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These world travelers might be, well, almost everywhere, but there is a still a lot we don't know about the famous paramecium.

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This paramecium is not having a particularly good day.

You wouldn’t be either if the contents of your body were being slowly consumed by a ciliate. But as you, hopefully, are watching this from a comfortable position, while not being consumed, we would understand if your attention is diverted from the microbial drama and instead focused on all the shiny things, like looking down on a city at night from an airplane, the moving lights a sign of life at a distance.

These shiny colors are actually crystals, visible this way under polarized light. So, we have to use a special filter to get these shots and they seem to be connected to the paramecium’s metabolism. We wanted to start with these crystals because, like, we think they’re stunning.

And stunning is not necessarily the first word that comes to mind when you think of paramecia. Of the early microorganisms discovered by scientists, paramecia have gone on to have perhaps the most distinguished career. Studied by scientists and students alike, these protozoans have been so useful as a model organism that one scientist called them the “white rat” of ciliates.

But while paramecia have played an important role in our understanding of genetics, microscopic movement, and more, importance does not always imply excitement or enthusiasm. After all, if we compare paramecia to their protozoan brethren, what does a hairy oval have next to the shifting shapes of the amoeba or the strange movements of the euglenoid? That doesn’t stop paramecia from wanting some attention though: there have been plenty of times where we’ve set up a recording of some beautiful organism, only to have paramecia come by and photobomb our precious composition.

Their reliability in labs and frequent appearance in classrooms might make paramecia seem like a known quantity. The things we know, we seem to have a good grasp on. We know that this is a paramecium, a unicellular eukaryote covered in tiny hairs called cilia that help it swim.

We know that when scientists first began describing paramecia, they divided them into two groups based on their morphology: Paramecium aurelia, which have a long, tapered oval shape; and. Paramecium bursaria, which are shorter and rounder. We also know that they have multiple nuclei: one macronucleus where transcription happens, and a number of micronuclei that vary across species that become important during sexual reproduction.

We’ve talked about paramecia in a previous episode, describing the endosymbiotic algae that reside inside of some species, providing sugar produced through photosynthesis in exchange for the movement and shelter of the surrounding cell. So much of their knownness rests on just how widespread they are. You can find paramecia in freshwater, brackish, and marine environments around the world, though the distribution, of course, varies by species.

The ones that we’ve come to know best seem to be the ones that are considered cosmopolitan, that’s a word we use for things that are found seemingly everywhere. Recently, scientists studying Paramecium biaurelia collected from 92 spots around the world over 62 years of time found very little genetic variation between the different strains. This kind of genetic similarity indicates not only that these organisms have travelled the world, but that they continue to, keeping their populations spread out and not diversifying into different species.

But the ubiquity that makes paramecia so known to us is itself a question: how did paramecia manage to find their way to all these different locations, especially bodies of water that are isolated from others? Unlike some of the other aquatic organisms we’ve discussed on this channel, paramecium don’t form protective cysts that keep them safe during a dry spell, which makes it that much harder for them to cross the globe and its many dry spots. Through experimentation and observation, scientists have confirmed the presence of hitch-hiking paramecia on the wet fur of raccoons and on the bodies of snails, showing that they might be using the surface of animals as moist taxis, but whether that explains the entirety of their travels remains to be seen.

Paramecia are also subject to what you might start to notice is a recurring theme when it comes to the microcosmos. While our early classification methods relied on what we could see of an organism’s morphology, the development of various molecular techniques is uncovering new patterns and complications in how we define the relationships between its species. These complications were hinted at by early observations, like the fact that the broad, morphologically-defined label of P. aurelia couldn’t capture the strange mating subdivisions that occurred in the species, groups of P. aurelia seemed reproductively isolated from each other.

In time, as scientists gathered more observations, P. aurelia stopped referring to one species of paramecia. Instead, it was redefined as a complex of fourteen siblings named P. primaurelia, P. biaurelia, P. triaurelia… all the way up to P. quadecaurelia. And a fifteenth subspecies named P. sonneborni was discovered later.

With more observation and genetic techniques, the Paramecium genus has come to encompass 19 morphospecies, comprised of what are sometimes called “cryptic species” these are species who morphological similarities belie their underlying differences. When we look at them in the microscope, there is no way for us to tell which species it is. We would need to sequence their genome.

With words like “cosmopolitan” and “cryptic” used as technical terms to describe paramecia, they start to seem less like a textbook organism and more like an international microbe of mystery. It might be fitting then to learn from the experience of the scientist Tracy M. Sonneborn, who began working with protozoa in the 1930s.

Sonneborn developed many of the techniques that turned paramecia into a model organism, his work so influential that it has been memorialized in the name of that 15th P. aurelia subspecies, P. sonneborni. While researching paramecia, Sonneborn became interested in the so-called killer trait, a subset of paramecia that seemed able to produce toxins that killed other paramecia. These killer paramecia all shared a trait: the presence of mysterious cytoplasmic particles called kappa.

Sonneborn tracked how these kappa particles passed on through generations, proposing the “plasmagene theory” to explain the patterns he saw, a hypothesis that these kappa particles were some special thing coded in the paramecium’s DNA, but that could self-replicate on their own in the cytoplasm. And he was wrong. It turned out that these kappa particles aren’t particles, they are bacteria: another example of endosymbiosis, except that instead of providing the nourishing promise of photosynthesis, these bacteria turned their paramecium homes into killers.

It took time for Sonneborn to accept that his plasmagene theory didn’t explain kappa particles. But if paramecia have come to model so much of what we learn in science, then looking more broadly, this microscopic organism also models us and our own use of the scientific process. In a letter to another researcher, Sonneborn reflected on this, on what it means to set aside his own ego for the reality of the microbe, to know that studying nature requires an understanding of our own selves.

He wrote, “‘It was awful of me to be so attached to a pet idea. That was an ordeal between my mind and my heart and it took a while for the mind to win and the heart to accept. Impersonal scientific objectivity is a goal to be sought by hard self-discipline; we are not born with it.’’ Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

Journey to the Microcosmos is produced by Complexly. We produce over a dozen different shows, including a new one, Ours Poetica, it’s a co-production between Complexly, The Poetry Foundation, and the poet Paige Lewis. Ours Poetica brings you a new poem three times a week, read by poets, writers, artists, and sometimes unexpected, yet familiar, voices like... like mine for example.

You can check out a video of me reading Edgar Alan Poe's "The Raven." We think there are times for YouTube to be high energy and times for it to be a little bit more relaxing. Journey to the Microcosmos and Ours Poetica are both going for that second thing.

I also want to say thank you for listening to my extra-gravely voice today. I have a cold. We haven’t talked much about viruses on this channel.

They are very difficult to observe, they’re tiny, probably aren’t really even alive. But they have colonized much of my upper respiratory system right now and I’m just pushing through. Also thank you so much to everyone who has signed up to be one of our patrons on Patreon.

We are extremely grateful to be able to make this content and you are the reason why we get to. And if you want to see more from our Master of Microscopes James, check out Jam and Germs on Instagram.