Amoebas: Occasional Brain-Eaters – Transcript

00:12There’s a chance that before you had an idea of what an amoeba was, you had an image of them inyour head, built on headlines describing invisible brain-eaters or pop culture references totheir simple, amorphous bodies.

00:31Tardigrades and hydras and stentors are magnificent,but amoebas…they’re classic. When asked how he named his iconic Californiarecord store Amoeba Music, co-owner Marc Weinstein said, “The word "amoeba" came up becauseit sounded good with ‘music.’

00:55We just wanted it to be a psychedelic kindof name that students would relate to.” Record stores aren’t alone in finding familiaritywith amoebas. They show up often in writing, like a culturalambassador of the microcosmos that serves as a handy reference for anything that isblobby, that splits, that is undeveloped.

01:19But as is the truth of many a muse, the realityof amoebas is so much more complicated and weird than the flattened image of a formlessancestor that we project our literary needs onto.

01:38The very idea of “amoeba” as a categoryis 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 tohow they move: sporozoa, flagellates, ciliates, and amoeba.

02:00According to this classification, amoebasare the ones with pseudopodia, extensions of the cytoplasm that help the amoeba eatand move around. Amoeboid protists were largely grouped togetherinto 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.”

02:29These classifications were built on the toolsof the time: namely, microscopes and what scientists could observe through them. And so the pseudopodia that defined amoebaswere also used to differentiate them from one another. Some of these pseudopodia are likely whatyou would imagine, loose and spreading.

02:50But heliozoa are also amoebas, their bodiesextending outward in distinct, spiking, rays that are actually pseudopodia held rigid byinner microtubules. And the bodies of amoebas, despite the collectiveblob we often envision, show the same range.

03:11Some amoebas, called testate amoebas, actuallybuild shells around themselves, they’re single-celled constructors. These shells vary across species in how they’remade and what they look like.

03:27This young Arcella is colorless, but as itages, the storage of iron and manganese in its shell will turn it brown. Unfortunately, while the shells of testateamoeba can be stunning, they can also be difficult to record because the weight of the glasscoverslip on the slide can break those shells.

03:57In terms of size, amoebas can range from micronsto centimeters, making them sometimes visible to our naked eyes. The amoeba Pelomyxa is somewhere in between. But even though they usually measure in alittle under 1 millimeter, they’re still considered a “giant amoeba”.

04:18To get a sense of just how big a giant isin the microcosmos, here’s a Pelomyxa…and here’s a tardigrade. Remember, tardigrades have thousands of cells,and that Pelomyxa has just the one.

04:38Inside of that one giant cell is a body lackingin 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.

04:59While some Pelomyxa might have only a fewnuclei, others have been found with several thousand. The simplicity of the Pelomyxa body was fora long time the basis of the belief that it is primitive, predating the advent of thoseother eukaryotic parts.

05:21The truth, as we will see later, gets murkieras 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.

05:44And as this single-celled creature makes itsway around in that particularly amoeboid way, you should know that this kind of movementis going on inside of you right now. It’s driving your white blood cells around,for example, as they monitor for any external threats.

06:05White blood cells share another amoeboid feature:phagocytosis. Amoebas don’t have mouths, so when it comestime to eat, they use their pseudopodia to trap their prey and engulf it, drawing thefood into its own compartment called a vacuole, where it will then be digested.

06:28And yes, some species will feed on nerve tissue,including your brain, if the opportunity presents itself. But the meal of choice for many amoebas isnot necessarily so terrifying. This Vampyrella, for instance, sounds menacingbecause of its name.

06:48And sure, when it wants to feed, the Vampyrellapokes a hole into its prey and begins to suck out the contents. But its primary food is algae and sometimesfungi, even the occasional microscopic worm.

07:03When 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 theway amoebas live, it’s easy to understand why they were used in our early attempts tomake sense of the microcosmos.

07:25But in more recent decades, as we’ve developedthe tools to investigate life based on DNA, our categorization of amoebas has changedas well. No longer lumped together in the Sarcodina,most amoeboid protists are now divided between two supergroups.

07:44One is Amoebozoa, which includes the Pelomyxa. The other group is Rhizaria, which includesHeliozoa. But there are some lineages of amoebas thatweave their way out of these groups. As these changing evolutionary trees shiftour understanding of amoebas, they raise questions about just what we call primitive.

08:08Pelomyxa, for example, was considered to beamong the earliest eukaryotes because they lacked mitochondria. More recent evidence, however, indicates thatthey’re actually the descendants of eukaryotes that had mitochondria.

08:24It’s just that for some reason, Pelomyxaseems to have lost theirs. And if someone would like to do some researchto try and determine why Pelomyxa is better off without mitochondria, please do it, becausewe are very curious.

08:42Even our understanding of how amoebas reproducehas had to be corrected, our vision of a simple organism simply dividing is now complicatedby interwoven lineages that mix sexual and asexual reproduction.

08:59At the risk of getting too circular with ourreferences, our understanding of amoebas is…it’s well…a lot like amoebas themselves: shifting,full of splits and divisions, and yet somehow encompassing.

09:17Amoebas are scattered now: left, right, up,and down the evolutionary branches, sometimes as single cells, sometimes as parts of otherorganisms, even us. It’s fitting then that we see amoebas asuseful touchstones or metaphors in books or music or wherever else they show up; our bodies,after all, are built out of the lessons that

09:45nature learned with them. Thank you for coming on this journey withus as we explore the unseen world that surrounds us. And thank you especially for all of our patronson Patreon who let us do this wonderful thing.

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