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﻿If I asked you to picture a single cell,
I bet you’d imagine something super tiny that you had to look at through a microscope.
Which is totally valid, considering the vast majority of cells on this planet
are microscopic—especially if we’re talking whole organisms that consist of just one cell.
But, there are some huge exceptions to this rule. And I really do mean huge.
The six big single-celled creatures we’re going to talk about today are not your average cells.
And because of that, they’ve challenged our ideas of how life works—at
the cellular level and beyond! When scientists first spotted the
trumpet-shaped Stentor coeruleus in pond water in the late 1700s, they mistook it for a hydra
—the anemone relative, not the mythical monster.
Hydras are made up of tens of thousands of cells, though,
and stentors are but one. Each can grow to be up to two millimeters
long—which, ok, is small. But, it makes it a colossus in the unicellular world.
And it’s big enough to be unwieldy. You see, one of the reasons cells are small is that large cells
are too hard to control; it simply takes too long to send information from one part to another.
But stentors can achieve their large sizes because they have an elongated
macronucleus which puts the critter’s genetic material everywhere it’s needed.
They also have lots of small micronuclei that help out when it’s time to make more stentors,
which they do the really old fashioned way: by simply splitting into two.
But perhaps the most remarkable thing about them is their ability to regenerate.
Most cells don’t fare so well if their membranes are ruptured. But cut a Stentor in half, and
you’ll soon have two totally fine stentors. In fact, thanks to the shape of its macronucleus,
almost any piece of Stentor can regenerate; it just needs part of the macronucleus and a small
piece of the original cell membrane. Researchers still have lots of unanswered
questions about this creature's unique healing abilities.
Like: Why don’t the cell’s insides leak out after it’s cut open?
How can multiple individuals regenerate from different fragments? And how does each fragment
know which structures they need? But the more researchers study these
weird little trumpets, the more they challenge our understanding
of what cells can and can’t do. Answering these questions will help us
understand the full capabilities of all cells—microscopic or not.
In the dark depths of the ocean, some 1200 meters down,
are weird little critters called Gromia. And when they were first discovered in the
early 2000s, researchers were completely puzzled. They’d simply never seen anything like them.
It was later determined that these hard, grape-sized cells are a kind of testate amoeba:
amoebas that make themselves porous, protective shells called tests.
Which is super cool; but what’s even cooler is that they move.
Scientists believe gromias crawl ever so slowly by extending tiny feet-like projections called
pseudopods through holes in their tests. And they leave behind noticeable
tracks in the sediment. These just so happen to look very
similar to fossilized tracks that date back as far as 1.8 billion years—tracks that were said
to be evidence for multicellular life. You see, many paleontologists argued that there
was simply no way for a single-celled organism to make gouges like that.
Now, of course, we know that’s not true. So the discovery of Gromia challenged the
evolutionary timeline. And now, most paleontologists
think multi-celled creatures didn’t come on the scene until about 600 million years ago—just
before the Cambrian, when multicellular life as we know it really took off.
Back in 2013, scientists exploring hydrothermal vents off the coast of Sicily found what they
thought was a new species of sponge. The several-centimeter long creatures each
had a stalk that penetrates into the sediment and holds it in place, and a globe-like top
with lots of thin spikes sticking out of it—much like carnivorous sponges.
And those spikes look a lot like spicules: the small, hard structures made by sponges.
It wasn’t until later that researchers realized the organisms weren’t animals at all.
They actually belonged to a group of single-celled amoeba-like organisms called forams.
And they decided to call them Spiculosiphon oceana.
The researchers were right to be thrown by those spicule-like structures,
though… because they are, in fact, spicules. Each Spiculosiphon collects and painstakingly
arranges spicules from carnivorous sponges to steal their look.
That’s because it allows them to capture prey, like small invertebrates,
in the same way carnivorous sponges do. So yes, these are single-celled organisms that
can devour both single- and multi-celled organisms by dressing up like a multi-celled
predator. Biology is weird. These unique creatures are only found
living close to hydrothermal vent systems. And that may be because they prefer their water
acidic and packed with the dissolved minerals that spew out of these underwater volcanoes.
In fact, researchers think they could tell scientists where undiscovered vents are.
Basically, if spiculosiphons are there, there must be recently active hydrothermal vents nearby!
And they may also be a canary in the coal mine of sorts for ocean acidification.
That’s because, while they like things acidic, they can’t tolerate super acidic conditions.
Their stolen shells would dissolve. So if they start struggling,
that could indicate deeper waters are feeling the effects of climate change.
Valonia ventricosa basically looks like what you would think a cell should look like,
just super sized. They can grow to be
four centimeters or larger. They’re also known by the nickname “sailor's
eyeball”, because back in the late 1800s, people thought that they were actually eyeballs staring
back at them from underwater. But I think they look much more
like giant underwater grapes. And, in fact, they’re kind of like
grapes… at least structurally. The outer shell, or cell wall,
contains a lot of cellulose—the same fibery substance found in all sorts of plants,
including the skin of grapes. In fact, 19th and early 20th century
scientists were able to work out the structure of cellulose by studying Valonia.
