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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
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