This episode is sponsored by Fabulous,   the #1 self care app that helps you start  building better habits.

The first 100 people   who click on the link in the description  will get 25% off a Fabulous subscription. These dinoflagellates didn’t come to us like this,   spread out and diffuse across a slide.

When  they first came to us, they were living a   very comfortable life, tucked into the body  of a coral. So how did they end up here? There’s a short answer and  a long answer to that question.  So let’s start with the short answer.

James, our master of microscopes, recently  received a package from a coral farm in Germany.   We’ve explored some of the microscopic creatures  and bristle worms that were living and thriving   in those packages in previous videos. But today  we’re here to focus on the main event: the corals. Unfortunately, it turns out that imaging corals  under the microscope is a little bit difficult.   They don’t fit under the lens.

So  instead of trying to squeeze them in,   James tried cutting thin slices out  of them that would be thin enough   to put under the microscope. But as he cut,  the corals began to bleed dinoflagellates. So that’s the short answer.

The long  answer would take about 240 million years,   and we don’t have that kind of time. So let’s go  with something a little bit in between the two.  At particular spots in the ocean, where the  temperatures are just right, you can find coral   reefs spread out like a bony but colorful garden.  The twists and turns of the corals within these   reefs create spaces for other animals to swim  in and around, forming a stunning scenery that   complements the incredible array of oceanic life. Because of the forms they take on, corals look   like plants.

Even the ones from the coral  farm look like a weird, cute neon plant you   might buy from a home decor store designed by Lisa  Frank. But corals are not plants. They’re animals,   members of the phylum Cnidaria.

Which  means that they are relatives of such   former Journey to the Microcosmos stars  like hydra and the starlet sea anemone. And like their Cnidarian relatives, corals  have simple bodies. Through the middle of the   animal is a gastrovascular cavity that opens to a  mouth, lined with a ring of tentacles that sweep   through the water in search of food.  To help them in their search for food   are special cells on their tentacles called  cnidocytes, which are loaded with toxins.  The types of corals that we’re showing you today  are different from the types that assemble the   vast majority of reefs.

Those corals are  often called hard corals or stony corals,   thanks to the hard exteriors the corals make  for themselves out of calcium carbonate. But the corals that James looked at  under the microscope represent the   softer side of their family.  Like this Xenia, with its bluish   arms moving with the water around it.  These soft corals still live in reefs,   but they don’t form the calcium carbonate  structures that their stony counterparts do. Many species of corals—stony and soft  alike—share another trait, the dinoflagellates,   though in this symbiotic context they’re  known by another name: zooxanthellae.  You can see their brown bodies  inside the soft coral Xenia.

In some corals, the presence of these  zooxanthellae might not be immediately   obvious. Like this zoanthus, which is under white  light. Those brown dots that were so immediately   obvious when we zoomed in on the Xenia aren’t  so obvious now.

So where are the zooxanthellae? To find them, we can take advantage  of the photosynthetic pigment inside   of zooxanthellae: chlorophyll, which  glows red when it’s excited with red   light. When we illuminate the coral with  this red light, we see the algae then   as a bright red version of themselves  tucked into different parts of the coral.

The oldest coral fossils that have been found  are around 400 million years old. However corals   haven’t had algae living inside of them the whole  time. Now it’s hard to pin down the exact date this   relationship began because dinoflagellate fossils  are hard to come by.

But in 2016, a team of   scientists studying coral fossils for the chemical  and physical evidence of zooxanthellae came up   with a rough estimate for when the two may  have found each other: 210 million years ago. That would be around the late Triassic  period, a time when the first dinosaurs   were emerging. The waters that the corals  were living in weren’t particularly rich   in nutrients.

And yet corals were able  to expand and diversify in those waters.   And they probably have the tiny algae living  inside of them to thank for that success. The corals either inherit  their algae or attract them,   releasing signals and communicating chemically  until they can essentially eat the algae.   Except instead of eating, the corals  tuck them away, housing them in special   compartments called symbiosomes within  the lining of their gastrovascular cavity. And from there, the exchange is simple but  powerful.

The zooxanthellae live their algae   lives within the coral, absorbing light  and photosynthesizing it into nutrients.   But those nutrients aren’t  really for them. About 90%   of what the zooxanthellae makes  gets sent to the coral instead. That’s a lot for the zooxanthellae to give up,  but the corals put that nutrition to good use.   Scientists studying stony corals found  that when the zooxanthellae were removed   from the corals, or when it was darker and the  zooxanthellae couldn’t do their photosynthesis,   the corals were much slower at making calcium  carbonate.

And while there are corals that don’t   form symbiotic relationships with zooxanthellae,  they’re also slower at making the calcium   carbonate structures compared to symbiotic corals. In exchange, the zooxanthellae get the protection   of their coral host. And they also  get the waste that the coral produces,   things like ammonia that are useless to the  coral but nutrition to the zooxanthellae.

This exchange benefits far more than just the  coral and zooxanthellae. It’s also allowed many   other animals to thrive as well. It’s estimated  that coral reefs cover less than 1% of the ocean   floor, and yet they are home to around one third  of all marine species.

Also, the overall effect   is beautiful, with zooxanthellae providing  much of the color we associate with corals. But it’s also a delicate relationship. When waters  become too warm or pollutants contaminate the   water, or any other myriad of things happen that  become too stressful for the corals, they expel   their algae, losing their brilliant color in the  process in what’s known as “coral bleaching,”.

And with our changing climate, these stresses  are starkly rendered in our oceans, as corals   turn white and face new challenges without their  dinoflagellate inhabitants. The relationship   they formed with zooxanthellae may have been  one of the defining features of their past,   but the loss of those relationships may be  one of the defining features of their present. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.

And thank you to Fabulous  for sponsoring this episode. Fabulous is an app that was created  by behavioral science researchers   to gently support your personal goals. Habit changing and habit building is hard.  So, if you’re looking for ways to add   a new thing to your routine,  Fabulous is the way to go.   Their Journey feature makes it easy to set  goals and track your progress using the roadmap.

With over 30 million users, Fabulous can help  you whether you’re looking for ways to stay   focused at work, need a reminder to take a break  once in a while to stretch and drink some water,   or if you’re looking to develop  a whole new post-work routine.  The app is 100% personalized to your goals and  with their daily coaching and goal setting,   you can start building your ideal daily  routine today! The first 100 people   who click on the link in the description  will get 25% off a Fabulous subscription. I like to think that we have formed a symbiotic relationship with the people on the screen right now.

They give us a little support. We give them and also everyone else who has access to a high speed internet connection just really chill, good videos about microorganisms. This is absolutely a mutually beneficial relationship, And if you would like to enter into a relationship like that, you can go to Patreon.com/JourneyToMicro where you can find out more about becoming a patron and all the cool stuff you can get.

If you want to see more from our master of microscopes, James Weiss, you can check out Jam & Germs on Instagram, and if you want to see more from us, there is always a subscribe button somewhere nearby.