IndiaAsksWhy | Why does rain smell good?

IndiaAsksWhy Episode 5 | S1

☁️It’s a cloudy evening. You are sitting at your window and, suddenly, it starts raining 🌧 Along comes that earthy smell 😌Ever wondered, why does rain smell so good? 😍

In the last episode of season 1 of IndiaAsksWhy, Utsuka and Jigyasa talk about the origins of the smell of rain!🌦

They discover how microbes and the winds together lead to this lovely Aroma! 💨

👨‍🔬In the Ask A Scientist segment, they speak to Samay Pande (https://​www​.redqueen​lab​.com), who studies the evolution of bacteria to understand why organisms help each other! 🧪

To get instant updates on all the upcoming episodes of IndiaAsksWhy, follow IndiaBioscience on Spotify.

You can send your questions to Utsuka and Jigyasa at indiaaskswhy@​gmail.​com, or join their Telegram group for updates and fun science discussions.

Join Utsuka and Jigyasa as India becomes more curious, one question at a time!

Podcast Duration: 17 min 4 sec

Stay tuned for more updates from IndiaAsksWhy!

Utsuka — U, Jigyasa — J

Transcripts with Timestamps

[00:09] Both- Hi Everyone”

U- I am Utsuka

J- I am Jigyasa

Both- And you are listening to IndiaAsksWhy

U- A science podcast supported by IndiaBioScience

J: Where we do the research for you to get smarter.

U: Join us, as India becomes more curious,

Both: one question at a time!

[00:34] U: One evening when Utsuka and Jigyasa were enjoying the winds at their window, it suddenly started to rain. Along with it came the sweet smell of rain which made them wonder, Why does rain sometimes smell so good?” While exploring the reasons why they found out that a chemical produced by a specific bacteria is behind the smell. In the second segment, they talk to Dr. Samay Pande, a researcher who studies bacteria to know more about the fascinating chemical molecules that the bacteria produce.

[01:35] J: It’s that time of the year again, monsoon and lots of rain. Doesn’t it feel so good to hear the raindrops hitting the roofs and windows?

[01:46] U: Hmm.. The good smell of the rain brings me back so many memories. Umm.. What is this smell called again? Petrichor, yes.

[02:00] J: Pterichor? That sounds poetic. I wonder why rain sometimes smells so good?

[02:11] U: So the smell of wet earth is not because of rainwater itself but because of various chemical molecules. The main one is called Geosmin’ which is produced by spores of a bacteria living in the soil called Streptomyces. [02:29] J: Wait, wait, wait. That’s a lot to unpack here. So many big words like geosmin, streptomyces, spores, and bacteria. Go slow, and tell me everything.

[02:40] U: Hehe.. okay. I’ll slow down. So Streptomyces is a spore-producing bacteria that live in the soil.

[02:48] J: What are bacterial spores? Are they different from bacterial cells?

[02:53] U: So Jigyasa spores are tough structures that are dormant meaning they are not performing any life-like activities or consuming nutrients. Spores help in dispersion. They can move from one place to another to…

[03:09] J: But why do these streptomyces bacteria have to produce spores to move from one place to another? Can’t the normal bacterial cell move?

[03:18] U: No, No. Streptomyces is very unique. Unlike other bacteria which are single-celled or have special structures which can help them move, streptomyces cells are joined together with each other to form a thread-like structure. These bacterial cells are entangled and run inside the soil.

[03:39] J: But first tell me, why do these bacteria have to move places?

[03:44] U: Well imagine yourself as streptomyces, okay? You are stuck at a place that is so running out of nutrients. What would you do?

[03:55] J: Yeah… If the conditions at the current residence aren’t favorable, I should be looking for a better place and move out. Shouldn’t I?

[04:04] U: Exactly. These bacteria love food. So that’s why spores come into the picture. Streptomyces threads give out a special structure called hyphae that is made up of multiple cells. Hyphae grow out of the soil surface which gives rise to multiple spores which can then get dispersed. So you can escape from that unfavorable condition.

[04:32] J: So bacteria make the spores on their hyphae for dispersion. Okay. That makes sense. But what has this got to do with that geosmin?

