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This is kentrophoros, a ciliate that James—our master of microscopes—had been searching for, receiving samples from all over the world in the hopes of finding it gliding around. When you first look at it, it doesn’t seem particularly special. But there are two things that the kentrophoros is famous for. The first is its lack of a mouth. The second is its coat of bacteria.

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You can also sign up to make a monthly contribution to offset your emissions or support rainforest protection projects by clicking our link in the description. In our episode last week, we introduced you to a particular type of relationship between two creatures called epibiosis, which is when one organism takes up residence on the surface of another.

And we asked which one you would prefer to be, the one living on, or the one being lived upon. And there are plenty of pros and cons to either lifestyle, but these interactions can vary a lot based on the species involved. So we promised to dive a little deeper into one specific example of epibiosis where it might be a little bit easier to decide because choosing wrong means death.

This is kentrophoros, a ciliate that James—our master of microscopes—had been searching for, receiving samples from all over the world in the hopes of finding it gliding around. And finally, it turned up in some sand samples he received from the Adriatic coast of Italy. When you first look at it, this kentrophoros probably doesn’t seem particularly special.

It’s just slowly gliding around the slide, looking a bit more like a worm than a fascinating case study in epibiosis. But there are two things that the kentrophoros is famous for. The first is its lack of a mouth.

And the second is its coat of bacteria. As we zoom in, you can see those bacteria poking out on one side of the ciliate’s body. Scientists have actually estimated that one Kentrophoros can have around 4,500 individual bacteria living on it.

At least 15 kentrophoros species have been described, but they can look very different from one another. Some are like ours—flat and ribbon-like. Others are built more like a tube.

Some have their bacterial coat laid out on one side of their body, while others tuck the bacteria into pockets made out of their own self . The fact that these different species look so different from one another—and that the only things really tying them together were mouthlessness and bacterial companions—meant that scientists had to ask the question: are these different species actually related? It wouldn’t be the first, or last, time that we’d been led astray by a few coincidental similarities.

But when they studied the Kentrophoros’ DNA, they found that the species are in fact related to each other, suggesting that they may have evolved this relationship with their bacterial friends long ago and spread the partnership all over the world. When you look at the bacteria on their own, you can see their rod shape more clearly. These bacteria are primarily thiotrophs, meaning that they live on sulfur.

When researchers dug into the genetics of these bacteria, they found that they were all members of the same clade, Gammaproteobacteria—a type of bacteria that includes more famous sulfur bacteria like Beggiatoa. But additional work found evidence of another type of bacteria in that coat as well called Muribaculaceae—a branch of bacteria that includes residents of our own guts. Now we mentioned earlier that James found these Kentrophoros in samples that came from the Adriatic Coast, and that’s no accident.

Kentrophoros are usually found in marine sediments, and that is thanks in part to the thiotrophic bacteria that live on their surface. Their relationship reminds us a bit of Paramecium bursaria and the green algae Chlorella that live as endosymbionts inside of vacuoles specially designed to hold them. Chlorella love sun, using it to drive their photosynthesis so they can make their own food.

And so, to suit the needs of their internal residents, Paramecium bursaria are also drawn to well-lit areas. It is an exchange that works very well for both the algae and the paramecium. The algae gets the safety of the paramecium’s body, and the paramecium gets some of the nutritional benefit of whatever the algae has made.

The Kentrophoros’ relationship with their thiotrophic bacteria is similar, except that the bacteria live on the Kentrophoros instead of inside them. And instead of being drawn to light, the Kentrophoros are drawn instead to that band of water that hovers above the sediment where oxygen is low and there is just enough hydrogen sulfide for their bacteria to convert into food. By letting themself be drawn to these areas, the Kentrophoros benefits as well, taking in some of the metabolites made by the bacteria.

And it seems that this exchange has paid off enough for both the bacteria and the Kentrophoros. The kentrophoros’ cilia is distributed in a way to make sure that there are specific places kept free and clear for their surface tenants. And the pockets we described some kentrophoros species as having maximize the amount of space available for the bacteria to attach themselves too.

Meanwhile, the kentrophoros’ bacteria divide in a way that is not typically seen, but that makes sense when you remember how it lives. Typically, a rod-shaped bacteria would divide down the middle, along its shorter axis. But these bacteria tear apart down the longer axis, breaking apart into a sort of “V” shape that then splits into two.

And why would it do that? Because it lets them stay attached to their host as they divide, hooked on through the root of that “V” until it’s ready to completely separate into two. And this all seems to be fairly balanced, well-managed relationship of the microcosmos.

The bacteria gets access to sulfur, the Kentrophoros gets access to nutrients. What could possibly go wrong? Well, a lot, if you are the bacteria.

Because sometimes, the Kentrophoros gets hungry… too hungry. A few metabolites from their bacterial buddies just aren’t enough anymore. But the bacteria themselves?

Well, that is a handy snack to have on hand. The kentrophoros may be mouthless, but it is not harmless. It has a trick up its sleeve: phagocytosis.

It simply absorbs the bacteria into itself and begins to eat. In some descriptions of the relationship between Kentrophoros and bacteria, the scientists Bland Finlay and Tom Fenchel have used the phrase “kitchen garden” to describe them, which is a quaint image from the Kentrophoros’ point of view. It's kinda sweet, after all, to imagine the ciliate raising a field of bacteria, fertilizing it with trips through anoxic waters and tending to its needs.

But from the bacteria’s point of view, it must feel like a bit of a betrayal. Like the whole ground beneath their feet has just opened up and swallowed them whole. And in our imagination, that seems like the far worse place to exist.

If you were to ask the simple question of “who would you rather be,” the answer feels to us like an easy one. It seems much more appealing to be the creature who chooses to consume the other, not the one who goes from friend to phagocytosed without warning. But maybe we are being too human about this.

The relationship between these bacteria and the ciliate has gone on for a long time, covering oceans on opposite sides of the planet. There is something about this that must work for the bacteria, even if we don’t have the capacity to imagine it working for us. Perhaps there is something to this life—attached to the side of another creature’s body, surviving as countless copies of yourself endure—perhaps there’s something to all that makes sense when it’s all you’ve known.

And perhaps it is our life…our way of imagining ourselves as whole, complete individuals rather than as a single note in the broader symphony of our species that is truly bizarre. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Wren for sponsoring this episode.

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