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Euglenoids have had a very, very long time to evolve, and that has led to the things they have evolved into being extremely diverse—so diverse that, combined with the varied shape-shifting abilities of its member species, euglenoids have proven challenging to both identify and classify

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Sources:
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This is Euglena ehrenbergii, and as you can see, it is a bit of a shapeshifter.

It’s not just changing its shape though…it’s moving. Across the surface of this unicellular eukaryote, a series of strips made out of protein run the length of the cell, working with a system of molecular tubes to drive this cellular contortionist around in a form of movement known as metaboly.

Those strips create a ridged surface along the plasma membrane called the pellicle, this is the structure that unites the otherwise incredibly diverse set of organisms called euglenoids. Euglenoids are mostly found in freshwater, where many species get their food by engulfing it through phagocytosis. Millions, or maybe even a billion or so years ago, the thing that got engulfed was a photosynthetic green algae.

This marks a major division. Many euglenoids, like this peranema, don’t have chloroplasts. Instead, these non-photosynthetic species rely on feeding mechanisms like phagocytosis or osmosis.

But the euglenoids we’re going to talk about today are the ones whose ancestors not only ingested the green algae, they took on the algae’s chloroplast and the rest of the photosynthetic apparatus, evolving the ability to make their own food in addition to consuming nutrients from their environment. As we mentioned earlier, the common link between euglenoids is that striped surface called the pellicle. Other than that though, euglenoids can look pretty different from each other.

The most common shape among Euglenoids is this sort of elongated almond shape, where the end comes to a tapered point. Then there are these Monomorphina pyrum, whose bodies are more rounded and have a clearer tail coming out at one end. You can see Monomorphina pyrum here as well, along with Phacus tortus.

Almost all Phacus species have this sort of flat, diamond body, resembling a leaf, and some—like this Phacus helicoides—can twist that leaf into an amazing and beautiful corkscrew-like structure. Phacus species do have a pellicle, but they are rigid and can’t actually engage in metaboly, that shape-shifting method of locomotion we saw in the beginning that is so common to euglenoids that it’s also called euglenoid movement. In general, the flexibility of euglenoids corresponds to the number of protein strips in its pellicles.

Species that are less rigid and thus more likely to use metaboly have more strips, while more rigid species have fewer strips. So you might be wondering how these rigid species get around if they can’t rely on wiggling their bodies to different places. The answer is simple: flagella.

All photosynthetic euglenoids have two or more of those hair-like structures that they use to move, even if they are capable of metaboly. The flagella are a little hard to see…if you squint, you might see them here towards the front end of the cell, which is also where you’ll find that distinct reddish spot that looks kind of like an eye. That red spot is, appropriately enough, the eyespot, though it doesn’t actually see.

Instead, the eyespot works as a sort of “shading device”, that along with photoreceptive proteins at the base of the flagella, tells the cell where it can find light to drive photosynthesis. Now, despite their ancient green algae origins, the chloroplasts in photosynthetic euglenoids are a bit different from their ancestral counterparts, particularly in how they store their energy. Green algae make a starch that is stored in the chloroplast for their later energetic needs, because the sun never shines all day. but euglenoids produce a different carbohydrate stock called paramylon that is kept in the cytoplasm of the cell instead of in the chloroplast.

Different euglenoids keep their paramylon stored in different shapes, ranging from small discs to larger rods. As this euglenoid turns, you can actually see its paramylon bodies. It’s those oval ring structures.

And when the cell uses the paramylon for energy, those rings will become thinner. Of course, as we’ve seen in other episodes, things in the microcosmos can get tough, so some euglenoids build their own defenses. This is Trachelmonas.

And I should say here that I am fumbling through some of these pronunciations so don't take them as gospel. It's encased in a protective structure called the lorica, a sugary matrix that the cell produces when its young. The lorica starts out as a softer, more flexible covering.

But over time, it picks up minerals like iron and becomes increasingly solid. You can tell that compared to the other very green euglenoids we’ve shown, this little guy is more on the yellowish side. That color actually comes from those embedded minerals.

These Euglena sanguinea are demonstrating another use for sticky encasements under stress. These cells have entered what is called the palmelloid stage. On the right side of this clip, you can see four cells enclosed in a clear membrane, but when this little clique was started, it was originally just one cell that had lost its flagella and enclosed itself in a capsule of its own making.

In time, that one cell divided into two, and then those two cells divided into four. When conditions are right, these cells can leave the capsule and reform their flagella. Euglenoids have had a very, very long time to evolve, and as we’ve seen, the things that they have evolved into are diverse—so diverse that, combined with the varied shape-shifting abilities of its member species, euglenoids have proven challenging, both to identify and to classify.

In the past few centuries, attempts to categorize them have resulted in their inclusion with animals, with plants, and with protists, as well declarations of their own kingdom. The microscope is a powerful tool, but this is where molecular tools like gene sequencing take over, helping scientists better understand just how the species of this vast group are related to each other and to everything else. And even as these threads of evolution get untangled and old questions get answered, there will only be more questions to ask and opportunities to wonder.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. If you want to see more from our Master of Microscopes, James Weiss, check out Jam and Germs on Instagram. And if you want to come back to see more of the beautiful, weird world of the micro there's probably a subscribe button that you can click on.