[04:46] U: Geosmin, yes. So these bacterial spores are sitting on the hyphae and are looking to hit a ride to a different place, right? So they are stuck in the soil and can’t move.

[04:59] J: They need a vehicle of some sort, right?

[05:04] U: Exactly. So that’s why the latest research says they produce geosmin to attract their vehicle. The vehicle here is called a spring tails’ which is a tiny-winy six-legged animal.

[05:18] J: Woah! I’ll look up its image online.

[05:22] U: So as you see the geosmin that streptomyces produce attracts springtail. Springtails think it’s food and come to feast on these bacterial spores.

[05:35] J: Wait.. You say springtails are vehicles for these bacteria to hitchhike, right? Now you say the spores get eaten?

[05:43] U: Don’t worry, don’t worry. So bacteria produce spores in large numbers. So while springtails are eating those spores, some latch onto the surface of the springtails’ body and when the springtails move to a different place, they drop off these spores that were once stuck. Some spores even make it out of the springtail’s digestive system, unharmed, and get dispersed that way too. And then the spores restart their new life at the new place. That’s pretty cool, right?

[06:19] J: That’s a very interesting interaction between springtails and streptomyces. But what I still don’t understand is, if the geosmin is produced in the soil, how does it reach my nose?

[06:34] U: So when the rain hits the soil, at the point of contact larger water droplets form smaller air bubbles which rise upwards and burst into tinier aerosols. So do you know what aerosols are?

[06:51] J: Yes, aerosols are solid or liquid droplets suspended in the air, like the droplets which come out from the deo bottle when I spray it on my body, right?

[07:02] U: Yes, and these aerosols contain geosmin and other aromatic elements which gives petrichor the smell. The wind then carries these aerosols to your nose.

[07:14] J: hmm.. This makes me wonder, in a different condition, would the bacteria produce a different molecule and would it do something else?’

[07:23] U: You’ve got to hold on to know the answer to that question, Jigyasa. We are going to ask Dr. Pandey in the next segment about this very same question. So, stay tuned!

[07:39] J: And now, it’s time to ask a scientist.

[07:44] J: Hi everyone! Getting back to the second segment of the interview, we have with us today Dr. Samay Pandey from IISc, Bangalore. Utsuka, why don’t you go ahead and ask your question to Dr. Samay Pandey?

[07:57] U: So Dr. Pandey, we just learned that geosmin which is the chemical behind the smell of the rain is produced by bacteria. And that it is produced to attract springtails to help for its dispersal. We also learned that geosmin is what is called a secondary metabolite. So can you talk to us a little bit about what secondary metabolites are and why do bacteria produce them?

[08:25] S: That is a deep question but now there are compounds that many organisms make which are not necessarily giving any growth benefits or energy. For any life form to function, you need energy. So that’s why you make many compounds. Some compounds are giving energy that is primary metabolites, some compounds you produce but there is no obvious reason why you would produce them as a bacterial cell because they are not giving any energy. So those are secondary metabolites. Now, what are the different types of secondary metabolites? So I’ll just give some examples. And I think probably one of the compounds that I would highlight most importantly which has probably changed the course of human evolution with mild yield is antibiotics. Now another example of secondary metabolites can be what is now famously known as coral sensing molecules’. So these are the molecules that are used by bacteria to talk to each other, to communicate.

[09:24] U: So how do you know these bacteria are communicating with each other using this secondary metabolite and so on?

[09:30] S: In our lab, we are interested in sort of, more of why questions. We are interested in, Why should you be social?’ Why should they communicate with other bacteria? Why should they help each other? So what we do is, study something called experimental evolution’ or evolution in a test tube’. Now think of an approach that will allow you to go back and forth in an evolutionary timeline, right? The simplest way one can do it is, the simplest experiment that we do in our lab is, let’s say we are working with a bacteria that makes a secondary metabolite that kills other bacteria. And we know that making the secondary metabolite is very co.. So what we do is, we grow the bacteria that makes the secondary metabolite, have another bacteria that is now susceptible (i.e can be harmed by the secondary metabolite), and put them together in a test tube, okay? Now what these bacteria do is, grow and make multiple copies of themselves. Then we transfer a fraction of these populations, let’s say out of one billion, we transfer a million to new test tubes with fresh food. And we keep growing it every day in our lab. Now when we transfer one million to a new tube, the remaining almost billion cells, we also freeze them at ‑80 degrees. That’s a temperature we are talking about at which the bacteria can stay alive without changing their biology. So after 50 days of doing this experiment, we have something which we called an evolved bacteria’ which has changed itself to acquire new ways of behaving with other bacteria that are harmful or helping and all the ancestors are frozen. So when you go back and forth in this evolutionary time, you observe a change in biology to understand how an aggressive bacteria which is making the secondary metabolite is changing to make that metabolite more efficient in killing whatever it wants to kill or harm. And at the same time the bacteria that is being harmed, how is it changing over time to protect itself better? That’s how we study these behaviors.

