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The microcosmos surrounds and engulfs, invisible to us until we peer through the lens of our microscope to witness the incredible diversity of life at what seems like the smallest of scales.

But there are much, much smaller scales than this, creating a kind of sub-microcosmos.

And while you may find some exceptionally tiny life here, this is also the realm of viruses, existing in their own limbo between what we consider living and non-living.

Viruses are built of packaged genetic material, just like any living cell.

But unlike microbes and our own cells, viruses lack the ability to replicate that genetic material.

So instead, they infect, relying on their hosts to take in their genetic material and follow their instructions to make more virus.

While there are some large viruses, like the amoeba-infecting megaviruses that can get as large as 750 nanometers, most are much, much smaller.

For context, the little crystals you see illuminated at the tip of this closterium are about 2 microns, or 2000 nanometers long.

That's almost 3 times the size of the largest megavirus, and almost 17 times the size of most coronaviruses.

So it is difficult to see viruses with the equipment we have on hand.

But this sub-microcosmos has a long reach, extending into the microcosmos and as well into our own world.

So for today, we want to talk more about some of the ties that bind across scale.

Let's start with one example of how viruses shape life in the microcosmos.

These paramecium bursaria are very, very green.

They get that color thanks to hundreds of little green algae called Chlorella that are housed in vacuoles.

This relationship between the paramecium and its internal algae is an example of endosymbiosis.

And when we see examples of endosymbiosis in the microcosmos, it's easy to focus on what the larger microbe is getting out of the relationship.

The green algae are photosynthetic, which makes them a great internal nutrition provider to the paramecium.

But symbiosis is a complicated relationship to maintain.

The Chlorella have to be housed in a special perialgal vacuole membrane.

And the paramecium can control the number of chlorella based on how much light there is or based on when the paramecium is getting ready to divide.

So why would the algae agree to all of this?

Well, protection.

The microcosmos, as we've seen, is full of predators both small and a little less small.

But just as worrisome to the Chlorella are the viruses, particularly the Phycodnaviridae, a group of large, DNA viruses that target eukaryotic algae.

From this group, Chlorella are particularly susceptible to two types of viruses called NC64A and Pbi.

Look, once you get down to these scales, nobody's thinking of creative names anymore.

But when they're inside the paramecium, the Chlorella are safe.

It's like the paramecium is a kind of fence, keeping the algae in and the viruses out.

These viruses only target the chlorella when they are released from the safe confines of the paramecium.

Of course, viruses are also crafty.

Scientists studying chlorella viruses found that they were able to wait along the surface of paramecium bursaria, most likely binding to receptors that keep them anchored to the microbe.

These videos were produced by the researchers using confocal microscopy and fluorescent stains.

The big red blobs you see are actually the Chlorella's chloroplasts, distributed around the paramecium and making their own red autofluorescence.

The green dots are produced by adding a stain that binds to the DNA of dead cells, showing us potential locations for the virus along the outside of the cell.

If the paramecium dies for whatever reason, the Chlorella inside of them get released.

And now, not only is the Chlorella's protector gone, its membrane is lined with the very thing the algae was trying to evade.

Terrifying.

The goal for viruses is to infect, to find something that will make more of them.

Sometimes they're looking to infect an algae, sometimes they're looking to infect one of our own cells.

And in a way, you can think of our skin the same way chlorella might see a paramecium.

It's the initial line381.3f defense against pathogens like viruses, protecting the many cells inside us.

There are other ways that viruses infect us, but in addition to our own body's defenses, humans have created a very simple but powerful tool for dealing with pathogens--one that we didn't even realize was so powerful until a few centuries ago: soap.

Beautiful, beautiful soap.

Soap has been around for many, many millennia, made from various oils and alkali.

The goal was cleanliness: whether of objects or of a person's own body.

But it wasn't until the mid-1800s that a few doctors began to realize that washing their hands kept their patients safer and healthier.

But even as we began to understand more about microbes and people continued to push for soap as a way to contend with pathogens, it wasn't until the 1980s that anyone established some sort of national hand-washing guideline for hospitals.

So what is so magic about soap and washing your hands?

Well, some of it is just the movement of it all, of water and friction scrubbing away at things that you might not even be able to see.

But part of the power of soap is simple chemistry.

We can't really demonstrate the effect of soap on viruses because, as we said earlier, we don't really have the equipment to showcase them.

So we're going to use bacteria as a proxy instead for a simple experiment.

While bacteria and viruses are different in a number of ways, they also share one very important trait.

These bacteria were obtained most professionally from the saliva of some very cute kittens named Lupin and Sirius.

That's right, we swabbed our kitties.

The membranes of these bacteria are made up of lipids, which have a phosphate head which dissolves in water and two fatty acid chain tails that don't.

The structure of this membrane is built on keeping these fatty acid chains away from the water that is both inside and outside the bacteria, and that creates what's called the lipid bilayer.

And while viruses aren't technically living, they can find themselves reliant on a similar chemistry.

Their genetic material is enclosed in a protein capsid.

And then for many viruses, that capsid is coated with a viral envelope that, like the bacterial membrane, is made up of a lipid bilayer.

Soap has a structure similar to these lipids, with a fatty acid tail that wants to avoid water.

And as we pipette diluted liquid soap onto the sample, those hydrophobic tails also want to avoid water.

Some do this by attempting to break into the bacteria's lipid bilayer, ultimately breaking up the membrane entirely.

Others form bubbles called micelles around bits of bacteria and other debris.

Of course, we can't see these little chemical disruptions in action, that's far too small a scale.

What we do see is the bacterial colony quickly melting away.

Our own cells also have lipid bilayers.

But we can wash our hands without them completely dissolving away into nothing because our skin is a complex combination of protein and cells that protects us.

Instead, the soapy water destroys and carries away the microbes that cover them, making it safer to eat and to touch and whatever else you might be using your hands for.

Viruses are tricky.

They're so small and so capable of adaptation.

So sometimes you have to hide out like an algae, and other times you can rely on the basic chemistry of soap.

But these are only two examples of the many, many strategies that have evolved in nature or been developed by humans to contend with the enormous impact that viruses can have on our lives.

Nothing in this world is so small that we cannot find a way to understand it, and nothing is so large that we cannot seek to confront it.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

And thank you as well to Audible, for supporting this episode.

There are many audiobooks about the Microcosmos that are available on Audible.

Author David Quammen explains, in The Tangled Tree, how recent discoveries in molecular biology can change our understanding of evolution and life's history, and that is, yes, available as an audiobook on Audible.

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