Thank you to Skillshare for supporting
this episode of Journey to the Microcosmos. If you are one of the first thousand
people to click the link in the description you can get a two-month free trial
of Skillshare’s Premium Membership. As we observe the world around us, what we see
is shaped by the way our eyes process the light that’s bouncing and absorbing and passing
through our surroundings. For many of us, what this means is that when we look at this clip
of cyanobacteria, we see green strips sliding along a blue background. For others, depending on
their eyes and any other corrective measures, the colors may be different, or the edges blurred, or
the image may not exist at all. The final product will come down to the physical and chemical
reactions that are taking place in our eyes in response to light, and how that informs
the biological processing that comes next. Microbes don’t have this sort of complex visual
processing, but many still need to know where the light around them is--whether that’s
to seek out food or avoid hidden dangers. They may not see things the way we do,
but the mechanisms they have in place allow them to respond to light in extraordinary
ways to meet the most ordinary of needs. Perhaps the most primitive of these methods are
light-sensitive pigments called phytochromes. But when we say “primitive,” we don’t
mean to diminish their importance or their complexity. If whole ecosystems are
built on photosynthesis, then phytochromes are essential to life as it currently exists.
Scientists first discovered phytochromes in plants, where they helped answer longstanding
questions about how plants respond to seasonal changes in light. But the ancestors of those
plant pigments are found in cyanobacteria like oscillatoria, whose phytochromes help
regulate growth, movement, and photosynthesis. These phytochromes respond not just to the
amount of light, but also the color of it. And remarkably, cyanobacteria phytochromes
can respond to a wider range of colors than plant phytochromes, which
makes sense if we think about where cyanobacteria live. They live in water.
Because of how water absorbs light, there’s usually more red light at the surface
and then more green and blue light the deeper you go. If a cyanobacteria were to have a pigment
that detects only red light or only blue light, it would end up restricted in where it could live.
So instead, these simple little organisms perform a complex calculation called chromatic
adaptation, calibrating the amount of their various pigments so they can maximize how much
light they’re getting from their environment. But cyanobacteria are unicellular organisms
that want light. What about those that don’t? Both blepharisma and Stentor coeruleus are
well-known for their photophobic nature. The reaction is straightforward: when confronted
with light, these microbes start backpedaling. This response is set off when light
interacts with their pigments, the pink blepharismin in the blepharisma,
and the blue stentorin in Stentor coeruleus. These pigments are used as a toxin against
predators. But they can make their own organism sensitive to light to a dangerous degree,
providing excellent motivation to avoid said light. And these photophobic responses
may also help the blepharisma and stentors avoid predators with better vision than their own. These photosensitive pigments are like nature’s
way of unlocking the foundations of vision. They are not themselves a specialized structure.
To see an example of a specialized structure, let’s look at that bright red eyespot on the
euglena. Also called a stigma, the eyespot is not actually an eye. It’s more like
a sunglass for the true photoreceptor. That structure is located close to the euglena’s
flagella, and it’s made up of around 50 layers of stacked membranes that hold hexagonal arrangements
of roughly a million photoreceptor proteins. The stigma shades the photoreceptor,
but just on one side of the euglena. And as the microbe rotates and different
wavelengths of light filter through and then get shaded by the stigma, the euglena is actually
getting information on where it can find light. As we shift from unicellular to multicellular
organisms, we start to see what microscopic eyes look like. One of the important evolutionary
developments was the formation of a cup shape in these eye-like structures,
which helps provide spatial information. Some of these animal eyes are mysterious,
like those beady little buttons atop many a tardigrade’s heads. Sure, scientists have
documented that many tardigrade species have what they call “inverse pigmented eye-cups,”
but what do they see? Well, we do not know. Others, like the planarian, have a pigmented
eye-cup connected to photoreceptor cells, which actually provides enough similarities to our
own eyes to make them useful for medical studies. And as the animals get larger, so does their
potential for complex visual systems. Similar to many insects, Daphnia have compound eyes that
are made up of units called ommatidia, which focus light onto photoreceptor cells to form images.
The larger the eye, the more information the daphnia and other animals with these kinds
of image-forming eyes can get. But size and complexity come with trade-offs. Sure, these eyes
help the animal navigate and find food. But these are costly organs to build and maintain. One
study found that the retina in a blowfly’s eye took up 10% of its resting metabolic rate.
Eyes and these analogous structures have evolved in response to what we need to process from our
surroundings. Needs inform evolution, which in turn, informs new needs. And light can provide for
those needs, but it can also pose its own dangers. In The Origin of Species, Charles Darwin
famously describes the eye as an organ of “extreme perfection.” He introduces it as an
example of biological complexity that may at first pose some consternation to those
considering natural selection. After all, how could something so remarkable be
made through iterations and iterations of tiny, gradual changes? But in Darwin’s search through
natural history, the case for stacking gradual change upon gradual change seems less improbable,
and science has continued to support his theory. With that said, it is tempting to get caught
up in this story that has been cultivated over eons and relate it entirely to the structures
that make up our own eyes. That was our entry point into this discussion after all. But one of
the most remarkable aspects of evolution is the understanding that it is not something that was
leading to us. Evolution has led to a whole world. All of these remarkable structures and chemicals
have their own unique properties that suit the organisms they reside in, allowing creatures
that, yes, may be simpler than ourselves to use systems that are not just good enough
for their survival, they are ideal for their survival. These organisms are thus able to survive
in ways that you and I could never be capable of. Thank you for coming on this journey with us as
we explore the unseen world that surrounds us. If you’d like to learn more about using
your eyes to observe the world around you in different ways, Skillshare has you covered.
Thanks to classes like “Frame a Great
Shot: Exploring Photo Composition” you can learn how to better take photos whether you’re
shooting with your cell phone camera or a DSLR. Photographer Porter Yates will walk you through
the process of finding the right environment, choosing your subject, and he’ll
also share some insights and tips about what makes a good, compelling photograph.
Skillshare is an online learning community that offers membership with meaning. With so much
to explore, real world projects to create, and the support of fellow-creatives, Skillshare
empowers you to accomplish real growth. And it makes it easy with short classes
that will fit into your daily routine. A Premium Membership will give you
unlimited access, so you can join classes and communities that are right for you.
And an annual subscription to Skillshare is less than $10 a month, and if you’re one of the first
1,000 people to click the link in the description, you can get a 2 month free trial
of Skillshare’s Premium Membership. And thank you of course also to all of the
people whose names are on the screen right now. These are the people who
make this channel possible. So, if you like it, you have them to thank.
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, there’s always a subscribe
button somewhere nearby.
