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Go to Squarespace.com/microcosmos to save 10%  off your first purchase of a website or domain. What makes something rare?

Sure, we can define “rare,” and come  up with some kind of threshold that   distinguishes between something that’s seen  often enough versus something that isn’t. But it gets arbitrary and subjective  at some point, doesn’t it? And especially in the microcosmos— where the organisms vastly outnumber us,   where what we find in a single pool  of water can change from day to day— what does it mean for a microbe to be rare?

James, our master of microscopes, is  also a bit of a specialized collector. While his countless samples have turned  up all sorts of fascinating creatures,   his real specialty is the hairy single-celled  eukaryotes known better as ciliates. He gathers water from ponds  and lakes and distant lands,   and pores over texts both old and new, all to  see the vast corners of the ciliated cosmos.

And thanks to him, we have the chance  to see ciliates that are so rare that   the last time they’ve been seen was in the 1930s. We spent our last episode focused  on one of those rare ciliates: the tailed Bryophyllum caudatum. And today we’re going to take a look at a few more   so we can better understand what  it is that makes them so rare.

This is Apertospathula, a predatory ciliate that  feeds on other protists and microscopic animals. And of the ciliates we’re looking at today,   it’s one of the only ones that wasn’t  last reported back in the 1930s. And that’s because it’s never been reported at all: it’s a new species in this genus.

Buried in its bodies are thousands  of prokaryotic endosymbionts. When you look at it in its resting cyst phase,   the apertospathula still manages  to seem somewhat in motion. That’s the motion of the prokaryotic  endosymbionts swirling against each other.

It’s hypnotic, moving just  so slowly that it’s hard to   tell if the collective movement is  real or just a trick of the eyes. That movement might be a way to distribute   nutrients around the cyst so that the  prokaryotes are treated more evenly. And under a UV light, the individual bodies  of those endosymbionts stand out even more,   their autofluorescence lighting up the image.

Altogether, it makes the  apertospathula rare and spectacular. Looking across all microbes, meaning all  microscopic prokaryotes and eukaryotes,   it’s estimated that around 1.5-28% of those  organisms are rare, marked by low numbers   and abundance across most time periods  and in most environmental conditions. For James, finding rare microbes  feels like he’s found a dodo or   a Tasmanian tiger frolicking  around in his neighborhood.

And it also feels like a tribute to his microscopy  heroes, scientists like Eugene Penard and Alfred   Kahl who described and classified the microbial  world in the beginning of the 20th century. For James, the passion in their work is  still apparent, even nearly a century later. It’s a familiar feeling to  our master of microscopes.

So when he finds something that was last reported  by them in the 1930s, it means so much to him. This species, Penardiella undulata, was described  for the first time by Kahl in 1930, but has never   been reported again until James found it in  a small pond full of decaying organic matter. The individuals he found were  always in the low oxygen sediments.

Kahl also reported this related species,  Penardiella interrupta, in 1930. And it had been found once in 2009 before  James was able to find four more individuals,   all of whom have green endosymbiotic algae. These species all belong to the Penardiella genus,   which Kahl constructed and  named for Eugene Penard.

So being able to find any feels like  a tribute to both scientists at once,   all the more special because  sightings tend to be very rare. Who knows, perhaps Kahl would  have managed to find more of   these ciliates in his favorite sampling areas. But Kahl stopped publishing work in 1935,   which may have been due to issues  with other members of his field.

He briefly returned in 1943 with a  publication that combined revisions   of his previous work alongside new  species found between 1934 and 1940. But a planned follow-up was  lost due to World War II,   and Kahl died in 1946 of  causes that remain unknown. One of the reasons it’s so valuable to continue  finding these rare microbes decades later   is that we have so many more techniques  available to us to study and record them.

We know so much more about  DNA and how to study it,   which has given us so much more insight into  how these organisms function and evolve. But these older microscopists were  diligent with the tools they had: their microscope, and the ability to combine words   and illustration to describe  a world that seemed so alien. Penard described the rare Legendrea  bellerophon in 1914, taking immense   care with the details and illustrations so that he  could capture the various forms of the organism.

The publication ended up being 27 pages long. We can’t help but wonder though, what it would  have been like for microscopists back in the   day to have the same ability to record high  quality footage of microbes like we do today. What would they have chosen to record?

How would they have chosen to film it? What would be the details they  would want an audience to see later? We don’t know what they would have wanted to see.

But we do know what James wanted: he wanted to know what would happen if he left   Legendrea bellerophon on a  sealed slide with no food. He found a cell, pipetted it on a slide with  no food, and left it in a humidity chamber. And then he watched and watched and watched… for 34 days.

The organism started out  with its tentacles extended,   at times moving around in  search of something to eat. But by day 34, it was mostly static, surrounded  by a constellation of its own parts. Its shape was slightly different,  perhaps because it was in need of food.

Unfortunately, the Legendrea  eventually disappeared when   the Vaseline seal on the slide broke. We don’t know where its future took it. So what is it that makes these ciliates rare?

There are a few traits that James and his  collaborator, Professor Genoveva Esteban,   have noticed that seem to be consistent  across the rare species they find. In general, these rare microbes are likely  to have something that limits their growth,   keeping their numbers small overall. There are various ways that can happen:  one is that they might be anaerobic, which is less  efficient compared to aerobic metabolism, and  also reliant on food that is harder to come by.

The Legendrea bellerophon is  like this. It can spend ages— 34 days even— waiting for its very specific diet of microscopic  animals like rotifers and gastrotrichs. Those animals aren’t thriving in the anoxic  environments Legendrea prefer to live in,   so the ciliate must be willing to wait.

And in turn, it will have a slower growth rate. So the fact that they can survive so long   without food might actually be  connected to why they’re so rare. At least, that’s one possible explanation.

And in a world filled with so many  organisms and so many ways to be rare,   there are likely many more reasons  and ways for an organism to be rare. We can only work at uncovering more of them. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.

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