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When you look out at a pond, you are rarely ever staring at just a body of water,
some sort of liquid mass separate from the trees and vegetation around it. The pond itself is
usually full of life, sometimes as large as a fish, sometimes as small as a bacterium.
But sometimes those tinier organisms come together to form large colonies that coat the sediment
in and around the pond. Like these beggiatoa, which might look like bits of string unfurling
underneath the microscope, but are actually hundreds of individual bacteria that have
attached to each other to create a filament. Beggiatoa are sulfur bacteria, and you can
see the sulfur they’ve all stored shining as the filament glides past,
glittering like a diamond-studded bracelet. And just as the bacteria gather to form a
filament, the filaments gather to form a mat, weaving together tens of thousands of
beggiatoa into a white carpet that lines surfaces found in marine and freshwater habitats alike.
And that is great news for James, our master of microscopes, because one of his sampling ponds
happens to have some of these beggiatoa carpets, making it very easy for him to just collect a bit
from the mat to check out under the microscope. Or at least, it used to be easy.
Recently, for some reason, the pond hasn’t yielded quite as much beggiatoa as it used to.
In fact, it’s barely produced any. We don’t know quite what happened,
but James suspects a new resident of the pond: a family of beavers that could have potentially
changed some of the chemical composition, making it less habitable for the beggiatoa.
But in its place, another bacteria has come to take over the thick biofilms of the pond:
filamentous cyanobacteria. And while there’s plenty to distinguish one from the other,
there’s one very important trait that unites them and makes them
similarly mysterious: it is in the way they move. Maybe you’ve picked up on it already. The long
lengths of beggiatoa and cyanobacteria gliding across the slide. There are only a few groups of
bacteria that do this kind of gliding, but they’re found across a plethora of environments, including
ponds, soil, and, surprise, in our own mouths. Those fast-moving bacteria are largely
moving with the aid of flagella, a helical structure powered by ATP
that propels the bacteria in different directions. But gliding does not involve those flagella.
Gliding involves something much more slick than flagella: complex polysaccharides, which
scientists call by a much cooler name, slime. A thread gliding across slime sounds more
like an arts and crafts project gone wrong than a complex form of motion that shapes
entire ecosystems. But of course, that is exactly what gliding turns out to be.
To appreciate the importance of this movement, let’s continue with cyanobacteria,
which have been photosynthesizing and oxygenating our world for billions of years.
They are small, but the impact they are able to have on the world around them
is huge. And all this without the aid of flagella to move them around.
Of course, it would be a massive challenge for cyanobacteria to survive if they couldn’t move
anywhere. They are photosynthetic organisms, after all, so even if they’re not hunting
down prey, they are on the hunt for sunlight. Fortunately, they already have the tools in
place to sense sunlight. The pigments they use to perform photosynthesis can also—when they
detect light— set off the reactions that drive the cyanobacteria in the right direction. But not all
light is the same: some species of cyanobacteria, for example, will glide away from blue light,
while others ignore color and instead gauge their response based on the intensity of light.
These responses will vary with the species, as will the way they glide.
Some can bend, others can rotate around the axis of their filament. But the end result is the same:
a slow, but directed movement in search of light that will one day be food.
But these filamentous cyanobacteria have hit on more than just a steady, graceful path towards the
light. They are collective organisms, gathering into filaments and mats. They can even aggregate
into toxic blooms that can span hundreds of square miles, suffocating or poisoning the unfortunate
organisms that come in contact with them. And one of the challenges of forming
these kinds of collections is that it requires diverting resources towards
making the extracellular structures that keep the cells together. And that can come at the expense
of dividing and making more cyanobacteria. So if there are disadvantages, there must
be an advantage, and there’s a big one. By working together, these colonies can allow the individuals
to better pursue a collective goal. And gliding happens to just be one of those collective goals.
In the case of the model cyanobacteria species Synechocystis, researchers have found that while
individuals of the species can navigate in response to light, they’re better able to
orient themselves to light when they’re attached to each other. It’s like they go from stumbling
individuals to a well-choreographed dance line. And the mathematical models suggest that even
if a fraction of this colony can’t respond to light, the overall movement around them
will stay the same—taking everyone towards light and towards photosynthesis and continued life.
So this seemingly simple movement brings complexity to these
seemingly simple organisms. And scientists have been trying to decipher the mechanisms
that drive gliding for many decades now. We mentioned earlier that where there’s gliding,
there’s also slime. But we don’t know what that slime is actually doing. There have been theories,
of course. In one potential mechanism, the gliding cyanobacteria oozes slime to power it along the
surface. In another, waves travel down the length of the filament to push it along the surface.
But even as scientists have identified more of the components driving the movement of
filamentous cyanobacteria, the process—or even whether there is just one process—remains unclear.
It seems like it should be so much more obvious,
doesn’t it? That there should be an easy answer. We know so much about flagella,
and our own system of locomotion with leg, well, that seems pretty obvious.
But then, maybe that’s what makes this so beautiful and infuriatingly difficult to explain,
despite how long we’ve been working to explain it. For now,
at least, it is a movement without an obvious metaphor. one day, we’ll think of something.
Thank you for coming on this journey with us as we glide through the unseen world that surrounds us
Thank you also to Endel for sponsoring this episode.
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There are now, arriving on your screen, a bunch of names. These are people all across the world,
who support us on Patreon. We have a little, weird show here, and it’s not designed to go super duper
viral and get a huge huge audience so that it can buy lots of expensive microscope equipment. So,
instead, we have Patrons on Patreon. They’re lovely. They’re the reason we can make this show.
Thank you to all of you, so much. If you want to learn how to become a patron,
you can go to patreon.com/journeytomicro. If you want to see more from our Master of
Microscopes, James Weiss, check out Jam & Germs on Instagram.
And if you want to see more form us, there’s always a subscribe button somewhere nearby.