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There are plenty of ways to get food in the microcosmos, but the cilia-driven vortex seems
particularly charming.
The microbes who use this method can graze in place if they want to, stirring up whirls
and whirls and whirls of water to draw food in towards them.
Rotifers, like this philodina here, use the crown of cilia around their head to create
that movement in the water and cycle food like algae into closer proximity.
But watch carefully: if you tap the slide and momentarily shock the rotifer into curling
inward, it’ll stop juggling its algae meal.
And more strikingly, the algae itself immediately stops moving.
Any additional movement you see in the algae is because the rotifer’s movements are displacing
the water.
When the rotifer’s cilia were in motion, the algae looked like they were caught up
in a fast-moving whirlpool.
Now, it’s almost like they’re stuck in molasses.
And in a way, they kind of are.
It is not that the algae are actually in molasses.
Like all our other samples, they’re living it up in good old H20.
And of course to us, liquid water is a flowing thing.
We can swim through it, jump into it, run our hands through it, gather samples for this
show from it--all with relative ease.
But for microbes, water isn’t like that.
It’s like a syrup.
And while life in syrup sounds whimsical, the mechanics and math that explains it is
much less so.
And thus we are going to give you just a very broad idea of what’s going on here.
Warning, fluid dynamics ahead.
The way things move in a fluid is an extremely complex, dynamic thing that’s characterized
and defined in many different ways.
But the most useful description for us is what’s called the Reynolds number, which
tells us whether or not the movement we’re interested in is dominated by inertia or viscosity.
When the Reynolds number is higher, the inertial forces dominate, which means that an object
or fluid that is gliding is more likely to keep on gliding.
And when the Reynolds number is lower, the viscous forces dominate, and that brings things
to a halt.
If we’re comparing things moving in water, then one of the most useful things to note
is that an organism’s Reynolds number will generally increase with its size.
A person swimming in water might have a Reynolds number that’s on the order of 10,000, which
lets us glide a little between each stroke.
A blue whale might be somewhere on the order of 100 million, which lets it
travel quite far with just a flap of its tail.
Bacteria, by comparison, they’re working with a Reynolds number that’s more on the
order of 0.00001.
And where a whale or a person can travel multiple body lengths depending on how we generate
power, a bacteria that’s been pushed by something else will only travel about a tenth
of the diameter of a hydrogen atom before coming to a stop.
That is not far.
So for the microcosmos, water is not the lovely flowing thing that we know and love.
It’s thick and syrupy, and it should be a challenge to move through.
But as you may have noticed, many of these microbes are still plenty fast.
Sometimes we get questions about whether we’ve sped up the footage, and the answer is that
unless we’ve otherwise noted any changes in speed, that is the real-time speed of these
organisms.
Even in this clip, the various flagellates and ciliates are moving quite fast--and that’s
after we slowed down the footage 500%.
So like…how?
How can they move so fast in syrup?
We went into more detail about how single-celled eukaryotes do this many episodes ago.
Some use cilia, others use flagella.
And while these are different structures with their own motions, the effect is the same.
Cilia and flagella generate thrust, which keeps the organism moving.
So protists have evolved these various appendages that allow them to move in a viscous environment.
And that’s because movement is essential, whether it’s within a confined area or freely
through space.
It takes hunters to their prey, and it takes prey away from their hunters.
So for microbes, the syrupy world around them is actually quite important, and it’s one
scientists are working to understand better.
They’ve studied how these didinium chase their prey in changing temperatures and viscosity.
They’ve found that increasing the viscosity of a rotifer’s surroundings slowed its population
growth.
And they’ve tested how viscosity affects bacterial movement too.
These responses are important for understanding not only the individual lives of microbes
and the ecosystems they live in.
They’re important for understanding our own bodies because to many microbes, we are
just another strange geographic feature to live in.
We’re a living bag of fluids that bacteria often find their way into.
And how they navigate our body deeply impacts our health.
Spirochetes, for example, cause Lyme disease and syphilis.
But to do that, they have to get through blood and other fluids, which they do in this erratic,
corkscrew motion.
The bacteria is generating thrust with flagella (though it’s important to note that bacterial
flagella are a completely distinct thing from the eukaryotic flagella we mentioned earlier).
And spirochetes use their flagella in a way that’s weird even for bacteria.
They keep their flagella inside them, sandwiched between their membranes.
And as the flagella rotate internally, they drive the entire spirochete to rotate and move.
This mechanism doesn’t just move the spirochete, it also helps them dig deeper into the dense
bits inside us that are more difficult for other bacteria to penetrate.
And that’s just one example of how a life in syrup impacts us.
Sometimes the syrup is water, and sometimes it’s you.
But for microbes, it’s just home.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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