They were able to look at it up close using x-rays and fancy microscopes to see that
cellulose is made up of microfibrils: little parallel strands of cellulose
bound together through hydrogen bonds. It’s those bonds that make the cellulose rigid,
which is how it gives Velonia the toughness it needs to grow so big.
This deeper understanding of cellulose gained by studying Velonia likely played
a role in the industrial development of cellulose-based products, which includes
everything from plastics and lacquers to paper, cardboard, and more famously, film.
Mermaid’s wine glasses, or Acetabularia if you want to be scientific,
can grow up to 10 centimeters tall. And they kind of look like fungi that you
might find carpeting the forest floor, with their stems and rounded caps.
But they’re actually single-celled algae that can carpet the seafloor in
subtropical shallow, rocky waters. Acetabularia has one giant nucleus
located at the base of its stem. And because of that, it helped biologists
understand what the nucleus actually does. During the 1930s and 1940s, German scientist
Joachim Hammerling performed a series of experiments on Acetabularia to prove that the
nucleus was the control center of a cell. He cut multiple specimens in half and found
that only the ones containing the nucleus could regenerate.
He also grafted together two species of Acetabularia, and showed that the
combined cell would take on the characteristics of whichever species its nucleus came from.
Those experiments laid the groundwork for our modern understanding of the nucleus.
It is a bit weird, though, that it’s able to grow so large, since all of its genetic
material is down at the base. Like, those Valonia we just talked
about? They have lots of nuclei. In fact, once you get larger than a Stentor,
many single-celled plants have multiple nuclei which work together to manage the huge cell.
Acetabularia gets around this by sending some of its genetic material to the upper
parts of the cell as messenger RNA; it has all the machinery it needs up there
to turn those into the proteins that that part of the cell needs.
And in a surprising turn of events, it actually does have multiple nuclei … but
only when it’s about to reproduce. When a mermaid’s wine glass wants to make
more wine glasses, its big nucleus goes through several rounds of division to
make little nuclei. Then all those little copies make their way up to the top of the cell.
From there, they bud off into numerous spore-like reproductive cysts
which can float about for a ways before they settle down and grow to their full size.
At first glance, you would probably have no idea that Caulerpa algae are individual cells.
Individuals in some species can grow to carpet a square meter or more!
And they sure don't look like your average cells. Each has a complex structure that
consists of a stem, fronds that look like leaves, and root-like parts that anchor it in place.
The reason a Caulerpa can grow so large is because it has loads of nuclei to keep
all parts of it functioning properly. Caulerpa species also use a process called
cytoplasmic streaming, which is essentially a plant’s solution to a circulatory system.
They always keep their cytoplasm moving in an organized fashion,
which allows them to quickly transfer molecules and other structures around the cell.
Lots of scientists have studied these algae over the decades.
Researchers were particularly interested in learning more about how they get their intricate
structures, because it was long assumed that kind of complexity required multiple cells.
Now, they believe it all boils down to when and where different genes are active.
For instance, research published in 2015 showed that different genes are
active in the nuclei in different parts of the cell, and these seem to be telling
those regions what shapes to be. This has deepened our understanding of
how all cells figure out their final shape and structure.
And it’s actually calling into question whether any plant is truly multicellular.
You see, some researchers argue that all plants should be thought of as single-celled organisms,
because their cells are interconnected and communicate with each other through
channels called plasmodesmata. And if cells are super connected,
are they really separate cells? It’s an intriguing question, anyway!
While humongous cells like Caulerpa are the exception, not the norm, because of their size,
they’ve given researchers throughout history the incredible opportunity to dig into the typically
microscopic world of cells. And because of that they can help
us understand the inner workings of all cells, big and small.
In fact, you may have noticed that all of these giant cells have something in common:
they are all found in water! While it is possible for a large cell to exist
on land, they are usually living in humid areas that have recently received heavy rains.
Life in the water means less stress for cells of any size — no drying out,
nutrients and waste products easily diffuse in and out, and the fluid pressure gives
some extra support to their walls. So even in their placement on this planet,
these cells have taught us a lot! Plus, they’re just unbelievably cool.
Thanks for watching this episode of SciShow, which is produced by Complexy.
If you loved learning about these weird big cells, I think you’ll love our Complexly
sister channel: Journey to the Microcosmos. Journey to the Microcosmos is a relaxing journey
to the world where stentors really are fearsome giants, with incredible microscopic footage by
James Weiss, music by Andrew Huang, and the soothing voice of Hank Green.
Plus, they’ve done whole episodes on stentors. So you might consider checking those out next.
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