This episode is sponsored by Squarespace.

Go to Squarespace.com/microcosmos to save 10%  off your first purchase of a website or domain. When James, our master of microscopes,   was looking through samples he’d received  from Spain, he didn’t expect to see this— a creature straight out of a horror movie,   with dark reddish brown eyes and  tentacles streaming out of its mouth.

We believe this is a tube worm, and the  fact that it seems to just be sitting   there makes it maybe even more unsettling, like at any moment, it could change its mind   and use one of those many appendages to  reach out from the screen for your face. It would be weird to look at this tube worm  and expect to find common ground with it. But even anxiety-inducing creatures from the  microcosmos have something in common with us.

We are, both of us, entirely beholden, every moment of our lives to a tiny little chemical called oxygen. We’re going to return to tube worms in a bit. For now, let’s talk about oxygen.

Not every organism is as  dependent as we are on oxygen. Anaerobic organisms like diplomonads  make energy through different chemical   reactions than what you will find  in aerobic organisms like us. In fact, many anaerobic organisms have trouble  surviving in oxygen-filled environments.

Billions of years ago, anaerobic  organisms like these were the norm,   filling in a version of our  earth we would hardly recognize,   and would definitely have not of been able to  survive in, because there was hardly any oxygen. That would change some time before 2.5  billion years ago when cyanobacteria   began to take over the Earth,  bringing in a new innovation:   a form of photosynthesis that  produced oxygen as a byproduct. We’ve talked about this  Great Oxidation Event before,   and how it was simultaneously  catastrophic and fortunate.

Among the many changes this influx  of oxygen brought to our planet,   the emergence of chemical pathways  involving oxygen to produce energy   would set the stage for the vast array  of aerobic organisms we see today. And what’s fascinating is that as we look through  the different organisms in the microcosmos,  we can see the same underlying principles   leading to very different strategies  to get the oxygen each organism needs. For single-celled eukaryotes, or  protists, the task is seemingly simple: they just absorb oxygen from their surroundings, letting it diffuse across their membranes, into their bodies.

They don’t even need any mechanisms or special  organelles to get access to that oxygen. This reliance on diffusion means that protists  depend on the amount of surface area they have   exposed to the water around them to be  able to get as much oxygen as they can. But otherwise, this seems like a  pretty straightforward way to breathe.

It does get a little messier than that though. Different  species have different oxygen preferences. Loxodes, for example, like microaerobic  areas that have some—but not much—oxygen.

These preferences are likely shaped by a number  of factors, like the habits of preferred prey,   the ability to rely on other chemical pathways  for energy, and the fact that oxygen itself is— despite all that it provides—quite damaging. In general, oxygen levels are lower  as you go deeper into a body of water. So for loxodes to find the  depths it can thrive in,   it relies on organelles called Muller vesicles  that help it essentially know where “down” is.

And that shows us one of the things  that’s useful for all organisms: we all need to find the right level of oxygen. Even some of the most famously resilient members  of the microcosmos are vulnerable without oxygen. As we look at meiofauna, which are tiny  little invertebrates like rotifers,   we start to see body parts that feel familiar,  like stomachs and reproductive organs.

But when it comes to oxygen, many of  these organisms are like protists. They rely on diffusion to deliver oxygen  from the water around them and through them. It might be a little more difficult if their  body is covered in a cuticle, but it works.

But while we’re staring at a tardigrade, we should point something out. They are not particularly good swimmers. All those legs are really better  coordinated for their waddling gaits.

This puts them at a disadvantage compared to  some of the other members of the meiofauna,   who can swim up and down  and around to find oxygen. But like, what’s the worst that can happen  to tardigrades without a little oxygen? Can’t they just curl up in  their resting tun state,   which seems to protect them from so  many other environmental threats?

Well, surprisingly…no. This is what happens to a  tardigrade without oxygen… it gets very, very still and very, very swollen. This is very different from  forming a compact little tun.

