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When you think about coral reefs, you probably imagine a sparkling tropical oasis that you can easily see while snorkeling or diving, but reefs can be found as deep as 8000 meters! As deep as they are, those reefs are still not immune to the effects of human activities, nor are we outside their sphere of influence.

Thumbnail credit: NOAA's National Ocean Service
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When you think about coral reefs, you probably imagine a sparkling tropical oasis that you can easily see while snorkeling or diving. But that’s just part of the picture when it comes to corals.

Reefs can be found as deep as 8000 meters. In many ways, the corals of the deep are similar to shallower corals, but they also form their own weird, wonderful ecosystems. And while they might seem really far away, they’re not immune to what happens above, nor are we outside their sphere of influence.

We’re just beginning to understand how important deep reefs are to the ocean and beyond. But already, we know that we rely on the health and wellbeing of all coral ecosystems, whether we can see them or not. And unfortunately, all corals are struggling as a result of human activities — even those tucked hundreds of meters below the surface.

No matter where you are in the world, there’s probably a deep reef nearby even if you’re in Antarctica! And the animals that build them have a lot in common with their shallow water cousins. Like, both shallow and deep corals are foundational species which create ecosystems and provide vital habitat for a huge variety of creatures.

Also, in both cases, what we’d call a single “coral” is actually a colony of identical anemone-like animals called polyps! . The biggest difference between deep sea and shallow water corals has to do with algae. Shallow water corals all have symbiotic relationships with single celled algae known as zooxanthellae.

These algae live inside the coral’s tissues. And in exchange for a safe place to live and access to plentiful sunlight, they provide some food to the coral through photosynthesis, as well as manage any waste produced by their coral hosts. In fact, they’re largely responsible for the corals’ vibrant colors.

If they leave or are ejected from the coral’s mostly clear tissues, we see the hard, white calcium carbonate skeleton below. This is known as coral bleaching. And it occurs when a coral gets stressed; like when the waters they live in become too warm.

Shallow corals are so dependent on these algae that if one goes too long without them, it will die. But sunlight quickly disappears the deeper you go. So things are a bit different with deep corals.

The shallower deep corals, the ones living between 50 and 150 meters down, do see a bit of sunlight. And they do have zooxanthellae in their tissues. Trouble is, the amount of sunlight that reaches these algae isn’t ideal for photosynthesis, which makes it hard for them to hold up their sugar-making end of the bargain.

The corals have evolved a nifty workaround though. They have fluorescent proteins in their tissues that convert the available light into more algae-friendly wavelengths!. Still, once you descend past about 200 meters, there’s basically zero sunlight.

At those depths, not even fluorescent proteins can drive enough photosynthesis. But most deep corals are found below this point! So, the majority of species have ditched their algal buddies altogether.

Since they don’t have any colorful symbionts, they can’t bleach like their shallower cousins. But, that’s not to say they’re all a boring white! Deep sea corals actually come in a stunning array of brilliant colors.

For example, the so-called black corals aren’t black in life; they’re pink, yellow, orange and red. They get their namesake from the color of their skeletons, which are different from the skeletons of most shallow water corals. Instead of using the white mineral calcium carbonate, they build their skeletons with protein and a fibrous substance called chitin which, when dried out, is black.

These chitin and protein skeletons are more flexible than carbonate ones, yet still tough enough to support the animals. So they allow black corals to live where they prefer: in strong water currents that provide them with more food. Other deep corals called octocorals, named for the 8 tentacles on each polyp, are also intensely colored.

Like, there’s one species that is a stunning shade of purple. These, too, are a little different than your average shallow coral. Most are considered soft corals, which means they don’t have a hard internal skeleton.

Instead, the polyps are supported by little needle-like pieces of calcium carbonate known as sclerites. No one is fully certain why deep corals are so colorful, but researchers have a theory. While pigments aren’t visually helpful in the deep sea, they may be acting as antioxidants and antibacterial compounds.

You see, the pigments found in deep corals are carotenoids the same class of compounds often responsible for the brilliant colors in the veggies and fruits we eat! In fact, much like us, corals have to get carotenoids from their diet. So it’s likely part of the reason deep corals vary so much in color is that the local snacks contain different pigments!.

Now, you might be imagining all of these colorful deep corals growing on top of one another, like shallow corals do. But in the deep, that kind of reef building is the exception. In fact, out of the over 3300 deep sea coral species described so far, there are only six that build that kind of reef.

Instead, most are found standing separate from each other, like trees in a forest. They can pepper the seafloor or cling to the sides of underwater structures like seamounts. We still call the habitats they create reefs, though.

And just like shallow reefs, they provide essential habitat for ecologically and commercially important species. And some are massive. Like, in 2018, researchers were exploring the Atlantic Ocean off the coast of.

South Carolina and discovered a deep reef that stretched for 85 miles! Others are much smaller. But even solitary deep corals create oases of life in the desert of the deep seafloor.

