With the exception of humans and possibly that one mole-rat, it’s weird to think of
living things as naked.
This is particularly true for microbes, who often seem to be made up of well-arranged
squish that requires very little in the way of ornamentation.
But we have met a few members of the microcosmos who have figured out how to brighten up their
exterior, like the silica frustules made by diatoms that harden their walls.
These structures don’t satisfy any kind of internal, diatomaceous vanity, but rather
offer protection against things like UV light.
And then, there are the amoebas.
The classic amoeba of popular imagination is generally considered—scientifically speaking—naked.
Their pseudopodia, or “false feet,” transport their shifting, unadorned bodies throughout
the microcosmos.
But as we’ve discussed before, amoebas are a polyphyletic group.
We’ve grouped different species together because in the early days of microscopy, they
seemed to resemble each other in a way that suggested a close relationship.
But with molecular tools that reveal the hidden history encoded in their DNA, we’ve learned
that these relationships are actually much more distant, and that the term “amoeba”
does not define a single taxonomic group.
Rather, it’s a descriptive term, encompassing organisms that exhibit that distinct, crawling
“amoeboid movement” using its pseudopodia.
So, saying something is an amoeba is more like saying something is a flyer than saying
it is a bird.
Birds are all related to each other, but a flyer might be a bird, might be a bat, might
be a ladybug, might be a pterodactyl.
But this distinction remains important as we venture into the lives of testate amoebas,
the more modest compatriots of the naked amoeba.
Much like amoebas as a whole, the grouping of “testate amoeba” is a polyphyletic
one, lumping together amoebas who encase themselves in a shell called the “test.”
The test acts as protection from changes in the environment, as well as from potential
predators.
But it also contains a hole, or “aperture,” from which the amoeba’s pseudopodia can
come out and do what it needs to do.
Different testate amoebas assemble their shells out of different materials.
In some cases, the test is autogenic, meaning that the amoeba produces its own building
material.
For example, the shell of this Arcella here is made out of a single layer of what is called
“proteinaceous alveoli,” which is comprised of a keratin-like protein.
This shell is shaped like a dome, with a central cavity that holds the aperture.
The Arcella produces this shell during binary fission, readying it for the newly produced
daughter cell.
Before starting division, it will collect the protein building blocks inside its cytoplasm.
And as division starts, the Arcella will extend its cytoplasm through the aperture to create
a bud, where those building blocks then get gathered and assembled and converted—first
into the right materials, and then into the right shape.
And when the new test is done forming, the cell itself begins to divide.
The Arcella’s shell starts out colorless, but with time, they may become colored with
accumulating iron and manganese.
Other autogenic tests may be made up of different materials, like silica or calcium carbonate.
But there are also amoebas who gather the materials to make their shells from their
environment, making what are called xenogeneic tests.
One example of this are the Difflugia amoeba, whose shells are composed of various materials
including quartz or even diatom frustules that are stuck together using the organism’s
own special glue.
Their tests, also called agglutinate shells, come in a variety of shapes: elongated, lobed,
horned—there’s an elaborate set of descriptors used to specify this beautiful morphology
different species take on.
While some of their structures may appear at first glance to be random, the aperture
in particular often shows a much more orderly arrangement.
Some species have a tooth-like pattern lining the aperture, while others might have a more
rounded structure.
At first glance, Difflugia might not seem particularly dexterous given the hardened
shell they choose to live in.
They survive largely on bacteria and algae anyway, so nimbleness may not be their primary
concern.
But they have still managed feats of agility that have astounded their observer.
In one case, a scientist documented a Difflugia attacking a rotifer, the amoeba stretching
six long pseudopods through its aperture to trap its prey by the feet.
How it managed to detect and quickly snare its prey remains a mystery.
But of course, amoebas are always teaching us just how deceiving appearances can be,
a lesson that is constantly reinforced as they demonstrate the limitation of trying
to establish evolutionary relationships between these organisms just by looking at their morphology.
Amoebas are a polyphyletic group of blobby organisms.
Testate amoebas are a polyphyletic group of blobby organisms with a shell.
And in 2012, scientists established that Difflugia, which share this common characteristic of
how they make their shells and thus seem pretty closely related, are in fact a polyphyletic
group of blobby organisms with a particular kind of shell.
Even within this group, distinguishing between species has been a major challenge because
it relies so heavily on our observations of shell size and shape.
Slight changes in shell structure might make one species appear to be different.
But we see so little of the organism itself, and thus we can’t readily rely on other
cellular features to distinguish them.
It’s only with genetic techniques that we can begin to unravel more about the creatures
inside the shell.
And unraveling these things is important not just to our understanding of amoeba, but also
how we study the world around us.
Testate amoeba are abundant and distributed in waters well around the world, and they’re
quite sensitive to conditions like moisture or pH.
Perhaps most importantly, their shells are well-preserved, a rarity in the microbial
world.
And the accumulation and distribution of their shells allows scientists to study not just
current testate amoeba populations, but also ancient ones whose remains can reveal what
their environments may have looked like.
Even with the vagaries of establishing different species and evolutionary relationships, testate
amoebas have been extremely useful paleobioindicators.
But learning more about their diversity and taxonomy will sharpen the kind of inferences
scientists working in this field can make, providing more insight into both microbial
evolution and ecological change.
For all their influence and long, storied impact on our earth, microorganisms have a
pretty ephemeral vibe.
Their life cycles are often short, and when they die, they usually leave behind very little.
While the legacy of ancient microbes encompasses every level of life, the only traces of their
individual identities are encoded in DNA.
But sometimes, as with the testate amoeba, they leave behind a little something extra.
The shells that protect the amoeba while alive, and remain evidence of their existence long
after they are dead.
Perhaps no one mourns the amoeba, but we do study them.
And in doing so, they teach us many things, including just how complicated such a seemingly
simple organism can be.
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