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This episode is sponsored by Wren, a website where you calculate your carbon footprint.

You can also sign up to make a monthly contribution to offset your carbon footprint or support rainforest protection projects. Blepharisma have appeared on our channel several times before.

In fact, this channel got its start thanks to a video that James, our master of microscopes, once posted of a Blepharisma dying. Around 3 million people watched that video, including me, your host Hank Green. So if you enjoy this channel, you can thank that dead Blepharisma.

But perhaps you should wait for another day to thank them. Because in about 10 seconds, you’re going to watch a Blepharisma explode. Here it is, glowing with autofluorescence underneath UV light.

You can see its oblong shape and oral groove outlined in red…but not for long. The red becomes brighter and brighter, but it also looks like it’s starting to expand. And then suddenly, the walls of the blepharisma burst, the organism popping like a crimson balloon.

The blepharisma bubbles and pours into its surroundings and it all happens within a matter of seconds. Let’s watch it again. Dead or dying microbes are a common enough sight in our journey through the microcosmos.

And there are many potential culprits behind these deaths: predators, accidents, environmental changes, the inevitable march of life into death. But the culprit this time… well, it was us. Us and the UV light that is part of our new fluorescence microscope upgrade.

And our UV light has been very exciting for us. In particular, it’s allowed us to look for methanogens, or Archaea, which sometimes take up residence inside protists. Under normal light, it’s hard to tell the tiny archaea and the tiny bacteria apart.

But under UV light, the archaea will shine blue. So UV can reveal new aspects of the microcosmos. But if you’ve ever fallen asleep on a beach or just stayed out in the sun a bit too long, you may have also experienced the darker side of UV light.

No one wants a sunburn, but fortunately, we have defenses, like hair, and melanin, and sunscreen which can block or absorb UV rays before they cause further damage in our cells. We also, and this is crucial, have more than one if some of them die, which when you get a sunburn they do, the rest of our bodies can live on. Not all organisms have these sorts of protections.

Or if they do, they’re designed for exposure to the sun, not the intense scrutiny of our UV light. So when James wants to hunt Archaea, he has to be careful. He can quickly shine the UV light to see if anything blue appears.

But he has to quickly shut it off. Because as we’ve seen, even a few seconds of exposure to the UV light will kill off his pond buddies. We want to note that as we said earlier, death is a common reality of the microcosmos…we just usually prefer to walk in on a microbe dying rather than being the cause of death.

But for this episode, we decided to make an exception and use our UV light for an extended period of time, with the knowledge that it would kill the microbe we were watching. Because these explosions illustrate the cost of doing business with light. The word for this business is phototoxicity.

Death by light. And while it can happen under other monochromatic lights, the particular wavelength and intensity of our UV light makes it much more harmful to our organisms than our other red, blue, or green light sources. This death starts with excitation.

When the light hits the organism, it can potentially excite chemical structures inside the cell, sending electrons up and down, and producing fluorescent colors in the process. But colors aren’t the only thing that gets created. If there’s oxygen around, it will react with the excited fluorescent molecule, creating what are known as reactive oxygen species.

In biology, reactive oxygen species are byproducts of different cellular processes that metabolize oxygen, which can make them part of normal life. There are even reactive oxygen species that are involved in signaling pathways. But the “reactive” in their name is key to what makes an excess amount of them dangerous.

If you are an organism, and you are, there are a lot of reactions you want to have happen in your cells. You want your DNA to link together correctly, you want your enzymes to find the right substrates. But reactive oxygen species are happy to react with all of those molecules too, damaging them and getting in the way of the chemistry that we need to survive.

What phototoxicity will look like depends on the organism and the light being directed at it. For the organisms we’ve been showing here, like this homalozoon, the overall effect of this intense UV light seems to be unanimous: the cell swells up and bursts open, like a galaxy erupting on our slide. But while the overall effect is the same, the internal machinations are likely different, triggered by a complex interplay of different chemicals that nonetheless react to our light source in a similar, catastrophic fashion.

While we’re not sure of the culprits behind the homalozoon’s death, we can identify one of the chemicals that likely sets off the blepharisma’s death. It’s the reddish pigment molecule called blepharismin that gives the ciliate its color under more normal circumstances. Outside of the UV light, you can see the membrane-bound pigments neatly distributed along the rows that stretch from one end of the blepharisma to the other.

But under our UV light and with oxygen in the environment, the blepharismin reacts to form reactive oxygen species, and death follows quickly from there. But while toxic in our experiment, we should note that the blepharismin serves a key purpose for the blepharisma: defense. These pigment molecules are toxic to some of Blepharisma’s predators in both the light and the dark.

That makes the pigment somewhat like UV light: necessary for survival, yet also a delicate negotiation. But in the same way that we manage our relationship with the sun, scientists have learned ways to manage these phototoxic reactions. They’ve had to in order to understand how we can use fluorescence microscopy to study cells and organisms.

They’ve learned how to modulate wavelength and intensity and duration, along with many other factors, to wield light in a way that better serves their purposes. In the case of the blepharisma, for example, scientists found that using a moderate light for around 1 hour wasn’t much of a problem for them. But with more time under the light, the cells would eventually die.

It’s easy to think of the microcosmos as a separate world from us, even when we know that the microscope is a bridge between large and small. But these deaths at the hand of our supposed bridge are a cautionary sign that we are encountering microbes in a world that is both natural and manufactured at the same time. The way that we light that world impacts the way we see the organisms, and it also shapes their lives—reminding us that they are stronger often than we can fathom, but fragile nonetheless.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Wren for supporting this episode of Journey to the Microcosmos. Wren is a place where you can calculate your carbon footprint, then offset it by funding projects that plant trees and protect rainforests.

We’re gonna need a lot of different approaches to stop the climate crisis, and this is one way that you can learn more about your carbon contribution and take some action. I took their climate quiz, answering a few questions about my lifestyle so that I could see what my carbon footprint was. Then, they should be some ways I could start reducing it.

But no one can reduce their carbon footprint to zero. So, by using Wren, I was able to offset what I had left. Once you sign up, you’ll receive updates from the tree planting, rainforest protection, and other projects you support.

And also we have partnered with Wren to plant 10 extra trees for the first 100 people to sign up using the link in the description! The people on the screen right now, those are our Patreon patrons. They’re the people who support this channel every single episode so that we can continue diving into this wonderful and mysterious world.

So, if you love what we do here, these are some of the people to thank. And if you want to become one of those people, all you got to do is go to If you want to see more from our Master of Microscopes, James Weiss, and why wouldn’t you, you can check out Jam and Germs on Instagram.

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