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There’s something you probably heard a lot in biology class. And no, it's not “mitochondria is the powerhouse of the cell”...

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To get started, go to or text microcosmos to five hundred five hundred. There’s something you probably heard a lot in biology class.

And no, I do not mean, “mitochondria is the powerhouse of the cell,” which, I mean, it is. Instead I’m talking about these two words: “structure” and “function.” In biology the way bits and pieces of an organism’s body are built - from the materials to the shapes to the arrangements—all these factors that impact what an organism looks like, its structure, are connected to what those bits and pieces do. The function.

Now this seems very simple, but the point is that structure and function are linked. To observe one is to make predictions and inferences about the other. And this is something we can see across all levels of biology, from the shape of a bird’s wing to why mitochondria are the powerhouse of the cell.

And of course, structure and function relate together in the ways that an organism eats. In animals, you might think of the different types of teeth and mouths that reflect both how and what a species eats. But this, of course, extends into the microcosmos as well—not in terms of teeth, of course, but in other structures that influence an organism’s behavior and function as they pursue their meal of choice.

And to see more of this, let’s focus on one major group of eukaryotes that encompass a multitude of eating styles: the euglenoids. The euglenoids, or euglenids, are a large group of flagellates made up of well over 1,000 species. The primary trait uniting them are the strips of proteins and microbes that line their bodies, creating a visible and striking structure called the pellicle.

And some euglenoids are green—very, very green, as we’ve explored before in a previous episode. Their color comes from the chloroplasts inside their body, which enable them to generate their own nutrition via photosynthesis. But euglenoids weren’t always phototrophic.

Now, we’re used to thinking of plants as in some way less developed than animals. That things that make their energy from the sun, are more primitive than the things that get their energy by eating other stuff. This actually is often the reverse in the microcosmos.

As is the story for several eukaryotic lineages, at some point a long, long time ago, a non-photosynthetic euglenoid consumed a green algae. But instead of merely digesting that algae and turning it into food, it formed an endosymbiotic relationship that would eventually turn the algae into a chloroplast and turn the phagotrophic euglenoid, or one that survives by eating stuff, into a phototroph. So, then the question becomes, what happened to the organisms in those lineages that didn’t make this dietary switch?

Well, in the case of euglenoids, they continued evolving, just with a little less color. And by comparing these different euglenoids, we can not only see how different forms of eating connect across evolutionary webs, but also how they become reflected in the structure of the organisms themselves. Let’s start by explaining what the non-photosynthetic euglenoids do for food.

And you will notice here that I did not say “What they eat” because they don’t all eat. We can divide these colorless euglenoids into three groups. You’ve got your bacteriovores, your eukaryovores, and your primary osmotrophs.

Bacteriovores and eukaryovores are both phagotrophic, meaning they rely on phagocytosis to engulf and consume their food. Where they differ, of course, is what they eat, which you can probably guess. Bacteriovores, like this Dinematomonas, which I have no idea if I’m pronouncing correctly, usually go after smaller organisms like bacteria, while eukaryovores like to super size their meals and feed on larger microbial life like eukaryotes.

The last category of euglenoids, the primary osmotrophs, are the ones who survive by absorbing nutrients from their surroundings. While phototrophs and phagotrophs are able to get food in this as well, for primary osmotrophs, this is it—food-wise, it’s absorption or bust. So, they do not eat, they just allow stuff to ooze into them.

There is an evolutionary trajectory to these categories. The bacteriovore euglenoids came first. Eukaryovores evolved from bacteriovores.

And then phototrophs and primary osmotrophs evolved independently from eukaryovores. There are, of course, a few exceptions to this neat narrative, including a few osmotroph species that evolved from phototrophic euglenoids. But for the most part, molecular phylogeny has held up this order of dietary evolution.

And what these organisms eat dictates how much they interact with their environment, which in turn requires different things out of their bodies. Perhaps the most obvious divergence compared to phototrophs is that these colorless euglenoids are colorless. They don’t have the green chloroplasts that, through their own chemistry and structure, help phototrophic euglenoids convert sunlight into food.

In the case of the phagotrophs then, they need a way to grab onto the organism they want to consume, and they do that with the help of a feeding apparatus, which unfortunately we can’t quite make out in our footage. This apparatus is made up of microtubules that extend out from the front of the cell and twist open to grab their prey. As the organism retracts its feeding apparatus back, it drags the food in with it.

But osmotrophs and phototrophs don’t need that apparatus. They’re not hunting anything down. And so it might not be much of a surprise that these species tend to be marked by a reduction or even a loss of the feeding apparatus.

Absorbtion or making your own nutrition also requires a different sort of motility compared to when you’re on the hunt. Osmotrophs and phototrophs tend to swim, which allows them to explore and adjust to sunlight and nutrition availability along a water column. Meanwhile, phagotrophs tend to glide along surfaces, which enables them to search out and find prey nearby.

This behavioral difference has its basis, of course, in structural differences. Most primary osmotrophs and phototrophs have two flagella. But one is much more apparent and active than the other, moving in a figure-eight shape that helps the organism swim through the water.

Meanwhile, phagotrophic flagella lie on different ends of the cell and are lined with hairs and rods that help the organism glide. Their front flagellum extends straight out, constantly moving to sense the world around them. And for some species, their flagella is multipurpose: they can use it to harpoon prey and move it towards their feeding apparatus.

But even though these phagotrophs may all ingest via phagocytosis, not all food is the same—differences that again are reflected in the bodies of the hunters themselves. Eukaryovorous euglenoids tend to be larger compared with their bacteria-consuming relatives, and they also have more pellicular strips, which allows them to be less rigid. Their size and stretch lets them not only consume bigger food, but also to morph their bodies according to what they digest.

The premise of “structure and function” is one of those things that is so obvious that it’s easy to dismiss sometimes. Of course the way a thing is built impacts what it is able to do! That’s the basis of how we make things that are useful, whether they’re our homes or our cars.

But what makes this concept so fascinating and powerful in biology is that these pieces weren’t consciously assembled together—they were evolved. At one point, a bacteriovore euglenoid changed in some way that gave way to a eukaryovore. And eventually, some descendant of that eukaryovore consumed a green algae that set the stage for phototrophs while another lost its feeding apparatus to become an osmotroph.

These differences in structure and function link together a whole history of small changes upon small changes, connecting organisms both past and present through the much bigger differences we observe today. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you as well to Audible, for supporting this episode.

There are many audiobooks about the Microcosmos that are available on Audible. In I Contain Multitudes, The Microbes Within Us and a Grander View of Life, author Ed Yong takes us on a grand tour through our microbial partners and introduces us to the scientists on the front lines of discovery. And he does this with an enthusiasm for microbes that just might rival our own.

And that is, yes, available as an audiobook on Audible. We here at Journey to the Microcosmos know that seeing the world in different ways opens us up to new ideas and new ways of thinking, and we know that compelling stories have the ability to do that same thing. If you go to the link in the description, you can listen to I Contain Multitudes, along with many Audible Originals which are exclusive audio titles created by celebrated storytellers from the worlds of sci-fi, journalism, literature, and more.

Get your first audiobook for free, plus access to all of the Audible Original monthly offerings when you try out Audible for 30 days by visiting or texting “microcosmos” to 500 500. And thank you to all of these people whose names are on the screen right now. They are our patrons on Patreon, which you can find at, if you would like to help us keep making this thing.

If you want to see from our Master of Microscopes, James, check out Jam and Germs on Instagram. And if you want to see more from us, there’s always a subscribe button somewhere nearby.