This is Science Friday. I’m Manoush Zomorodi, host of the podcast Note to Self from WNYC, filling in for Ira Flatow. Later in the hour, we’ll take a look at content moderation, the work being done behind the scenes to keep offensive photos and videos out of your sight online. But first, a favorite topic here at Science Friday is the microbiome.

We have talked about the microbiome of babies, buildings, even werewolves. But there is a place where bacteria lurk that may still surprise you. The clouds and the sky have a microbiome too.

This week, a study in the journal Nature Communications showed how soil bacteria can hitch a ride on rain drops and float up into the sky. It sounds picturesque, right? But once those bacteria are in the atmosphere, what do they even do up there? What are they interacting with? And how do they affect the weather and climate?

Well, my next guests are here to talk about the microbiome above. Cindy Morris is research director at the French National Institute for Agricultural Research in Avignon, France. Hi, Cindy.

CINDY MORRIS: Hi, Manoush.

MANOUSH ZOMORODI: Hi. Athanon– I knew I was going to do this, Dr. Nenes. Athanasios Nenes is Professor of Atmospheric Sciences, and also Chemical and Biomolecular Engineering at Georgia Tech in Atlanta.

ATHANASIOS NENES: Hello.

MANOUSH ZOMORODI: Hi, sorry about ruining your name. People do it to me all the time, so forgive me.

ATHANASIOS NENES: No, no, you did a great job.

MANOUSH ZOMORODI: And by the way, neither of them were involved in the study published in Nature Communications. And if you, listeners, have questions about how bacteria in the atmosphere might affect us here on the ground, you can call us at 844-724-8255. That’s 844-SCI-TALK. Or you can tweet at us @scifri.

So, Cindy, let’s start with you. What are the ways we know that bacteria can get transported into the sky. How does it happen?

CINDY MORRIS: Well, bacteria are very, very light. And so almost any air movement can lift them up. And generally air, especially during the day, there is a net upward movement. And so they’re riding these currents from the heat of the soil upward. And they’re so light, gravity just doesn’t have much force on them. And so that’s probably the main driving factor to get them up in the sky, the natural convective movements of the air turbulence.

MANOUSH ZOMORODI: And so where are they coming from? Are they just hitching rides on raindrops? Is that what’s happening?

CINDY MORRIS: Well, the rain drops is called splashing. And probably much of that will come right back down to the ground because those drops are rather heavy. And so most of the bacteria that are going upward are coming either from soil surfaces or mostly plant surfaces. Plants are covered with many, many, many bacteria. Healthy plants are covered with bacteria. And there are bacteria on everything, our skin, all sorts of open surfaces. So these bacteria lift off, sort of like dandruff, and float up into the atmosphere. And they can go all the way to the stratosphere.

MANOUSH ZOMORODI: So, Athanasios, I understand that, as part of a study a few years ago, you actually flew through a hurricane to collect samples of bacteria in the atmosphere. That sounds exciting and dangerous. You were about five miles up. What did you find up there?

ATHANASIOS NENES: So we were actually flying pretty much on the top of the hurricane, the air that comes at the outflow at the top of one of those systems. And just to see what’s there, whenever you get to fly through one of these systems, it’s a huge opportunity. You don’t often go through these. And we just wanted to see what’s up there. And maybe we could see things like biological particles.

So we sampled the air masses. And, to our surprise, we found that a lot of the larger particles that were around were, to a large extent, biological, bacteria. And so, yeah, that’s what we found. So pretty much it was a surprise to us and a surprise to a lot of the community.

MANOUSH ZOMORODI: And so there were different types of bacteria up there?

ATHANASIOS NENES: So, yeah, we collected DNA from these bacteria. And we sequenced them, and we identified somewhere around 100 classes of bacteria types. And some of them were from ocean. Some of them you can identify them as terrestrial bacteria. And some of them you tend to find them all over the place, about 20 types of them. Around 100 we found, and about 20 were from all over the place.

MANOUSH ZOMORODI: You’d think that the conditions would be pretty harsh up there. How do they survive?

ATHANASIOS NENES: So a lot of these, by the time they reach those altitudes they’re completely frozen. So the idea is that perhaps they have mechanisms to cope with these stressors. If you freeze some ice around them, they may form a protective layer. Water can help stop some of the stresses from affecting these bacteria. But it’s actually unclear at this point what exactly they do. Some bacteria are known to have mechanisms to help cope with UV exposure. Some are known to withstand droughts. Some are known to withstand very high oxidant levels. So perhaps the 20 types that we saw, which you can find everywhere, have just the right sort of machinery to withstand the atmospheric conditions.

MANOUSH ZOMORODI: Cindy, I wonder if you could tell us– some of the reading I did, talked about this idea ice nucleation. You have worked with bacteria in cloud chambers. Does this mean that bacteria might play a part in the rain making process? Is that what this is about?

CINDY MORRIS: Yeah, this is what the big question is. It’s been driving research for the past decade. People are very much excited about it. So ice nucleation, roughly– if I were to ask you what temperature water freezes, most of us have learned in school that it freezes at zero degrees Celsius, which is the temperature above which water cannot turn into solid ice. But below those temperatures you need a catalyst. And that’s what this ice nucleation is. It sort of kick starts the freezing process at temperatures warmer than it would happen– then would happen without it. And without it water can cool down to about minus 39 degrees Celsius, which is about minus 39 degrees Fahrenheit.

