How coral reefs teem with life in nutrient-poor water

HostWhen you look at those bright blue waters in the tropics, they look like a dream. But for most fish and plants, that clear water is basically a desert. There's almost nothing in it to eat. Why do these busy, crowded reefs grow in places that should be empty?

GuestIt's one of the biggest mysteries in the sea. People who study the ocean call it a paradox because it feels like it should be impossible. Usually, if you want a lot of life, you need water that's murky and green, full of the tiny bits of food that plants need to grow. But the water around a reef is crystal clear because it has almost no food in it at all. It's a blue wasteland. Yet, the reef itself is like a big, loud city. It's packed with color and movement. The reason it works is that the reef doesn't wait for food to come to it. It has learned how to build its own world by recycling every single scrap of energy it can find.

HostHmm. If the water is clear, it means there's nothing there. So where does that first bit of energy come from to start the whole thing?

GuestIt starts with a very tight deal between two very different living things. You have the coral itself, which is a tiny animal that looks like a little tube with a mouth and arms. But inside the skin of that animal, there are millions of tiny plants. These plants use the sun to make sugar. But they don't keep that sugar for themselves. They give almost all of it—maybe ninety percent of what they make—straight to the coral animal. In return, the coral gives the plants a safe place to hide and feeds them its own waste. It's a perfect trade. The animal gets energy from the sun through the plants, and the plants get a home and food from the animal.

HostThat sounds a bit too easy. If the coral is an animal, why does it not just eat the plants? It has them right there inside its body.

GuestWell, if it ate the plants, it would be full for a day but then it would starve the next week. The coral has learned over a long time to be a farmer instead of a hunter. It protects those plants like they're the most valuable thing it owns. The coral even builds these hard stone skeletons that act like little greenhouses. They're shaped to catch as much sunlight as possible. The animal actually moves its own body around to make sure the plants get enough light. It's a very careful balance. If the water gets too hot or the coral gets stressed, it kicks the plants out, and that's when the coral turns white and dies. It simply can't live without that constant flow of sugar.

HostSo they have this internal farm. But a reef isn't just coral. You have sharks, turtles, and thousands of fish. They can't all be living off a little bit of plant sugar.

GuestThey aren't. And this is where the second secret comes in. For a long time, people saw that corals leak out this thick, slimy snot. It just floats away into the water. It seemed like a huge waste of energy. But it turns out that sponges, which are those lumpy things that just sit on the rocks, are the secret heroes of the reef. They suck in that slime and they eat it. Sponges are very fast at growing new cells. They grow so fast that they have to shed their old cells constantly.

HostOkay, but the sponges are just eating the coral slime. That still feels like it's just moving the same energy around in a circle. You can't just keep recycling the same stuff forever. You have to get something new into the system.

GuestYou're right. A circle on its own would eventually run out of steam. The reef is also a giant trap. Think of the reef as a massive, lumpy wall in the middle of the ocean. When the big ocean currents hit that wall, they get pushed around and slowed down. This forces the water to drop any tiny bits of food or salts it was carrying. The reef also reaches out and grabs things. At night, the corals come out and use their tiny arms to snag tiny swimming bugs from the open sea. They bring that new energy into the system, and then the recycling loop takes over to make sure it never leaves.

HostSo it's like a city that's so good at using its trash that it barely needs to bring in new supplies.

GuestYou can see it that way. The sponges take that liquid slime and turn it into solid food—those shed cells—which the crabs and worms then eat. Then the fish eat the crabs. It turns a waste product into the base of a whole food chain. Even the poop from the fish gets caught by other things living in the holes of the reef. Nothing is allowed to drift away into the empty blue water. The reef is an island of life because it's incredibly stingy. It holds onto every single atom.

HostIt's strange that the very thing that makes the water look so beautiful and clear is the reason the reef has to be so tough and smart about how it grows.

GuestThe clear water is actually a sign of how well the reef is working because if there were any loose food floating around, someone on the reef would've snapped it up already.

HostThe desert isn't just outside the reef—the reef is what keeps the water empty by eating every last drop of life it can find.