But tuns are just one of several ways that  tardigrades enter what’s called cryptobiosis,   where their metabolism comes to a pause. Tuns are the cryptobiotic response  tardigrades turn to in the face of a particular environmental challenge, like dryness. But we don’t know as much about how  this response to a lack of oxygen works,   though one explanation is that mechanism controlling the amount of water in the   tardigrade’s body begins to fail when there  is a lack of oxygen, leading to this swelling.

The tardigrade can emerge from  this state when oxygen returns. In fact, James sometimes finds that  if he blows gently on the slide,   the tardigrades return to life. In one recent study, scientists found  that tardigrades were able to come   back to life after even a severe lack of oxygen.

But the longer the tardigrades went without,  the less likely they were to survive. And that brings us back to tube worms,   which are bigger than the meiofauna we’ve been  watching and yet face their own challenges. In particular, tube worms are, as  the name suggests, encased in a tube.

We’re guessing here as non-tube worm  experts, but the white spiral tubes   these worms live in look a bit like the  tubes that Spirorbis tube worms live in. But if anyone can help us identify these worms, please let us know.  Spirorbis tube worms are found around  north west Europe, and at their head   is a crown of tentacles, sometimes  better known as the tentacular crown. This crown helps the worm get food,  filtering water in search of nourishment.

But it also doubles as a tool for respiration,   with thin epithelia that are able  to take in oxygen through diffusion like the other organisms we’ve seen. On the one hand, it makes sense to get as much use out of a single part of the body as you can. On the other hand, that’s a vital  apparatus just hanging around outside,   where it is vulnerable to predators.

So to protect themselves, the worm  can quickly retract its tentacles   back into the safe confines of its tube,  leaving it able to breathe another day. Looking at the tube worm’s tentacles,  James was reminded of something else: of us. Our bodies have their own branching  organ to help us take in oxygen: the lungs, which are their own  complex take on many of the same   ideas of diffusion and surface area that  we’ve seen play out in these organisms.

And when we look across these organisms,  all these adaptations come down to one thing oxygen— a tiny little molecule of gas that was once barely a part of this world. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us. And thank you to Squarespace  for sponsoring this episode.

Squarespace offers a robust online platform that   empowers small businesses to  create their own websites. Whether you're just starting out  or looking to expand your existing   venture, Squarespace provides all the  essential features you need to succeed. Maybe you are a talented  yoga instructor and want to establish a  captivating online presence for your studio.

Squarespace provides you with all the tools to  design a beautiful and professional website that   reflects your studio's tranquil ambiance and  captures the essence of your teaching style. Utilizing Squarespace's third-party extensions,  you can effortlessly manage class schedules,   streamline bookings, and offer online yoga  sessions to students around the globe. And if you're interested in sharing your yoga  knowledge and inspiration with a broader audience,   Squarespace makes it easy to integrate  a blog seamlessly into your website.

You can enlighten your readers with  insightful articles, yoga techniques,   and mindfulness practices, fostering a  deeper connection with your community. And you can even engage with your students through threaded comments and replies as  you encourage discussions, offer guidance,   and celebrate their progress as  they embark on their yoga journey. You can sign up for free today at Squarespace.com.

And when you're ready to launch, you can visit squarespace.com/microcosmos to   enjoy a 10% discount on your first  purchase of a website or domain. The people on the screen right now,  they are some of our Patreon Patrons. They make it so that we can continue to explore  all of these bizarre little bits of our universe.

Isn't it nice, that the cyanobacteria  created oxygen for all of us to breathe,   and isn't it nice that our Patreon  patrons helped us create this show? If you would like to join them, you  can go to Patreon.com/JourneytoMicro. If you want to see more from our  Master of Microscopes, James Wiess,   you can check out Jam and Germs on Instagram.

And if you want to see more from us, there's  probably a subscribe button somewhere nearby.