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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.
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