The Science Behind That Fresh Rain Smell : Short Wave Scientists have known for decades that one of the main causes of the smell of fresh rain is geosmin: a chemical compound produced by soil-dwelling bacteria. But why do the bacteria make it in the first place? It was a bacteria-based mystery... until now! Maddie gets some answers from reporter Emily Vaughn, former Short Wave intern.
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The Science Behind That Fresh Rain Smell

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The Science Behind That Fresh Rain Smell

The Science Behind That Fresh Rain Smell

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MADDIE SOFIA, HOST:

You're listening to SHORT WAVE from NPR.

(SOUNDBITE OF MUSIC)

SOFIA: Maddie Sofia here with a very special guest Emily Vaughn, our show's first-ever intern, who has left us and is now out in the world making radio.

EMILY VAUGHN, BYLINE: (Laughter).

SOFIA: Vaughn, welcome back to the pod. I'm glad to have you on.

VAUGHN: Hi, Maddie. Yes. And to start us off, I want to play a little game.

SOFIA: Ooh, OK.

VAUGHN: So here's how it works. I'm going to play a few seconds of a sound, and I want to see if you can smell it.

SOFIA: I'm not sure that's how that works. But go on.

VAUGHN: (Laughter) OK. So close your eyes. Listen. Take a deep breath.

SOFIA: (Inhaling).

VAUGHN: And try to smell the smell of this sound.

(SOUNDBITE OF RAIN, THUNDER ROLLING)

SOFIA: Oh, I can do this, actually. That is fresh rain smell.

VAUGHN: Ding-ding-ding. You're a winner. Now, bonus question - do you know what causes that smell?

SOFIA: Oh, God, no - absolutely not. Game over.

VAUGHN: (Laughter) So there are a few different things that contribute to the fresh rain smell, but the main one is a chemical compound called geosmin. It's made by a bacteria, and our noses are incredibly good at detecting it. And besides fresh rain, you might also associate it with something else.

KLAS FLARDH: This compound causes the - for many people, the attractive smell of soil.

SOFIA: Oh, yeah. I do love the smell of soil.

VAUGHN: So do I. And so does Klas Flardh. He is a professor of microbiology at Lund University in Sweden, and he studies the bacteria that make geosmin in soil.

SOFIA: OK, OK. So it's not anything about the rain that smells that way. The raindrops are churning up this stuff, this geosmin, that's been sitting on top of the soil. And then, once it's in the air, we can smell it.

VAUGHN: Yes, exactly. But the real question, as always, is why?

(SOUNDBITE OF MUSIC)

FLARDH: The substance has been known for a long time, but nobody really understood why the bacteria make this. What is the role of it?

SOFIA: Vaughn, are you telling me that we are solving a bacteria-based mystery today?

VAUGHN: Oh, I am, Maddie.

So today on the show...

SOFIA: Hey.

VAUGHN: We're going to...

SOFIA: Wait a minute.

VAUGHN: ...Get to know one of the soil-dwelling bacteria behind the smell of fresh rain and learn the origin story of one of earth's most beloved perfumes.

SOFIA: You're just going to do the whole thing, huh?

VAUGHN: I'm Emily Vaughn...

SOFIA: These are my lines.

VAUGHN: ...And this is SHORT WAVE from NPR.

(SOUNDBITE OF MUSIC)

SOFIA: OK. Emily Vaughn, first-time SHORT WAVE reporter, we're talking geosmin, which we associate with the smell of fresh rain and fresh soil. And you said it comes from a bacteria.

VAUGHN: Yeah. It primarily comes from a big group of bacteria called Actinomycetes. And the geosmin in soil mostly comes from a single genus called Streptomyces.

SOFIA: Ooh, Streptomyces - I love that. Keep going.

VAUGHN: Streptomyces have evolved an uncommon survival strategy for bacteria. I mean, you're a microbiologist.

SOFIA: Indeed.

VAUGHN: What do bacteria do when conditions are good, when they're happy?

SOFIA: Well, when bacteria are happy, they make more bacteria. That's their whole vibe. So one cell splits into two; it's called binary fission.

VAUGHN: Right. And if you're a type of bacterium that can swim or sort of wiggle around, before you divide, you'll maybe try to situate yourself somewhere with a lot of good food for your offspring. But for most of its lifetime, Streptomyces isn't mobile. So if it's in a location with not a lot of food or if it's starving, it has to do something else to be sure its offspring have a good shot at survival.

MAHMOUD AL-BASSAM: This bacterium is actually very weird.

VAUGHN: That's Mahmoud Al-Bassam. He's a researcher at University of California, San Diego, and he worked on the same study as Klas.

AL-BASSAM: As it starts to make sure that its offspring will survive, it starts to produce dormant cells. And these dormant cells can withstand dryness, lack of food. And then once the environment is favorable - so they have food; they have water - they can grow again.

SOFIA: Dormant cells. OK. That means we're talking bacterial spores, which is very exciting. Basically, when some bacteria are stressed, they can produce these tough little cells called spores. And those can withstand things that normal bacterial cells probably couldn't survive, like extreme temperature or drought.

VAUGHN: Yeah. And because it's kind of unusual, Mahmoud's lab decided, we should figure out how this works, like, at the genetic level. And he found that, indirectly, one of the genes that tells the bacteria, hey, it's time to make spores - also tells it that it's time to make geosmin.

