Microbes Hitchhiking On Hailstones

Snow and ice usually form around a nucleus of dust, which helps the ice crystals aggregate—but pollen, spores and bacteria can serve as the nuclei of precipitation too. Montana State University Ph.D. student Alex Michaud discusses his search for microbes in hailstones.


This is SCIENCE FRIDAY. I am Ira Flatow.

This spring, thunderstorms, tornadoes brought not only deadly winds and rain but intense hailstorms. I actually got caught up in one, myself, this week. And, you know, as you watch these giant chunks of ice pelt everything in sight, you can't help but marvel at how all of this ice gets formed in the sky.

And you know what? It's a lot more complex and fascinating than what we all learn in earth science class, as my next guest will tell you. He's been collecting hailstones, literally picking them up off the ground. He stashes them in his freezer and then dissects them in his laboratory, piercing together and piecing together the journey that they've made through clouds by looking at their layers of ice.

And he's poking into their cores, too, and looking for the essence of their beginnings, and apart from the usual dust grains in the nucleus of the hail, he's found lots of other stuff, like pollen and spores and even living bacteria nestled inside the idea.

Yes, bacteria, lots and lots of bacteria in ice from all over the world, even in ice in Antarctica. What do bacteria have to do with hail and ice? That's what we're going to be talking about first. Our number, 1-800-989-8255, 1-800-989-TALK. You can also tweet us @scifri, @-S-C-I-F-R-I, or you can go to our website at sciencefriday.com and leave a note there.

Let me introduce my guest. Alex Michaud is the Ph.D. student, ecology and environmental science at Montana State University in Bozeman. Alex joins us by phone. Welcome to SCIENCE FRIDAY.

Mr. ALEX MICHAUD (Ph.D. Student, Montana State University): Glad to be here.

FLATOW: Hi. How did you decide - give me - to go around picking up hailstones?

Mr. MICHAUD: Well, it was kind of a sample of opportunity, a chance occurrence. There was a very large hailstorm that fell on Bozeman in June of last summer, and watching them hit the ground with some significant force was very interesting.

And so I went out after the storm was - right after the storm was over and decided to pick some up and put them in the freezer and see if I couldn't look at them in some more depth.

FLATOW: And how do you dissect a hailstone, and what do you find inside?

Mr. MICHAUD: Usually, you - the method that I used was to cross-section them. So cut a thin slice through them, and that allows you to reveal the layers, and then from there just dissecting with a razorblade, you can cut the different layers out and then analyze from there.

FLATOW: And you found bacteria in the center of the hailstones that you can culture?

Mr. MICHAUD: Yes, so that means that they are live or in some sort of state that allows them to persist through that ice.

FLATOW: And then what's fascinating is that in reading of your work, is how it appears - how common bacteria is at the center of hailstones.

Mr. MICHAUD: It is. There seems to be a fair amount of them, you know, on the order of - ones that are alive and can be cultured, about 1,000 per milliliter of sample water that you get from the middle of the hailstone.

FLATOW: And so what's the advantage to - if I'm a bacterium, what's the advantage to me of hanging out in the inside of a hailstone?

Mr. MICHAUD: Well, they need a way to get back down to the surface. It's a pretty harsh environment up there in the clouds, lots of UV, cold, dry, not the best place for a living thing to be, and so it's good to be brought back down to where you have protection in the soil or on a plant surface, much more suitable habitats.

And so whether they come down in hail or rain, it's a good thing to get back down to the surface.

FLATOW: Interesting, and there are certain bacteria that really have an affinity for really creating these ice. We know that ice sometimes needs a little drop of something to form around, needs a nucleus, right?

Mr. MICHAUD: Exactly. You need something to initiate the freezing of water. Everybody thinks, you know, water hits zero degrees Celsius or 32 degrees Fahrenheit, and it freezes, but it needs something to freeze onto.

And so normally water has to cool well below that, if it's pure water, in order to freeze. And bacteria actually do it really well at warm temperatures or more so than bacteria, but also biological particles really help to start that ice crystal formation at warm temperatures, say minus-two degrees Celsius to minus-eight degrees Celsius.

FLATOW: So when these bacteria get to the ground, if they can form ice at a warmer temperature than, let's say left alone, other particles of soot or whatever in the atmosphere, and they land on leaves, aren't they going to start getting ice into the plants themselves?

Mr. MICHAUD: Yeah, that's what's been studied for the last 40 years is these bacteria, as plant pathogens, because they don't allow the water in the plants to super-cool and cool down to the point of not freezing, so these bacteria cause frost injury in plants, and if you disrupt a plant cell, it's going to leak out a bunch of good nutrients and food for you to eat.

FLATOW: So this is a way of getting inside the plant and then freezing at a higher temperature and, sort of like water in pavement breaks open the pavement, water in the leaf breaks open the leaf structure so that the bacteria now have some lunch inside there.

Mr. MICHAUD: Exactly, exactly, and that's a very common - or it's a common strategy for a select few of bacteria.

FLATOW: And if there's a lot of these bacteria floating up in the atmosphere and making it easier for it to rain or for it to hail, would that not affect the weather? Would they not, you know, have some sort of influence in the weather?

Mr. MICHAUD: Yeah, the - some past studies have shown that there are about 40 percent of the nuclei or the initiators of drops of precipitation are biological in origin.

Now, that's not saying just bacteria but other types of biological particles do help begin the process of precipitation. And so yeah, it - biology does play a role in this precipitation process.

FLATOW: Do you think that perhaps certain bacteria evolve to be able to form ice crystals around them?

Mr. MICHAUD: Yes, that's a debate right now in the literature, you know, whether or not bacteria are - were evolved to cause frost injury in plants, or is this a mechanism for getting back down to the ground once they've been swept up into the air by strong winds?

So yeah, it's one of the big questions that we're asking right now.

FLATOW: Here's a tweet that came in from Noah Cort(ph), who says: Does regular, unfrozen precipitation have bacteria, too?

Mr. MICHAUD: In the form of rain?


Mr. MICHAUD: Yes, that's what Brent Christner and others have been studying. You do have live bacteria that do have unfrozen precipitation, such as rain.

FLATOW: So what do you now? Now that you've got all these hailstones, what's your next line of inquiry here?

Mr. MICHAUD: Well, because - hail is really good because it's a kind of record of the storm. And so continuing to analyze those embryos, or the cores, very middle of the hailstone, to show that there are these efficient ice nucleation active bacteria at the core, this allows us to link a function with a bacteria to show that they were present at the site of nucleation.

FLATOW: Well, good luck to you, Alex.

Mr. MICHAUD: Thank you very much. I appreciate it.

FLATOW: You're welcome. Thanks for taking time to be with us today.

Mr. MICHAUD: No problem.

FLATOW: Alex Michaud is a Ph.D. student in ecology and environmental science at Montana State University in Bozeman.

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