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Some of the ciliates we find in the microcosmos have a sort of stoutness to them,
like the declarative trumpet shape of the stentor or the oblong of the paramecium.
But then you have this little weird fellow, which looks more like a twisty, slithering worm
than a unicellular organism. This is a trachelocercid, and no,
I do not know if I am pronouncing that entirely correctly. It’s a member of a class of ciliates
called the Karyorelictea. We’ll be showing you a few different trachelocercids today, though
we can’t specify their genera any further without using other experimental and microscopy techniques
to see the smaller structures inside them. What they all have in common though
is that they come from a place that we have not really visited yet on our journey through
the microcosmos. They come from the beach. James, our master of microscopes, hired two PhD
students to help him with a very important task: every month, they bring him wet sand from the sea.
Yeah, maybe some of us might get a subscription box filled with items related to our favorite
hobby. James gets a monthly subscription of sand. And not just a little bit of sand here or there.
He gets liters and liters of it that he then sifts through to find the microbes living inside.
They’re tucked away in the space between the grains of sand,
in pockets of water called interstitial water. And while for many of us, a day at the beach might
sound like a nice, relaxing vacation, things are very different for the microbes who live there.
Those little bits of interstitial water are a chaotic habitat. Just imagine your life at that
scale, surrounded by sharp pieces of sand that to you probably would look more like giant boulders.
But imagine that those rocks aren’t just gigantic, they’re moving in a constantly shifting landscape
of sedimentary rubble, a cutting maze that’s always changing as waves crash upon you.
To survive, you can’t just rely on adapting in the moment. You have to rely
on countless generations of adaptation and evolution that have made it possible for you
to respond and survive in the world around you. And for the karyorelictids, evolution has
provided them with a number of tools to handle their tumultuous surroundings.
Most karyorelictids live in the sands of marine environments.
The notable exception are species of loxodes, which live in freshwater.
You’ve probably noticed that the trachelocercid, while very worm-like,
still has some of its own distinct shapes. If we were asked to compare it to another ciliate, we
might look at the lacrymaria olor, with its neck similarly extending and seeking and retracting.
And like the lacrymaria, the focus of the karyorelicted’s pursuit is probably food. They
particularly enjoy a diet of other organisms like bacteria, diatoms, and even rotifers.
But the karyorelictea doesn’t just get to focus on hunting. In a world that is always changing,
the ability for the karyorelictea to twist and turn their elongated body
helps them navigate their uncertain terrain and the narrow corridors that fill it. It also helps
that some species can grow as large as 5 mm long. Karyorelictids can also use their cilia to attach
themselves to grains of sand, giving themselves something to anchor onto if needed. But even then,
a little bit of damage seems inevitable. I mean, this is sand and waves. That can hurt,
even when you’re not a microbe. But the karyorelictids, they have
a solid back-up plan: regeneration. It’s a classic amongst microbes.
You cut one piece, and the organism grows that piece back, stitching itself anew. In
the case of the karyorelictea, some species can regrow parts of their body within 3 or 4 hours.
A lot of the traits we’re describing in the karyorelictids are traits shared by other ciliates
that live in their own environments, but just refined through generation upon generation of
interstitial life. But there is something that’s a bit more unusual, their nucleus doesn’t divide.
Like other ciliates, the karyorelictids have macronuclei and micronuclei,
sometimes multiples of them. The macronuclei provide the
genetic template for the cell’s day-to-day life, while the micronuclei provides the
genetic material that will be used and passed on during sexual reproduction. (We talked about
this process more in our recent conjugation episode if you’re curious how that works.)
And for most ciliates, when the organism divides, the macronucleus divides with it,
ensuring that the new daughter cells each have their own macronucleus. But this is
not the case for the karyorelictids. When they divide, their macronuclei
don’t. So the daughter cells use their micronuclei to make a whole new macronuclei. Karyorelictids
are usually observed with multiple macronuclei, which is probably a way to ensure that
there’s at least some functional nucleus around while the organism is making a new one.
It’s not clear what this strange macronuclei behavior and its specificity to karyorelictids
says about these organisms and their evolution. To some scientists, it means that maybe
karyorelictids mirror some kind of ancestral ciliate state of nucleus
non-division, while others consider it a potential evolutionary add-on that
helps offset mutational damage in the organism. One of the challenges of being able to study their
nuclei is the fact that most karyorelictid species have been difficult to cultivate in the lab. There
is something a little frustrating but also very true to our attempts to understand biology,
that these organisms are so difficult to house in the well-fed comfort of a flask,
and yet thrive in the harsh reality that makes them so fascinating to begin with.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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