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Go to Brilliant.org/microcosmos to learn how you can take your STEM skills to the next level! Now it’s not really hard to see how heliozoa got their name.

With their spheroid body and spiky rays, it seems almost impossible to not pick a name related to the sun. So why not refer back to the Greek god of the sun Helios? Of course, heliozoa aren’t exactly riding golden chariots across the sky, but they’ve been similarly difficult to pin down.

And that’s partly because we’ve come to understand that labeling things as heliozoa might not be as meaningful as we once thought it was. In the 19th century, when heliozoa were first described, the label was thought to apply to just about any spiky, freshwater organisms that looked like spiky, freshwater organisms. The similarity in their appearance suggested a proximity in their categorization, so it made sense to group them together.

That has of course changed in more recent years as scientists have turned to studies of organisms' DNA to revise those early classification systems so they better match genetic relationships. So today, “heliozoa” is less of a specific label and more of a description, referring to a set of organisms that are round with stiff pseudopodia or axopodia, the more technical term for those spikes. And they are not necessarily closely related to one other, they just look like it.

If a heliozoan’s axopodia seem threatening to you, that’s good. Those stiff spikes aren’t there as decoration, they play an important role in the heliozoan’s hunt for food, whether that’s other protists or even tiny animals. And you should probably think of them less as spikes and more as sticky skewers.

After the heliozoan’s prey gets caught in the axopodia, it is immediately immobilized and stuck to the adhesive surface. The prey then gets drawn in towards the heliozoan itself until it is close enough to be engulfed into a vacuole for further digestion. The central, driving life of the heliozoan is in that central body, where food is consumed and the important biological molecules that keep the organism alive are made.

And what a fine, spherical body it is. But let’s be honest: it’s the axopodia that makes a heliozoa so striking to look at. And for as long as we’ve been looking at heliozoa, we have wanted to know more about how those axopodia work and how they’re made.

After all, axopodia have quite a lot to do. They’re not simply stagnant spikes on a heliozoan. They’re an extension of the organism’s cytoplasm, carrying the extrusomes that make their surface adhesive.

And they contract when the organism needs them to, like when they’re drawing in their prey. It’s tempting to compare the axopodia to having just a bunch of arms, but the comparison doesn’t do the axopodia justice. After all, it’s not like we can retract and extend our arms out of our bodies at will.

While axopodia are, in a sense, the cytoplasm poking outwards, there needs to be something underlying those extensions to give them shape and structure. And that “something” are hollow filaments called microtubules. Made from a protein tubulin, you can think of microtubules as building blocks for eukaryotic cells, though they’re shaped like rods, not actual blocks.

Microtubules show up in mitotic spindles, cilia, flagella, and many other pieces of cellular machinery. And how they’re arranged inside an organism—whether they are unicellular or multicellular—impacts everything from shape to movement. And heliozoa’s axopodia are no different.

In the 1960s, scientists wanted to understand how the microtubules arranged and rearranged themselves, and how this impacted the heliozoa’s response to changes in the world around them. To get a much, much closer look than we can do here, they used electron microscopy, which revealed that the microtubules were arranged parallel to one another. And when they looked at the cross-section of the axopodia, they found that the microtubules formed an interlocking double-coil formation.

But scientists are rarely content with just knowing the structure of a thing. They also wanted to know how that structure responds to all sorts of stimuli. In the second half of the 60s, Lewis G.

Tilney and an assortment of different collaborators published a multi-part series of papers titled “Studies on microtubules in heliozoa.” We think some selections from the titles convey the goal of these studies…. We have part 2: “The effect of low temperature on these structures in the formation and maintenance of the axopodia.” Then there’s the sequel in part 3: the “pressure analysis of the role of these structures in the formation and maintenance of axopodia” And yes, there’s a part 4, titled “The effect of colchicine in the formation and maintenance of the axopodia.” That is a lot of analysis on the formation and maintenance of the axopodia. And these experiments helped to not only see what happens to heliozoa in changing conditions, they helped our understanding of the microtubules inside them.

For example, when the researchers lowered the temperature of a heliozoan, they found that the organism’s axopodia shortened until they disappeared, a result of the chemical connections holding the microtubules breaking down and the structure depolymerizing. And it was only around 30 to 45 minutes after the heliozoa were returned to room temperature that the microtubules reassembled and the axopodia grew back to their normal length. This result, combined with other observations they made with their other disruptions, helped to solidify the microtubule’s importance in the growth and structure of the axopodia, and provided us with more insight into the cold-sensitivity of microtubules.

These experiments were not the end of the “poke and prod and disrupt the axopodia” approach to heliozoan research. A later study would use what feels almost a poetic method—or at least as poetic as you can get with a science experiment and a microbe named after a sun god. The researchers subjected their heliozoan to ultraviolet light, watching as the ends of the axopodia broke down and shortened, or sometimes even broke off altogether.

And yet, even then, when the ultraviolet light had passed, the sun animalcule’s microtubules were ready to grow once more. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Brilliant for supporting this video.

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