Thanks to KiwiCo for supporting this episode
of Journey to the Microcosmos! Click the link in the description to learn
more and for a special offer. “Bursaria truncatella is a giant ciliate.” That is a quote, the first sentence, in fact,
from a scientific paper called “The Gravitaxis of Bursaria truncatella: Electrophysiological
and behavioral analyses of a large ciliate cell.” If you’re not quite sure what that title
means, don’t worry, we’ll get to it. I will occasionally have an argument with
my four year old about the size of microbes. We’ll look under the microscope and at a
mere 40x magnification, whatever we’re looking at might fill the whole view, I will shout
“It’s huge” and he will retort “it’s tiny!” and then we will laugh because, of
course, we are both right. But with their ability to grow to around 1
mm in length, Bursaria truncatella and its fellow Bursaria relatives are giant ciliates. Right? They are certainly giant /for/ a ciliate. But I would argue beyond that...i think, though
my son will fight me on it, that all things are relative. And if a blue whale is a giant of our world,
then Bursaria are giants of theirs. So giant that you can see them with the unaided
human eye. A single cell that emerges from the microcosmos
into our world. Now there are times when being a giant seems
to work out quite well for Bursaria, especially because its large body size is matched by
its large mouth, shaped like a horn or, as others have noted, like a baseball glove. And that large mouth comes in handy when it
comes to ingesting other ciliates If the word “bursaria” sounds familiar
to you, it might be thanks to Paramecium bursaria, another ciliate that we’ve come across in
our journey through the microcosmos. And While Bursaria and Paramecium bursaria
are technically related in the sense that they’re both ciliates, they are not closely
related. And a Bursaria will absolutely go ahead and
eat a paramecium when it feels like it. The origin of their shared name comes from
the Latin “bursa,” which translates to purse. Handbag designs these days come in all sorts
of shapes and sizes, but I suppose with a generous image of a purse in mind, you might
stare at these organisms and consider them bags full of biology. This naming, by the way, isn’t even restricted
to microbes. There’s a genus of plants named Bursaria
because of the shape their fruit comes in. So, apparently when it comes to nature, scientists
are always just seeing purses everywhere. Anyway. Returning to our current Bursaria of interest:
even giants have to find ways to protect themselves. And when you put a ton of resources into creating
that massive body, you want to protect that body from the environmental whims of the microcosmos. Maybe it’s getting dry, or cold, or there
isn’t much food. Many microbes, in situations like this, will
make a cyst...a little house it can use to ride out whatever storm has come their way. We’ve seen a bunch of different microbial
cysts so far in our journey, all with their own distinctive shapes and sizes. And if we were to compare these cysts and
award them superlatives, the Bursaria’s would have to be “most resembles the top
of a cartoon diamond.” The sphere in the middle is called the endocyst,
and it holds the resting body of the Bursaria. The endocyst is encased in a multi-faceted
exterior, connected by little bridges that hold the layers of the cyst together.. These cysts are reported to be around 100
microns long. And remember, the active Bursaria is around
1 mm, so 1,000 microns long. Just imagine the work that goes into collapsing
that giant ciliate into a cyst that is a tenth of its size, and also the work that goes into
remaking the giant when it emerges. The excystment process, it’s said to be
incredible to watch, taking anywhere from 2 to 6 hours as the Bursaria pushes against
an opening of the cyst until it can finally pop through and emerge, unscathed but also
unrecognizable. It takes about an hour for the cilia and organelles
to arrange themselves in their appropriate places, allowing the Bursaria to resume its
active life. Being a giant comes with other challenges
too, like gravity. Now, you and I are always dealing with gravity,
whether we like it or not. It’s, it’s just...there. Keeping us rooted to the ground, making us
fall down when we trip. But for free-swimming ciliates like Bursaria,
they’re not worried about tripping and falling. They’re worried about sinking, the density
of their cytoplasm pulling their bodies down through the less dense freshwater around them
and causing them to sediment. That is a verb in this case. It means getting dragged down to the bottom
and becoming, basically, sediment. To make sure they don’t sink, these organisms
have to be able to figure out what direction gravity is pulling them in, and then counteract
that force by swimming. And scientists have been observing unicellular
organisms doing this for centuries. There are a few different ways microbes move
through their environment. They may move in response to sensing chemicals,
which we call chemotaxis. They might move in response to light, which
we call phototaxis. And then there’s moving in response to sensing
gravity. Maybe you have a guess at what that might
be called...gravitaxis. We used to call this geotaxis, a term that
some people still use even though it is way more confusing because they are stubborn and
they do not like change. And yes, we are willing to fight this fight. Geotaxis is right out. We’re sticking with gravitaxis. When a microbe is traveling down but not swimming,
it is sedimenting. Scientists measure the sedimentation rate
of microbes by immobilizing them and watching them fall through water. And using that method, they’ve found that
the sedimentation rate of Paramecium tetraurelia is about 67.4 micrometers per second, and
the sedimentation rate for Paramecium caudatum was about 123 micrometers per second.. So that gives you a baseline for some ciliates. Bursaria truncatella’s sedimentation rate? 923 micrometers per second. I repeat: 923. That’s almost a millimeter every second. That is because of its large volume, which
is several hundred times that of Paramecium tetraurelia. So Bursaria really have to swim hard to make
sure they don’t just sink to the bottom of a pond. And that swimming is no joke. Bursaria may have cilia, but there’s a physical
limit to how many cilia they can fit on their, yes massive, bodies. So they really have to take advantage of what
they have to generate the forces necessary to counteract sedimentation. Of course, it’s not enough just to swim. They have to make sure they’re swimming
in the right direction, otherwise they may just be hastening their own descent. And that is why it is so vital for them to
be able to sense the direction of gravity. How do they do it? Well, we just don’t completely know how
Bursaria and other microbes do gravitaxis. Scientists have studied the organisms under
both microgravity and hypergravity to see how they respond. What is most important to realize though is
that there aren’t very many things microbes can sense, and adding gravity to that list
has enabled these microscopic giants to thrive in their worlds. Would we like to know how they do it? Absolutely we would. And we know how Paramecium do it better than
we understand bursaria’s gravitaxis. But to get to the bottom of it, we as a species
need to keep searching and learning and discovering. That’s what we do. The way microbes have chemotaxis and gravitaxis,
humans maybe have curiotaxis. We can sense what we do and do not know...and
we can allow ourselves to be drawn away from or toward those things. Thanks for being one of the people who is
drawn toward the things that we do not know, and thank you for coming on this journey with
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Awesome share! If you’re watching on mobile, you can zoom in pretty far into the video by pinching outward. Very cool stuff