries the smell to you.
- Check whether the smell seems different right after you arrive at the beach and again after about 10 minutes. You can notice your nose getting used to it.
- At home, try smelling a bag of green laver as you open it. You may find it somehow similar to the smell of the seaside.
How people sense smells varies from person to person. At the seaside, don't get too close to the water's edge, and watch your step as you observe.
For those who want to know more: terms, formulas and links to textbooksEach part is labelled by level, from junior-high science to university specialist courses
- Junior highCovered in junior-high school science
- High schoolCovered in high-school chemistry and biology
- High school+Advanced high-school material, or textbook sidebar content
- UniversityNot taught in high school; content from university specialist courses (marine chemistry, atmospheric chemistry, sensory physiology)
- ResearchNot taught even at university as settled fact; questions researchers are studying right now
Junior highTerms: this phenomenon has names
- Dimethyl sulfide (DMS): A small molecule made of one sulfur atom with two methyl groups (clusters of carbon and hydrogen) attached. It is thought to be the main component of the smell of the sea.
- DMSP (dimethylsulfoniopropionate): A substance that phytoplankton and seaweed store in their bodies. When it breaks down, dimethyl sulfide forms.
- Odour threshold: The faintest concentration at which a person can notice a smell. It differs greatly from substance to substance.
Junior highHigh schoolChecking with a formula: how many smell molecules enter your nose in one breath?
Suppose you breathe in one breath of air containing the faintest dimethyl sulfide a person can notice. Let's work out, from the gas equation, how many dimethyl sulfide molecules enter your nose.
| In symbols | N ＝ x × ( P × V ÷ ( R × T ) ) × NA |
| In words | Number of smell molecules ＝ mixing fraction × (moles of air breathed in) × number of molecules per mole |
| Where it comes from | The moles of air come from the ideal gas equation (pressure × volume ＝ moles × gas constant × temperature), and are converted to a number of molecules with Avogadro's constant |
| Symbol | Meaning and unit |
| N | Number of dimethyl sulfide molecules entering the nose (count) |
| x | Fraction of the air's molecules that are dimethyl sulfide |
| P | Air pressure (pascals) |
| V | Volume of air in one breath (cubic metres) |
| R | Gas constant (joules per mole per kelvin) |
| T | Absolute temperature (kelvin) |
| NA | Avogadro's constant (number of molecules per mole) |
| Gas constant R | 8.31 |
| Absolute temperature T at 20℃ (kelvin) | 293 |
| One atmosphere P (pascals) | 101300 |
| Volume V of an ordinary breath | 0.5 litres |
| Avogadro's constant NA (unit is "gai", 1 gai being 100 million times 1 trillion) | 6020 |
| Odour threshold x of dimethyl sulfide (said to be a value measured by the triangle odour bag method) | 3 parts per billion |
| Gas constant × temperature | 8.31 × 293 ＝ 2434.83 |
| Volume of 1 mole of air (cubic metres) | 2434.83 ÷ 101300 ≒ 0.0240 |
| Convert to litres | 0.0240 × 1000 ＝ 24 |
| Moles of air in one breath | 0.5 ÷ 24 ≒ 0.0208 |
| Number of air molecules in one breath (gai) | 0.0208 × 6020 ≒ 125 |
| Convert gai to trillions | 125 × 100000000 ＝ 12500000000 |
| If 1 in 1 billion (trillions) | 12500000000 ÷ 1000000000 ＝ 12.5 |
| If 3 in 1 billion, dimethyl sulfide entering the nose (trillions) | 12.5 × 3 ＝ 37.5 |
Even for the faintest sea smell, about 37 trillion molecules enter your nose in one breath. Yet out of all the air, that is only 1 in 300 million.
| Volume of the Tokyo Dome (cubic metres, as commonly cited) | 1240000 |
| One part in a billion | 1240000 ÷ 1000000000 ＝ 0.00124 |
| Three parts in a billion (cubic metres) | 0.00124 × 3 ＝ 0.00372 |
| Convert to litres | 0.00372 × 1000 ＝ 3.72 |
That is like mixing only about two plastic bottles' worth of dimethyl sulfide gas into a Tokyo Dome full of air. A human nose notices a dilution that thin.
High schoolHigh school+Why does it escape from water into air?
High schoolDimethyl sulfide has small molecules and almost no parts that mix well with water. So its boiling point is low, about 37℃, and it does not dissolve well in water. When it forms near the sea surface, it easily escapes into the air as a gas.
High school+The ratio of how much a gas dissolves between water and air is described by "Henry's law". Dimethyl sulfide is on the side that is more stable in air than in seawater, and the rougher the sea surface is whipped by wind, the faster it is said to escape into the air.
UniversityEnzymes, and the effect on clouds
The enzyme that cuts DMSP into dimethyl sulfide and acrylic acid is called "DMSP lyase", and both algae and bacteria are thought to have it. Once in the air, dimethyl sulfide is oxidised by highly reactive molecules formed by photochemical reactions, and ends up as tiny sulfuric acid particles. These particles become the cores of cloud droplets (cloud condensation nuclei). In 1987 this led to the proposal of the CLAW hypothesis: that phytoplankton regulate the climate through clouds. How a person senses a smell is set by which molecules bind to the olfactory receptors, and how much.
📖 Derivations and further reading: Dimethyl sulfide (Wikipedia) ／ Ideal gas (Wikipedia)
ResearchWhat is still not clear
- Do plankton regulate the climate through clouds? The CLAW hypothesis has been widely tested, but a 2011 review concluded that "the regulating effect is smaller than assumed", and no conclusion has been reached.
- Why do plankton store DMSP? Besides coping with salt, there are several ideas, such as neutralising substances that damage cells and making the plankton harder to eat.
- How does the nose sense sulfur smells so sharply? There are reports that receptors and copper ions are involved in detecting sulfur-containing smells, and the mechanism is still being studied.
In other words, this article too is "an explanation within what is known so far". The smell of the sea is thought to include other substances too, such as those given off by seaweed.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: "Links between living things" and "The nature of matter" | Eating and being eaten; properties of gases |
| High school | Chemistry: "Equation of state of gases" and "Amount of substance" | Calculating the number of molecules in one breath |
| High school+ | Chemistry: "Solubility of gases" | Henry's law and how the gas leaves the sea for the air |
| University | Marine chemistry, atmospheric chemistry, sensory physiology | DMSP lyase, cloud condensation nuclei, olfactory receptors |
| Research | Marine ecology, climate science | Testing the CLAW hypothesis; seabirds and plastic |
| ― | Links to daily life | Seaside smells; the smell of green laver and boiled vegetables |
- Japanese Wikipedia, "Dimethyl sulfide" (ジメチルスルフィド（硫化ジメチル）)
- Charlson, Lovelock, Andreae, Warren (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326
- Nevitt, Veit, Kareiva (1995) Dimethyl sulphide as a foraging cue for Antarctic Procellariiform seabirds. Nature 376
- Savoca et al. (2016) Marine plastic debris emits a keystone infochemical for olfactory foraging seabirds. Science Advances 2
- Yoshio Nagata and Takafumi Takeuchi (永田好男・竹内教文), "Results of odour threshold measurement of odorous substances by the triangle odour bag method" (三点比較式臭袋法による臭気物質の閾値測定結果), Japan Environmental Sanitation Center Bulletin (日本環境衛生センター所報)
- Quinn, Bates (2011) The case against climate regulation via oceanic phytoplankton sulphur emissions. Nature 480
* This article is a general-audience science explainer. The figures given are rough guides for understanding the mechanism. Odour thresholds vary greatly with the measurement method and from person to person.