Moments ago, you made a decision to click
on this video. Maybe YouTube has been gently poking you with
suggestions to watch this. Or maybe it’s part of your routine. Whatever the reason, we hope the decision
was an easy one to make. But even if the decision was easy, the steps
it took to execute that choice, the information you needed to gather, were not simple. From reading the title to physically clicking
the link to settling back into a comfortable position, your body had to coordinate a ton
of processes using chemical and electrical signals to quickly send instructions all over
the place. These signals are rooted in your nervous system,
a reflection of your multicellular complexity. Life in the microcosmos comes with its own
choices though. And while they may not have to coordinate
multiple limbs, single-celled eukaryotes, or protists, still have decisions to make. They've got places to be, things to do--and
they don't have a brain to help them. All they've got is the one cell to deal with
all of life's complications. Microbiologists have been fascinated by the
way protists respond to their environment for more than a century. And so naturally, that has led some scientists
to try and provoke a response, like James--our master of microscopes--using a piece of hair
to poke these stentors. In 1906, a zoologist named Herbert Spencer
Jennings published Behavior of the Lower Organisms, a 366 page book containing sections with titles
like "The Daily Life of Paramecium" and "Reactions of Infusoria to the Electrical Current". Nestled among descriptions of the many organisms
and behaviors he observed was a section documenting the way stentors responded when he injected
carmine into the water around them. Carmine is a red dye and apparently it’s
something that Stentors don’t like very much. Like our stentors, the subjects of Jennings'
experiment did not particularly revel in this irritant. In our case, you can see the stentors trying
to act like everything is normal. But with repeated poking, they start to contract,
evading the troublesome hair but also missing out on opportunities to gather passing food. They stay contracted for a short period, but
eventually they emerge and extend back to their original length. In Jennings' case, the stentors also attempted
to ride out this irritant. But when that failed, those organisms then
began to contort and twist to the side. And in that way, the stentors could attempt
to evade the carmine while still focusing on what's truly important: food. This avoidance reaction is a common one among
protists. In the same text, Jennings describes the tendency
of some organisms to avoid light, similar to this trachelophyllum here. Those changes in brightness that you see are
coming from the microscope as the diaphragm on the condenser opens up, allowing more light
to enter the slide. As the slide quickly gets brighter, you can
see the trachelophyllum trying to swim away. This organism isn't that well-studied, so
we don't know exactly why it's trying to avoid the light. Some microbes definitely seek out dark areas
to avoid predators, so that's a possible explanation. The light itself may also be harmful to the
organism. Whatever the reason, this trachelophyllum
isn't alone among protists. One of Stentor coeruleus' many compelling
traits is its photosensitivity. Inside its unicellular, trumpet-shaped body
are photosensitive pigment granules called stentorin that, in the presence of light,
drive electrical changes in the plasma membrane and reverse the movements of its cilia, transporting
the organism away. Of course, there are also protists that seek
out light, like the photosynthetic Euglena that rely on sunlight to make their food. Even then, there are subtleties in their response,
which Jennings observed and described. While the Euglena in his experiments sought
out light as expected, if the light was too strong or brought on too suddenly, the euglena
sought shadier refuge. Another protist that captured Jennings' attention
was the paramecium, that seemingly simple organism that has captivated many scientists
because of its hidden complexities. Here, you can see the paramecium on this slide
in their own series of avoidance reactions. And the main thing they seem to be trying
to avoid is each other. That might sound fairly familiar to some of
us these days. As they navigate the crowd, you can see the
paramecium actually turn around, which is coordinated by the movement of their cilia. By changing the direction that the cilia move
in, the organism is able to reverse their direction. They can also spin, turn, or just come to
a stop. If they do a number of these reactions in
quick succession, the paramecium can even swim backwards. These movements are a joy to watch, and it
is astounding to consider how a unicellular organism is able to change its behavior based
on its surroundings. But Jennings' findings extended beyond just
the notion that an organism will react to its surroundings. As he continued to prod his stentors with
carmine powder, he noticed that the organism began to explore other strategies to deal
with the irritant. When avoiding the carmine failed to stop the
provocation, the organism would temporarily reverse the movement of its cilia, driving
the water current away from the stentor so it could essentially "spit" water out of its
mouth. And when that didn't work, the stentor tried
to contract away from the stimulus. And when that didn't work, well, then the
stentor just up and left, detaching themselves from their substrate and swimming away in
search of a quieter, less carmine-filled home. There's a pretty key implication to Jennings'
observations here: the stentors he observed didn't just react to stimulus, they worked
through a hierarchy of possible responses. Think of it the way a cat deals with the common
irritant of a loving human who wants to cuddle: they start with simple evasion tactics, but
they might then escalate to swiping and hissing before just picking up and running to a corner
where they can't be bothered. In the case of Jennings' stentors, which fortunately
did not have sharp claws, there seemed to be a ranked preference to the measures they
were going to take. There was just one problem with Jennings'
results: no one could replicate them. And so for more than a century, his observations
were set aside until a scientist named Jeremy Gunawardena decided to dig deeper and found
that these follow-up studies used a different Stentor species than Jennings had used. When Gunawardena and his team gathered the
original species, Stentor roeselii, they found that their subjects exhibited those same behaviors. Interestingly though, different specimens
of the same species seemed to have their own ranked preferences. Some might only contract, while others would
perform a mix of bending and cilia-reversal. We think the stentors we've been poking here
are Stentor roeselii. And if they are, they seem to have preferred
the contraction approach themselves, until they gave in and swam away. For the past few minutes, we've been following
one of them as it tries to find a new home, brushing up against possible substrates with
its oral cilia. We think this is part of their inspection
process, and if they don't like what they feel, they swim backwards and away. This little guy has tried a few locations,
but here, it seems to have found something intriguing. Even as other stentors come in and interfere,
this one persists, returning to inspect its newfound spot again and again like it’s
about to make an expensive purchase. And when it's satisfied with its decision,
the stentor releases an adhesive substance and moves in, ready to eat until the next
decision needs to be made. And if that isn’t life, I don’t know what
is. It is remarkable to think that just choosing
the wrong species of Stentor can give such different results, that decision-making--even
for the simplest of organisms--is such a complicated endeavor for us to untangle. We're still only at the early stages of understanding
what these actions and reactions require from different organisms, and for some, it is likely
to be much simpler than for others. What's most remarkable, as always seems the
case with the microcosmos, is just how many ways there are to do anything at all. Thank you for coming on this journey with
us as we explore the unseen world that surrounds us. If you liked it, the people on screen now
are some of the people you should be thanking because they are our patrons on Patreon. And they allow us to do this wonderful, weird
thing. If you want to be one of those people, you
can go to patreon.com/journeytomicro. We would really appreciate it. And thank you to everybody who supports us. We really really appreciate it. If you want to see more from our Master of
Microscopes James Weiss, check out Jam & Germs on Instagram. And if you want to see more from us, we are
youtube.com/microcosmos
