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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?

Well, there’s a short answer and a long answer to that question.

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 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. 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 aren’t plants. They’re animals, members of the phylum Cnidaria1. 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 carbonate2.

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 may 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 old3. However corals haven’t had algae living inside of them the whole time. 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 ago4. 

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 success5. 

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 cavity6.

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 instead7.

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 species8. And 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,”9. 

And with our changing climate, these stresses are starkly rendered in our oceans, as corals turn white and face new challenges without their dinoflagellate inhabitants10. 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.

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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/JourneyToMicrowhere 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.