[11:53] U: Oh wow! So you are basically studying this chemical warfare inside a test tube.

[11:59] S: Perfect. So we are studying evolutionary warfare or chemical warfare as you correctly point it out in a test tube. Though we are interested in the questions of why but that why has a mechanism underneath it. So why will a bacteria do this or a cell do this?’ is a great question to ask. To basically understand how life revolves around a planet.

[12:23] U & J: That is super interesting.

[12:24] U: So fascinating. So yeah, why do social interactions among the bacteria or the evolution of bacteria interest you so much?

[12:41] S: So what interests me most is the evolutionary process and social evolution in general. This started off with a very simple sort of question that I couldn’t solve and later on realized this is probably one of the cornerstones of evolutionary biology in general. Why should anything help anybody else? An organism is helping another organism and by helping I mean actually helping. You invest your energy or resources, right? In theory that energy or resources can be diverted towards making more offspring, making more babies of yourself. So why won’t you make more babies of yourself which is sort of fitness as darwin said evolution of fittest? So why should you invest your energy and resources in helping somebody else and not help yourself to basically be successful evolutionarily? Right? So this was a basic question that fascinated me and then I realized that microbes are easy and more convenient model organisms to ask these types of questions.

[13:50] J: Coming to the end of it now we would like to ask, what is this advice that you would like to give to our listeners on nurturing curiosity as well as taking science as a career in the future?’

[14:02] S: I think at the crux of it all if you are curious just try to answer your curiosity and you will automatically develop into a scientist. So you don’t have to be in a lab, you don’t have to be associated with active academia. As long as you have started asking questions and you try to find its answers based on available information and not on hypothesis, just basically predicting something, but rather relying on information that you have in your hand, you are already a scientist.

[14:39] U: Yeah.. Jigyasa and I always love asking questions and it is good to know we are already becoming budding scientists, right?

[14:47] U: So Jigyasa, do you now understand why rain smells so good sometimes?

[14:55] J: Yes, the rain smell is due to a molecule called geosmin which is produced by the spores of a specific bacteria living in the soil. They make this molecule to attract their transport vehicle, the springtails. When the rain hits the soil, they produce tiny aerosols containing geosmin that reaches our nose when the wind blows.

[15:19] U: Yes. And Dr. Pande spoke to us about many other molecules like geosmin which bacteria produce. He also described super interesting evolution in a test tube experiment he performs to study why bacteria produce such fascinating molecules.

[15:40] J: True, true. But listeners, what we know about geosmin and rain smells may change over time as we get more evidence.

[15:50] U: Who knows one of our listeners might be interested in finding the role of more such molecules like geosmin.

J: So U: Stay Curious Both: and ask questions

[16:04] U: So, that’s it for today. We would like to thank Dr. Samay Pande for being on our episode.

J: If you want to know more about him, we have linked his profile in the Show Notes!

U: If you have any questions that you want us to explore, shoot them away to Indiaakswhy@​gmail.​com

J: If you’d like to directly talk to us and join the fun, join the fun on our Telegram Group. Link in the show notes.

U: For updates on IndiaAsksWhy, follow IndiaBioScience @IndiaBioscience on Twitter, Instagram, Linked In, and Facebook! (link in the show notes as well!)

J: Shweata N. Hegde and Ruchi Manglunia are the hosts of the podcast.

U: Ira Zibbu helped us transcribe and design the interview segment.

U: And we are thankful to IndiaBioscience for hosting us.

J ‑So stay tuned and stay curious!