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Go to CuriosityStream.com/microcosmos   to start streaming thousands of  documentaries and nonfiction shows. Collecting microbes is a dirty business.  When you venture out into ponds and lakes   to gather samples, you don’t just bring  back water and microbes.

You bring back   bits of plants and other debris. And that can make it difficult   to find the organism we want to  talk about today: the pelomyxa.  Pelomyxa are giant amoeba, capable of growing as  much as 5 millimeters in length. So it doesn’t   seem like they should be that difficult to find.  In fact, James—our master of microscopes—found   hundreds of them filling up his pond tank,  completely visible without the help of a   microscope.

They were so large that he actually  took one and he touched it. So, if you would like   to know what it feels like to pet an amoeba,  he is now an expert. He says, “It’s squishy.”  But while they may be in our pond tank, they’re  often harder to pick out with a microscope.   They move incredibly slowly, and their cytoplasm  tends to fill up with grains of sand and other   particles, so they often blend into the background  with the other debris we find in our slide.  So despite being giants, the pelomyxa can  be deceptively difficult to recognize.   And as we will see, that is not  the only tricky thing about them.  Pelomyxa was first described by the German  scientist Richard Greeff, who named it Pelobius   in 1870, and then four years later decided  to change the name to Pelomyxa palustris.  The species is marked with a number of nonmotile  flagella and several nuclei, though “several”   covers a wide range of nuclei numbers.

Some  pelomyxa have been observed with two nuclei,   while others have had several thousand. The species is anaerobic, found in the   bottom sediments of freshwater where oxygen is  low. And like many anaerobic protists, pelomyxa   turn themselves into a home for prokaryotes in  exchange for help surviving their environment.  While these endosymbionts had been observed  by Greeff and many others after him,   they weren’t identified until more recently,  thanks to the development of molecular   techniques to extract and identify their DNA.

In describing the endosymbionts they observed how   they help the pelomyxa survive and make nutrients,  the scientists compared the group to a sewage   treatment plant. That might sound like an odd way  to describe eating until you realize that Pelomyxa   eat by just engulfing stuff around them. And  when you’re a giant amoeba living at the bottom   of a pond, the stuff around you is sludge.

Lots of pond sludge, though if you want   to get technical about it, that sludge is  called sapropel. And while sludge may not   sound particularly delicious, sapropel  is made up of decaying organic matter.   It is rich in nutrients, making it a  feast for amoeba and endosymbionts alike.  So while calling them a sewage treatment plant  may not seem the most glamorous comparison,   it is a life that probably works  well for all microbes involved.  In the early days of Pelomyxa discovery,  scientists identified more than 20 species   of the giant amoeba. But distinguishing  giant amorphous species from one another is a   subtle challenge, and it became apparent in the  1990s that many of these species were actually   just Pelomyxa palustris in some  phase of its life cycle that was   distinct enough to look like a  different organism altogether.  So for a time, it was thought that maybe  Pelomyxa palustris was the only Pelomyxa species,   and that its life was just so complex that  it encompassed all of these different forms.  But since then, scientists have managed to  identify a handful of other Pelomyxa species,   making out those subtle details that  differentiate them from one another.  But while we’ve been describing  the traits the pelomyxa do have,   whether that’s their giant  nuclei or their endosymbionts,   there is one very distinctive trait  that they all lack: a mitochondrion.  This lack of a mitochondrion has been a  fascinating question to those studying the   evolution of eukaryotes.

Endosymbiotic theory  tells us that the mitochondria were once their   own organism, until in a massive act of evolution,  they were consumed by another organism and stuck   around, eventually morphing into the organelle  that we know and love and depend on today.  But eukaryotes like Pelomyxa that do not have a  mitochondrion don’t fit neatly into that story. So   in 1983, the scientist Thomas Cavalier-Smith  proposed a new kingdom of life: the Archezoa.  The kingdom Archezoa were composed of eukaryotes  who were thought to have diverged before the   evolution of mitochondrial eukaryotes. And  even of this strange group, the Pelomyxa and   their fellow mitochondrion-less amoeba were  thought to be the most primitive because,   well, look at them.

They’re amoeba. They’re  simple. And it’s hard not to buy into the   assumption that just because something is  more complex, it must be more advanced.  But the Archezoan kingdom took a big hit from  these supposedly primitive amoeba when it became   apparent that they actually did have some of the  molecular remnants of a mitochondrion.

Which meant   that even if they didn’t have a mitochondrion  now, they evolved from something that did.  That makes Pelomyxa something more  than just simple: they are degenerate.   That’s not an insult, this is what they  are called. They are degenerate protists,   all because at some point in their lineage,  a mitochondrion was gained and then lost.  And since then, the Archezoa kingdom has  been abandoned, undone by the evidence   in its own members’ molecules. And the  undoing of this kingdom was, of course,   more than just the result of any one organism.  It took many scientists studying many organisms   to come to this conclusion, and it will take many  more to answer the questions that still remain.  But it’s lovely to think of the Pelomyxa—our  very own mysterious, hidden giant—helping to undo   some of the myths that we hold about  simplicity and evolution.

It’s not the   sort of story that gets passed down through  generations, but you know, maybe it should be.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  This episode marks the end of the third season  of Journey to the Microcosmos, but do not worry,   we will be back on February 22nd to start  Season 4! In the meantime, you might see   a bonus video or two popping up in the next  couple of weeks so keep an eye out for those. This episode has been brought to you  by CuriosityStream, a subscription   streaming service that offers thousands  of documentaries and non­fiction TV shows   from some of the world's best filmmakers,  including award winning exclusives & originals.   They cover topics like history, nature,  science, food, technology, travel, and more!

One piece of content that might be particularly  interesting to Microcosmos viewers is “The   Kingdom: How Fungi Made Our World” which is all  about the hidden kingdom that rules life on land. You can stream CuriosityStream’s library,   including their collections of curated  programs handpicked by their experts,   to any device for viewing anytime, anywhere,  and if you go to curiositystream.com/microcosmos   and use the code “Microcosmos” to sign up, it  will only cost you $14.99 for an entire year! Thank you as always and once again to  all the people who are on the screen   right now.

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