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[♪ INTRO]
Oxygen is pretty great stuff.
On a chilly morning in the mountains,
there’s nothing better than breathing a couple of lungs full of fresh O2, right?
But some organisms couldn’t care less about oxygen.
In fact, in 2021, scientists announced the discovery
of an organism, distantly related to us, that breathes nitrogen.
But that’s not all.
This single-celled critter offers a window into how the type of cells
that make up our bodies
may have evolved billions of years ago.
Our bodies are constructed from eukaryotic cells
a complex cell that contains a central pocket called a nucleus.
And because we’re made of eukaryotic cells, we’re classified as eukaryotes,
which is that big, noisy family that includes us, other animals,
plants and fungi, plus a few others.
The eukaryotes evolved a little under 2 billion years ago.
Back then, days were four hours shorter, continents were just becoming a thing,
and the population consisted largely of sunlight-eating,
oxygen-farting cyanobacteria.
Oxygen-farting was a good thing for eukaryotes, since we rely on oxygen to power our cells.
We can only do that thanks to mitochondria, which, as you may have heard,
are the powerhouse of the cell.
And mitochondria’s origin story is very cool.
Scientists think an early single-celled organism swallowed a bacterium
but didn’t digest it.
And amazingly, this was the beginning of a beautiful friendship.
Together, these partner cells formed the first eukaryotes,
and the ingested bacteria eventually evolved into mitochondria.
Eukaryotes let that bacteria-turned-mitochondria stay
because of their special talent:
They use oxygen to make adenosine triphosphate, or ATP,
the molecule that stores and transfers energy throughout the cell and the body.
Mitochondria can draw energy from sugars via a cascade of electrons,
which they transfer to oxygen.
Oxygen is a great tool for this because it’s good
at accepting those low-energy electrons.
So, when you’re breathing, you’re taking in oxygen
and using it to generate the energy that keeps you going.
But some eukaryotes developed ways to generate energy that don’t rely on oxygen.
Now, that makes sense because oxygen
may not have reached current atmospheric levels
until around 450 million years ago.
So ancient eukaryotes needed to develop alternate ways of creating energy.
Today, there are still eukaryotes adapted to living in areas with low or no oxygen,
like the bottom of the ocean or the digestive systems of some animals.
These organisms mostly rely on fermentation,
which is a less efficient way to produce ATP
by breaking down sugars into CO2 and hydrogen.
But some have developed a way to breathe something other than oxygen.
Like the organisms recently discovered at the bottom of Lake Zug in Switzerland
that breathe nitrate, a molecule with one atom of nitrogen and three oxygens,
which they convert to nitrogen gas.
Unlike most oxygen-breathers, who breathe in O2 and breathe out CO2.
The waters at the bottom of this lake are oxygen-depleted but brimming with nitrate.
So you can see why these little Swiss survivors
need to make use of the ingredients in their environment.
They still seem pretty strange, though, because they do not have mitochondria
and they do not do fermentation.
But they are living, breathing eukaryotes.
They just don’t breathe the same stuff we do.
So, how do they manage it?
Well, like the first eukaryotes, they aren’t doing it solo.
They have an endosymbiont,
a little buddy living inside them that performs functions that keep them alive.
This inner partner does the same job as mitochondria,
only instead of transferring electrons to oxygen, it transfers electrons to nitrogen.
The partnership is so successful that researchers think it’s been around
for two or three hundred million years.
We know of some other organisms that can breathe nitrate,
but it’s very unusual to find one that doesn’t also use O2.
Also, there’s an intriguing difference between the endosymbiont and mitochondria.
When mitochondria evolved, they started as swallowed bacteria
that gradually ditched some of their genome
and integrated more closely with their host cell.
And then eventually they became an organelle, or little organ, of their host cell.
But in the Swiss eukaryotes, the endosymbiont isn’t an organelle.
It’s still a bacterium.
And the researchers think it may be on its way to becoming an organelle.
We may be witnessing an evolution
similar to the one that took place billions of years ago,
when those mitochondrial ancestors
first teamed up with another cell to form eukaryotes.
There are a lot of things we still don’t know about this, though.
Like, this relationship could be totally unique.
Or maybe it’s not.
Maybe the fact that it’s similar to what we think happened with ancient mitochondria
suggests that this kind of relationship happened in other eukaryotes,
we just haven’t seen it yet.
And if endosymbionts can help their host breathe nitrate or oxygen,
then what about other compounds?
Also, where did this relationship evolve?
Like, probably not in Lake Zug, which is only ten thousand years old.
In fact, researchers have found the Lake Zug
endosymbiont’s genes in other parts of the world,
in much older lakes.
So, it’s a prolific little thing, with an origin story that’s still a gigantic mystery.
We do know, though, that his relationship has a lot to teach us about how life
can find ways of surviving in places where survival seems improbable.
And it might even shine some light on our own very distant origins.
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And thanks!
[♪ OUTRO]