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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?
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.
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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/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,
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