SOFIA: Oh, OK. So when it is spore baby-making time, it is also geosmin-making time. So like, maybe the two are linked somehow.

VAUGHN: Exactly. So Mahmoud and his team were like, we've got to explore this. That's when Paul Becher got involved - yes, another scientist. He's a chemical ecologist and a researcher at the Swedish University of Agricultural Sciences.

PAUL BECHER: It is known that certain insects respond to the smell. Our hypothesis was simply that it is used to attract organisms, so it might be an attractant.

SOFIA: So why would bacteria want to attract insects and other invertebrates at the same time that it's making these spores?

VAUGHN: Well, if you think like a Streptomyces bacterium...

SOFIA: I always do. Go on.

VAUGHN: ...You're making spores when the environment around you is harsh. And you want to spread your spores out in the environment to maximize the chance that some of them land somewhere really perfect to grow. But you can't move much. You're living in the soil. You're low to the ground. And you can't really rely on, like, the wind to spread the spores. So your best bet is hitching a ride.

BECHER: So our hypothesis was that they attract microorganisms that help them to disperse their spores. Basically, that's a phenomenon that you could compare to a flower that attracts bees to disperse pollen.

SOFIA: OK. So that makes sense. The attraction makes sense. So how do they go about testing whether or not they're attracting bugs?

VAUGHN: Traps.

(LAUGHTER)

BECHER: We put out sticky traps that we baited with bacterial colonies on the ground here at our campus, hidden in the vegetation.

SOFIA: Very sneaky, as traps should be. So what did they find? What do they think is the thing doing this dispersal of the bacterial spores?

VAUGHN: So what they found were these teeny tiny little invertebrates called springtails.

SOFIA: Ooh, I have a general idea about those. Tell me more.

VAUGHN: So these springtails look sort of like off-white ants - six legs, antennae but less curvy, like an ant and a grain of rice had a teeny tiny baby.

SOFIA: (Laughter) OK.

VAUGHN: They're small. They're about as long as a nickel is wide. And there's this detail, too.

BECHER: If you have a microscope, you will also see that it's a bit hairy. And that's an important aspect with respect to our study.

VAUGHN: Which is important because more spores might get stuck on a hairy body.

SOFIA: Amazing. OK. So how common are these little buggos (ph)?

VAUGHN: Paul says there are thousands of species of springtails worldwide. And they're found almost everywhere on the planet, even on the edge of the ocean. But most often we'll encounter them in the soil. My favorite thing about them is where they get their name. They have this sort of appendage at the end of their body called a furcula that looks like a tail tucked between their back legs. And they can use it like a lever with spring action to jump, like, a hundred times their body length.

SOFIA: I love that. OK. So when Paul put out traps baited with Streptomyces bacteria, the springtails could smell the geosmin in the traps, and they came running or springing. Do we know why they like it?

VAUGHN: For the snacks, Maddie, for the snacks. Springtails eat the part of the bacteria that produce spores. So to them, the smell of geosmin means it's dinnertime. And when they eat, those little hairs on the springtails, they get covered in spores that they then spread around. And...

SOFIA: Ooh, OK.

VAUGHN: ...They also spread the spores around by eating them and then pooping them out later.

SOFIA: (Laughter) SHORT WAVE, where poop always gets the credit.

VAUGHN: (Laughter) It's what the show is known for.

SOFIA: All right. I mean, a couple of things - it's known for a couple things. But OK. Anyway - so let me see if I have this right. When the bacteria Streptomyces gets stressed because maybe their environment isn't great, there's not enough water or whatever, they make these tough little spores. And then they also produce a smelly substance called geosmin. And that smell helps attract tiny soil organisms like springtails to come chow down. And then the bacteria get a ride on either their insect bodies or in their insect bellies to a new environment, hopefully a more bacteria-friendly one.

VAUGHN: Nailed it.

SOFIA: Yes. I mean, this is pretty awesome. I feel like birds and the bees get all this pollination street cred, but this is equally awesome on an astonishingly small scale.

SOFIA: I totally agree. And for my Mahmoud, this is a great example of things you can discover by asking basic research questions like, why does rain smell like it does?

AL-BASSAM: I think one important thing is that we focus on fundamental science. It doesn't have to always be for, like, a reason. Most of the very important findings in science were just because people wanted to know how things work. We have to be risky. We have to be adventurous and spend some money on fundamental science.

SOFIA: You know I'm here for pure research, Emily Vaughn. I am so glad that you brought us this story. Next time I smell fresh rain or soil, I will yell - thank you, bacteria; springtails, I see you out here.

VAUGHN: (Laughter) Absolutely. And to me, the smell is a reminder of how complex all the systems are that we take for granted just right under our feet. And I think that's beautiful, too.

SOFIA: All right, Emily Vaughn. Thank you for this story. Let's - come back with another round of Hear That Smell for us sometime soon. OK?

VAUGHN: You've got it.

(SOUNDBITE OF MUSIC)

SOFIA: This episode was produced by Rebecca Ramirez, edited by Viet Le and fact-checked by Berly McCoy and Emily Vaughn herself. I'm Maddie Sofia. Thanks for listening to SHORT WAVE from NPR.

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