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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.
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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.
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