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Click the link in the description to learn more and for a special offer. We’re often running into examples of endosymbiosis in the microcosmos, like this partnership between Paramecium bursaria and the green algae Chlorella.

The algae lives inside the paramecium, providing sustenance derived from its own photosynthetic machinery in exchange for protection from viruses that linger outside the living borders of the paramecium. This is one of those heartwarming tales of the microcosmos. One small organism finds another even smaller organism, collaborating to create a better life for each.

But not all tales of internalized organisms are so peaceful, and not all creatures found inside a paramecium are welcome guests. This, for reference, is a healthy paramecium. You can see the round, healthy nuclei towards the center of the organism.

And this is another paramecium. It looks similar to the one we just saw, only now the nuclei are filled with strange bumps. From some angles, one of the nuclei looks almost as if it’s full of crystals.

Except those aren’t crystals. When James, our Master of Microscopes came upon this frosted-looking nucleus, he recognized it from a paper he’d read three years before - a paper that contained a photo of a paramecium whose nucleus was filled with needle-like bacteria called Holospora. Holospora were first described by Waldemar Haffkine in 1890.

He’d been studying Paramecium caudatum when he noticed these parasitic organisms residing in the nucleus. Through his investigation, Haffkine would end up identifying three species: Holospora obtusa, Holospora undulata, and Holospora elegans. While his work would prove important to establishing the existence of these bacteria, Haffkine soon moved onto other research interests.

This pivot would put a temporary pause to Holospora discovery, but it would pay off in other ways: Haffkine would go on to create vaccines against both cholera and the plague. So, you know, not bad. Today, there are nine species known to us.

They differ not just in appearance, but in where they infect their hosts. In some cases, certain species will live in a paramecium’s macronucleus, while others prefer the micronucleus. And some Holospora species may also only target specific Paramecium species, while others will infect more indiscriminately.

But the process of infection is a shared one. It begins with getting eaten. The paramecium may just be casually munching on its surroundings when they end up phagocytosing some of the Holospora, which then end up in the ciliate’s food vacuoles.

The bacteria aren’t motile, meaning they can’t move on their own. But within an hour of getting into the paramecium, these surprising escape artists will make their way out of the vacuole, which is good for them because everything in there is going to get digested and use the paramecium’s own systems to get into their nucleus of choice. How do they choose their target nucleus?

We do not know. But when they get there, the Holospora invade. At this infectious point in their lives, the Holospora are shaped like rods and will be around 10-20 microns long.

But once they’ve settled into their nuclear homes, the trespassing bacteria switch modes, shortening to about 1-3 microns long. They also begin reproducing, dividing and filling up the nucleus with more and more of their own kind until the time comes to infect a new paramecium. In general, parasites have two choices when it comes to widening their infectious net: horizontal transmission or vertical transmission.

For horizontal transmission, the parasite has to leave their current host to find a new one, but for vertical transmission, the parasite just waits for their host to reproduce so they can infect the new offspring. Each method of transmission comes with its own set of advantages and disadvantages. Horizontal transmission, for example, is great if you’re an organism that wants to make as much use as possible out of your host, but you do run the risk of potentially killing them in the process.

Meanwhile, vertical transmission involves waiting for your host to reproduce and then infecting the new offspring. This might be less risky than venturing out into the unknown to find a new home. But it will require both delicacy and patience, ensuring that you take just enough advantage of your host to survive, but not so much that your host cannot survive long enough to replicate.

Different parasites balance the choice between these transmission styles based on a number of conditions, but the Holospora can actually switch between horizontal and vertical transmission based on how the paramecium seems to be doing. After it infects a new paramecium and switches to its reproductive stage, the Holospora will keep doubling and doubling inside the nucleus. And if the paramecium is growing rapidly, then it will be dividing a lot into daughter cells, and with each division of the ciliate, the bacteria will be distributed between the daughter cells as well.

This is vertical transmission, and it works great for Holospora….unless the paramecium’s growth starts to slow down, at which point the bacteria will switch back to their infectious form and leave the host to find a new home. The Holospora can’t survive outside their host, so this is the one time they will leave the safe confines of the paramecium. Despite Holospora’s infectious nature and paramecium’s prevalence around the world, it’s actually kind of difficult to find Holospora-infected paramecium out in the wild.

They’ve only been documented in the Northern hemisphere, and even then, sightings can be infrequent. So it was actually pretty exciting to find them in our samples, even if it means our paramecium are dealing with an unwelcome guest. Paramecium have developed various strategies to prevent Holospora infection.

Some can’t ingest the infectious forms, while others seem capable of blocking the bacteria from entering the cytoplasm or nucleus. And given that infection by Holospora has been associated with lower rates of sexual reproduction and other abnormalities in the host, it makes sense that the paramecium would evolve a defense. However, there have been cases where a particular species of Holospora will infect a particular species of paramecium, and the host actually benefits from the interaction.

In one case, the host was better able to survive abrupt changes in temperature, and in another, it was able to withstand changes in osmotic pressure. But this is not a universal experience, it depends on the combination of host and parasite. It’s hard not to look at all of this from the paramecium’s point of view, to not see this as a larger organism that finds itself contending with microbial invaders.

But the Holospora is also just trying to survive as well, in the one world it knows how to survive in. If only that world wasn’t another organism. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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