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The big Roomba of the microcosmos is fascinating to watch as it lives its sink or swim life.

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The biogeography, phylogeny, and dispersal of freshwater and terrestrial free-living ciliates in Florida, USA
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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 us as we explore the unseen world that surrounds us. Thank you also to KiwiCo for supporting this episode. KiwiCo creates super cool hands-on projects for kids that make learning fun!

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