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Yes, they might eat your brain, but there's a lot more to amoebas than that!

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Sources:
https://www.sfgate.com/business/article/Amoeba-Music-maestro-3212244.php
https://www.semanticscholar.org/paper/The-twilight-of-Sarcodina%3A-a-molecular-perspective-Pawlowski/e67c512b06950ae108313401216f9a7b1ea9e89a
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3107637/
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There’s a chance that before you had an idea of what an amoeba was, you had an image of them in your head, built on headlines describing invisible brain-eaters or pop culture references to their simple, amorphous bodies.

Tardigrades and hydras and stentors are magnificent, but amoebas…they’re classic. When asked how he named his iconic California record store Amoeba Music, co-owner Marc Weinstein said, “The word "amoeba" came up because it sounded good with ‘music.’ We just wanted it to be a psychedelic kind of name that students would relate to.” Record stores aren’t alone in finding familiarity with amoebas.

They show up often in writing, like a cultural ambassador of the microcosmos that serves as a handy reference for anything that is blobby, that splits, that is undeveloped. But as is the truth of many a muse, the reality of amoebas is so much more complicated and weird than the flattened image of a formless ancestor that we project our literary needs onto. The very idea of “amoeba” as a category is a relic from the 19th century classification of single-celled eukaryotes, or protists, which grouped the organisms into one of four types that seemed to largely correspond to how they move: sporozoa, flagellates, ciliates, and amoeba.

According to this classification, amoebas are the ones with pseudopodia, extensions of the cytoplasm that help the amoeba eat and move around. Amoeboid protists were largely grouped together into a class or phylum called Sarcodina, a name derived from the word “sarcode,” which was in turn a French version of the. Greek word for “fleshy.” These classifications were built on the tools of the time: namely, microscopes and what scientists could observe through them.

And so the pseudopodia that defined amoebas were also used to differentiate them from one another. Some of these pseudopodia are likely what you would imagine, loose and spreading. But heliozoa are also amoebas, their bodies extending outward in distinct, spiking, rays that are actually pseudopodia held rigid by inner microtubules.

And the bodies of amoebas, despite the collective blob we often envision, show the same range. Some amoebas, called testate amoebas, actually build shells around themselves, they’re single-celled constructors. These shells vary across species in how they’re made and what they look like.

This young Arcella is colorless, but as it ages, the storage of iron and manganese in its shell will turn it brown. Unfortunately, while the shells of testate amoeba can be stunning, they can also be difficult to record because the weight of the glass coverslip on the slide can break those shells. In terms of size, amoebas can range from microns to centimeters, making them sometimes visible to our naked eyes.

The amoeba Pelomyxa is somewhere in between. But even though they usually measure in a little under 1 millimeter, they’re still considered a “giant amoeba”. To get a sense of just how big a giant is in the microcosmos, here’s a Pelomyxa…and here’s a tardigrade.

Remember, tardigrades have thousands of cells, and that Pelomyxa has just the one. Inside of that one giant cell is a body lacking in many organelles we associate with eukaryotes: there is no mitochondria powering the cell, no Golgi body moving things around. It does, however, house endosymbiotic bacteria, and multiple nuclei.

While some Pelomyxa might have only a few nuclei, others have been found with several thousand. The simplicity of the Pelomyxa body was for a long time the basis of the belief that it is primitive, predating the advent of those other eukaryotic parts. The truth, as we will see later, gets murkier as we try to piece together two billion years of eukaryotic history.

Amoebas move in a way that is, very helpfully, described as amoeboid locomotion, a combination of extending pseudopodia and altered shape. And as this single-celled creature makes its way around in that particularly amoeboid way, you should know that this kind of movement is going on inside of you right now. It’s driving your white blood cells around, for example, as they monitor for any external threats.

White blood cells share another amoeboid feature: phagocytosis. Amoebas don’t have mouths, so when it comes time to eat, they use their pseudopodia to trap their prey and engulf it, drawing the food into its own compartment called a vacuole, where it will then be digested. And yes, some species will feed on nerve tissue, including your brain, if the opportunity presents itself.

But the meal of choice for many amoebas is not necessarily so terrifying. This Vampyrella, for instance, sounds menacing because of its name. And sure, when it wants to feed, the Vampyrella pokes a hole into its prey and begins to suck out the contents.

But its primary food is algae and sometimes fungi, even the occasional microscopic worm. When they’re done eating, they form a cyst, creating a wall around themselves as they digest their meal and then divide. Given how important pseudopodia are to the way amoebas live, it’s easy to understand why they were used in our early attempts to make sense of the microcosmos.

But in more recent decades, as we’ve developed the tools to investigate life based on DNA, our categorization of amoebas has changed as well. No longer lumped together in the Sarcodina, most amoeboid protists are now divided between two supergroups. One is Amoebozoa, which includes the Pelomyxa.

The other group is Rhizaria, which includes Heliozoa. But there are some lineages of amoebas that weave their way out of these groups. As these changing evolutionary trees shift our understanding of amoebas, they raise questions about just what we call primitive.

Pelomyxa, for example, was considered to be among the earliest eukaryotes because they lacked mitochondria. More recent evidence, however, indicates that they’re actually the descendants of eukaryotes that had mitochondria. It’s just that for some reason, Pelomyxa seems to have lost theirs.

And if someone would like to do some research to try and determine why Pelomyxa is better off without mitochondria, please do it, because we are very curious. Even our understanding of how amoebas reproduce has had to be corrected, our vision of a simple organism simply dividing is now complicated by interwoven lineages that mix sexual and asexual reproduction. At the risk of getting too circular with our references, our understanding of amoebas is…it’s well…a lot like amoebas themselves: shifting, full of splits and divisions, and yet somehow encompassing.

Amoebas are scattered now: left, right, up, and down the evolutionary branches, sometimes as single cells, sometimes as parts of other organisms, even us. It’s fitting then that we see amoebas as useful touchstones or metaphors in books or music or wherever else they show up; our bodies, after all, are built out of the lessons that nature learned with them. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

And thank you especially for all of our patrons on Patreon who let us do this wonderful thing. We hope that you enjoy it as much as we do. If you want to see more from our Master of Microscopes, James Weiss.

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