And so most clouds, cloud tops, rain clouds, are not at these low temperatures. And so in order to start the process that would get cloud droplets to aggregate and become heavy enough to fall, you generally need freezing. In the temperate regions you need freezing process where an ice crystal then will collide with droplets that are also cold, but they haven’t frozen yet. And this is how the heavy droplets can form.

MANOUSH ZOMORODI: I feel like I am never going to look out the window of an airplane and see the clouds out there, and see them the same way. Cindy, just to follow up on that, so this is a good thing that bacteria in the clouds can do. Can there be a case of too much of a good thing in this case? Can there be too much bacteria in a cloud?

CINDY MORRIS: Well, if they’re ice nucleation active– some of my physics colleagues have actually discussed this idea with me, which was intriguing. I didn’t understand this, but in fact, what happens is that, in order for rain to form, you have millions of tiny droplets in a cloud. And about 10 to the sixth, so one with six zeroes after it, have to get together to fall. But most of them need to be liquid to do that. So if all of them were ice crystals, if every cloud droplet had something that could cause ice, that ice would remain suspended in the atmosphere, and the cloud would not precipitate. And so, yes, it is possible if you have an excessive amount of ice nucleators in a cloud to actually, what we’ll say, constipate the cloud.

MANOUSH ZOMORODI: And when you get constipated [INAUDIBLE] will the clouds have lightning? I mean, is that something that could happen then?

ATHANASIOS NENES: So you’d need to have both water and ice to have lightning occur. And so it’s a very interesting situation having too many bacteria around. Yeah, you would just have ice clouds all over the place.

[CHUCKLING]

They wouldn’t be [INAUDIBLE]

CINDY MORRIS: But you do need ice, which is a really interesting phenomena. You do need ice to have lightning. And so we have colleagues in Brazil, atmosphere physicists, who actually injected into a model particles that had the properties of bacterial ice nuclei. And they generated more lightning with them than without them. And so this is another interesting idea, that perhaps biological ice nucleators, these microbial particles that can cause ice nucleation, could also have a role in lightning.

MANOUSH ZOMORODI: Yeah, do you want to add to that? Please.

ATHANASIOS NENES: Yeah, so one thing that actually is interesting is that the ice nuclei are always a lot less than the particles that make droplets. So when you have an enhancement device nuclei, say you add more bacteria, you would always tend to have that tendency of making more rain and more thunder and lightning because of that. It’s a great point.

MANOUSH ZOMORODI: Yeah, so I guess I’m wondering, what do the bacteria get out of all this? Why would it evolve this kind of life cycle? Is there any indication of that?

CINDY MORRIS: Well, we’ve looked at this– that’s part of my research, in fact. We’re interested in ecology. And so first, what happens is that if you look at rainfall precipitation that has been initiated by ice formation in a cloud, it enriches for ice nucleation active bacteria. So there’s more coming down with the rain than there are, on a proportion basis, per constant volume of water, in the cloud. And so the first ideas is that we think it just basically helps the bacteria fall out of the clouds. Because like you said earlier, the conditions up there are harsh, and if the bacteria remain too long in the clouds, they probably eventually will die. So this is a sure down elevator. The elevator is going down for them. And they need to get on it because otherwise they won’t fall on their own.

MANOUSH ZOMORODI: And so, Dr. Nenes, you actually are also doing flights, if I understand it correctly, over Crete. What are you hoping to find out there?

ATHANASIOS NENES: We’re hoping to find what sort of bacteria get blown with the winds when they come from the deserts, versus when they come from the continental European lands, as well as the Middle East. It’s a nice place to be because you have all kinds of air masses. And each air mass has their own characteristic biological particle. And so, if you sit around there for long enough, you can actually get a very good idea of who is going to be there at which concentrations, which time of the year.

And then, once you have a good idea of their properties, how easily they make ice, what are the conditions they need, how much they can withstand the atmospheric stressors, then can get a much better idea of what’s the role of bacteria on cloud formation and precipitation.

MANOUSH ZOMORODI: Is there any– you think of bacteria, and I think of disease. Is there any evidence that this could occur? That bacteria could spread disease through the air?

[INTERPOSING VOICES]

CINDY MORRIS: Well, the bacteria that– excuse me, Anathos.

ATHANASIOS NENES: Oh, sure, sure, go ahead.

CINDY MORRIS: The bacteria that cause the ice nucleation– the one that’s most well-known– there’s several types of organisms that do this. But there are bacteria and fungi, the rust fungi, that also can cause ice nucleation, they are known plant pathogens. Now it doesn’t mean that when they come into contact with a plant, at least for the bacteria, that they’ll cause disease. But this is part of their lifecycle. And they can cause disease to plants. So it’s really interesting because in the future, a dream for me would be able to ask a farmer, one day, OK, this season, do you want no rain but healthy crops? Or do you want a little disease and rain? Because I think there’s a–

MANOUSH ZOMORODI: Oh, we got to leave it there. Sorry, Cindy.

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