This episode is sponsored by Endel, an app that creates personalized soundscapes to help you focus, relax and sleep.

The first 100 people to click our description link will get a one week free trial. James, our master of microscopes, is not a farmer.

He is, to put it simply, fascinated by microbes. And that may lead him to strange places and cause him to grow tanks full of weird things. But he is not a farmer.

So why did James, master of microscopes that he is, decide to put a sweet potato in a tank? Well, I guess you could say he was trying to rescue it. This poor, neglected sweet potato had reached the point of inedibility, with sprouts growing all over it.

So it seemed right to give the sweet potato some space of its own, and let it live a little. For James, the inevitable conclusion was to put the sweet potato into a tank. And here it is, thriving as its sprouts grow tall and its roots bury deep.

There are times when James can even spot the sprouts trying to escape the seal on the tank. Maybe he would let them burst out, but the tank is currently full of mosquitos, so for now it stays sealed. The mosquitos are a story for another day.

Today, we’re here for the sweet potato because it has a very important job, one that it probably never anticipated having. This sweet potato is going to be a stand-in for one of the most important chemical and biological processes in our world: nitrogen fixation. Let’s start by taking a tour of the sweet potato’s roots.

Imagine that instead of the tank water that they’ve been in, that you’re instead watching the roots from where they would usually be: buried in the soil. You might see a scene like this, a crowd of countless bacteria traveling towards the root and away from it. Some are shaped like tiny rods, and others are spherical.

And some form filaments that trail off like threads. As you look around the root, you can see these bacteria latch onto the root, almost as if they’re a part of the plant’s own texture. And along the root, you might find organisms like this Nostoc, a cyanobacteria that can even be found on plants like cycads that have above-ground roots.

The clinginess of Nostoc is important to plants—and actually to all of us—because they can do nitrogen fixation. What is that? Well it's a concept that comes up often on our channel.

We talk about microbes that have an outsized impact on our world through their role in translating atmospheric nitrogen into chemical forms that plants can actually use. But we’ve never really dived into what that means and why it’s so important. It all comes down to the fact that nitrogen is both essential and abundant, and yet difficult to get.

When we think of the chemistry of life and of humans and of our planet’s atmosphere, our attention often gets focused on oxygen, and carbon-dioxide. But nitrogen is the majority of our atmosphere, and it is extremely important. Nitrogen is essential to making amino acids and nucleic acids, and those are important things.

So if all of the nitrogen in the world were to disappear in an instant, we would not have protein. We would not have DNA. We wouldn’t have the fundamental building blocks that make life on this earth possible.

Which, for the record, I think, would be bad. And this is true for all life. This is something that we have in common with the sweet potato.

We all need nitrogen. Fortunately, as I said before, nitrogen is abundant. Around 78% of our atmosphere is made of nitrogen gas.

So we should be good. Right? Well, no.

The problem is that nitrogen gas is diatomic nitrogen, two atoms of nitrogen bonded together, and that's just far too stable. It’s hard for our bodies to break the bonds between those nitrogens and convert them into the forms we need, which is why we need eat other animals and plants that have nitrogen. But plants have the same problem as us.

They can’t turn all that nitrogen in the atmosphere into forms that they can use. But unlike us, plants have a secret weapon. The things that we’ve been watching throughout this video: microbes.

These microbes—these nitrogen-fixing microbes—are special because they have an enzyme called nitrogenase. This enzyme catalyzes the break-up of that strong bond holding nitrogen gas together, freeing up the nitrogens to become ingredients in more accessible chemicals like ammonia. There's one catch though: nitrogenase stops working completely when it’s exposed to oxygen.

So nitrogen-fixing organisms have a few options for how to live. Many of them live in anaerobic conditions, where the lack of oxygen means their vital enzyme will be safe. And other organisms, like the cyanobacteria we’ve been looking at, create their own internal forms of protection, sequestering the nitrogenase into specialized cells called heterocysts.

You can see the heterocysts standing out like special green beads on this dolichospermum, which is a kind of cyanobacteria. They’re super obvious rounder and larger than the other cells within the cyanobacteria filament. And they look so different because they need thick walls so that oxygen can’t pass through, which allows the nitrogenase to do its thing.

But these nitrogen-fixing microbes don’t keep the nitrogen for themselves. Bound to the roots of plants, they give some of what they’ve made up in exchange for other nutrients that the plants have made. Nitrogen-fixation isn’t the only way that plants get their nitrogen.

There’s also fertilizer, the death and decay of other living organisms, and even lightning, which can shatter the bonds of atmospheric nitrogen and set up its conversion into other forms. Like any other element, nitrogen’s journey through our ecosystems takes many twists and turns, setting up delicate balances and potentially dire consequences. Too little nitrogen, and life can’t happen.

Too much, and you can choke off lakes as the thriving plants and algae eventually reach the limits of what their ecosystem can support. It’s this interplay of nitrogen among so many other elements that births, kills, and sustains. It travels through our atmosphere, our soils, our lakes, our bodies…and ends up, sometimes, at the root of a strange, rescued sweet potato sprouting in a mosquito-filled tank.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Endel for sponsoring this episode. Endel is an app that takes everything we know about sound, and combines it with technology, and creates personalized soundscapes to help you focus, relax, and sleep.

Their app was named the Apple Watch App of the Year in 2020 and they have a brand new soundscape called Wind Down that they made in collaboration with James Blake. The goal of Wind Down is to help you transition from an active day to a calmer state, so it’s great just before bed too. Sound has a direct impact on your physical and mental wellbeing, and by adapting in real-time to things like your location, weather, and heart rate, Endel creates simple, pleasant sounds that can help to calm your mind.

If you’re interested in trying out Endel, just be one of the first 100 people to download it using the link in the description and you will get a free week of audio experiences! Thank you so much to all the people on screen right now. They are our patrons on Patreon, and they are the reason we are able to make Journey to the Microcosmos.

The reason we are able to put a sweet potato into a tank of mosquitos. And the reason, that later you will find out, what those mosquitos are all about. If you want to become one of those people, check out Patreon.com/JourneytoMicro.

If you want to see more from our master of microscopes, James Weiss. You can check out Jam & Germs on Instagram and TikTok. And if you want to see more from us.

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