How mangroves filter salt from seawater to survive

HostI was at the coast recently and saw these thick woods growing right out of the salt water. It feels like they shouldn't be there, because we're always told that salt kills plants. How do they manage to stay alive when they're basically standing in poison all day?

GuestIt's a bit of a magic trick. Most plants are like us. If they drink salt water, the salt actually sucks the moisture right out of their cells. It’s like a sponge that pulls water away from where it’s needed. But mangroves have found three or four different ways to beat the salt. They basically turn themselves into a living factory that takes the salt out of the sea. The first and biggest way they do it's right at the bottom, in the roots. Most of these trees have roots that work like a very, very tight filter.

HostSo is it just like a screen that keeps the salt chunks out?

GuestWell, salt isn't really in chunks. It's melted into the water. So a normal screen wouldn't do anything. The tree has to use a lot of power. Think about trying to blow air through a straw that's almost entirely blocked. You have to blow really hard. The mangrove does the same thing in reverse. It creates a massive amount of suction inside its roots. That suction is so strong that it pulls the water molecules through the walls of the root, but the salt molecules are just a tiny bit too big or have the wrong charge to get through. They get stuck on the outside. About ninety-nine percent of the salt never even makes it into the tree.

HostWait, if it’s that easy to filter water, why don't we just build pipes that do the same thing? It sounds like they’ve solved the problem of not having enough fresh water.

GuestWe do try to copy them, but it’s incredibly hard work. For the tree, it’s a constant drain on its energy. It has to spend a huge part of the sugar it makes from sunlight just to keep that suction going. If the tree stops working for even a little bit, the salt would rush in and kill it. And even with that amazing filter, some salt always sneaks through. No filter is perfect. So, the tree needs a backup plan for the salt that makes it into the trunk and the branches.

HostSo what happens to that one percent? Does it just build up in the wood until the tree gets pickled?

GuestThat’s a good way to put it. If it just stayed there, the tree would eventually stop growing. So, some mangroves have these special little pumps in their leaves. If you look at the back of a leaf on certain types of mangroves, you can actually see tiny white crystals. That's pure salt that the tree has pushed out through its skin. It's almost like the tree is sweating. When the wind blows or the rain falls, that salt gets washed back into the ocean. It’s a way of cleaning itself out from the inside.

HostThat sounds like a lot of maintenance. You’ve got this high-pressure filter in the mud and then these pumps in the leaves. Is there a simpler way some of them handle it, or is it always that complex?

GuestThere's a third way that seems almost like a sacrifice. Some species take all the salt that’s trapped in their system and they shove it into just a few old leaves. They pick the leaves that are about to die anyway. They pump all the salt into those leaves until they turn bright yellow. Then, the tree just lets them go. The leaves fall off, float away in the tide, and take all that salt with them. It’s like taking out the trash.

HostI don't know if I buy the sacrifice thing. It feels like a stretch to say the tree is "choosing" which leaf to kill. Is it possible the salt just ruins those leaves first because they're already weak?

GuestWell, it’s more about how the tree moves its sap. It directs the flow so that the salt ends up in those specific spots. It's a very organized way of dealing with waste. But you're right that it’s not a choice in the way we think about it. It’s just how they’ve evolved to survive. And it isn't just about the salt, either. These trees are also growing in mud that has almost no air in it. That’s why you see those weird roots that look like snorkels or stilts. They have to breathe through their roots because the mud would drown them.

HostSo they’re fighting on two fronts. They’re trying not to drown in the mud and they’re trying not to get dried out by the salt. It sounds like an exhausting place to live.

GuestIt really is. They live on the edge of what’s possible for a plant. But because they figured out these tricks, they get the whole coastline to themselves. No other trees can handle the salt, so the mangroves don't have to compete for space or sunlight. They’ve traded a very hard life for a very quiet one where they're the bosses of the swamp.

HostThose yellow leaves floating away in the tide aren't just trash, then.

GuestThey're the price the tree pays to keep the rest of its branches green and healthy in a world of salt.

HostThe next time I see a yellow leaf in a swamp, I’ll think of it as a little boat carrying away all the things the tree couldn't use.