They’re found covered in all sorts of critters like brittle stars, squat lobsters, and anemones. Some sharks lay their eggs on them! Plus, grouper, snapper, sea bass, and rockfish all use deep sea corals as spawning grounds.

Even marine mammals like the endangered Hawaiian monk seal get in on the deep reef action, since these habitats are full of their preferred prey. Deeper reefs don’t just pop up,though. It can take hundreds of years to establish these vital ecosystems, because deep corals live life in the ultra slow lane.

The oldest coral that we currently know of is estimated to be over 4,000 years old. It was just a little polyp when the Sumerians were inventing writing! And that 85 mile long coral reef I mentioned?

On reefs like that one, the corals at the top are still young a mere 700 years old! But they built on the skeletons of dead corals. And that structure is likely to be at least 20,000 years old.

One of the main reasons these corals grow slowly and live a long time is that they inhabit much colder waters. Below 200 meters, the average water temperature is around 4 degrees Celsius similar to the inside of your refrigerator. And cold slows all sorts of biological processes, including growth, because it literally slows down molecules.

And there’s also the matter of food — or, I should say, the lack thereof. Because deeper corals don’t have algae to help with food, they must subsist on whatever floats by or rains down from above. And that’s not always a lot.

Many position themselves in strong water currents to get more food more consistently. Still, they tend to get less than shallower corals, which means less energy for growth. And, unfortunately, food isn’t the only thing floating down from above.

Researchers have found that microplastics are making their way into the deep sea food chain, which seem to be slowing the corals’ growth even more. I wish I could say this is the only way our species is hurting deep reefs, but it’s not. Because of their beautiful skeletal colors, we’ve harvested deep sea species to wear as jewelry for hundreds of years.

That’s led to lower diversity and fewer coral babies in the species we covet most. Also, there are several fishing practices which cause tons of damage to these fragile communities. For instance, during bottom trawling, a massive weighted net is dragged along the seafloor, demolishing entire ecosystems.

And since the corals take decades to recover because of their slow growth, the diversity of life they support is also slow to rebound including the productive fishing grounds the trawling was targeting in the first place. Deep corals are largely spared from one of the major threats their shallow cousins face: warming-induced coral bleaching. Like I mentioned before, since they generally don’t rely on algae, most deep sea corals cannot bleach.

However, the majority are still susceptible to another climate-related threat: ocean acidification. When we put more carbon dioxide into the air, more also ends up in the water. There, it reacts with seawater to form carbonic acid which makes the ocean more acidic.

And researchers have recently found that as our ocean becomes more acidic, the skeletons of corals are becoming porous and fragile. This is similar to the weakening observed in human bones undergoing osteoporosis. So, they have coined this process coralporosis, and it’s happening in both shallow and deep corals.

And it’s not just that coral skeletons are becoming weaker acidification also means there’s less material in the water for them to build their skeletons out of in the first place. Many corals rely on a dissolved form of calcium carbonate called aragonite. But argonite reacts with carbonic acid to form a chemical called bicarbonate.

So more carbonic acid means less aragonite. Deep sea corals are also often impacted by deep water oil drilling, because they frequently live near places where oil and gas naturally seep out of the seafloor. That’s because corals can’t attach to soft substrates like sand and mud.

And while most of the seafloor is soft, microbes living near seeps break down oil and gas into carbonate. And that carbonate settles down, creating nice, hard mineral deposits. Unfortunately, that also means these corals are in danger when something goes wrong during the drilling process.

For example, the 2010 Deepwater Horizon accident spilled 210 million gallons of oil into the Gulf of Mexico. Up to 10 percent of this oil settled in the deep, coating the corals living near the wellhead. The sticky, oily residue suffocated polyps as well as the animals that depend on them.

Scientists estimate that 90% of the corals in the area were impacted, and it could take up to 30 years for them to recover. What’s interesting, though, is that we know some deep sea corals live right next to natural seeps. And they don’t seem to be bothered in the slightest by the oil bubbling up into the waters around them.

This oil is nowhere near the volume of spill, but it’s still more than you’d expect a coral to tolerate. So researchers think they may have a unique set of microbes to help them break down the oil into less toxic components. And that means learning more about these corals could one day help us develop new ways of cleaning up oil spills in the ocean!

In fact, microbiologists are keen to study all of the unique microbes living with deep sea corals, as new microbes often lead to discoveries like novel antibiotics and antivirals. And more generally, further research on deep corals will better equip us to help them stick around — which means we keep all the species that love them and get to learn more about how organisms adapt to extreme environments like the deep sea. All sorts of technological advancements in recent decades have made it easier for researchers to visit deep corals and study them.

And that’s made us realize just how much they do for us and how much they could continue to teach us going forward. If you enjoyed this dive into deep reefs, I bet you’d enjoy our other episodes on the animals that live in the depths of the sea. We even put a bunch of them together into a compilation, so maybe watch that next!

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