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Uploaded:2020-03-31
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In the South Pacific, there is a rare tree so rich in metal that its sap runs blue.

Hosted by: Olivia Gordon

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Sources:
https://www.chemguide.co.uk/inorganic/complexions/colour.html
https://mitchalbala.com/nickel-titanate-the-coolest-yellow/
https://www.ncbi.nlm.nih.gov/pubmed/9433812
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6359332/
https://www.nature.com/articles/srep41861
https://link.springer.com/article/10.1007/s11104-017-3523-3
https://link.springer.com/article/10.1007/s11104-016-2910-5
https://www.mdpi.com/2304-6740/7/7/89/htm
https://www.purdue.edu/uns/html4ever/2004/040901.Salt.antioxidant.html
https://link.springer.com/article/10.1007/s00425-013-1983-0
https://bsapubs.onlinelibrary.wiley.com/doi/full/10.3732/ajb.89.6.998
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https://www.ncbi.nlm.nih.gov/pubmed/17244045
https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.15105
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Images:
https://commons.wikimedia.org/wiki/File:Pycnandra_acuminata_05_-BH-_aspect_g%C3%A9n%C3%A9ral.jpg
https://commons.wikimedia.org/wiki/File:Pycnandra_acuminata_01_-BH-_Feuilles.jpg
https://commons.wikimedia.org/wiki/File:Pycnandra_acuminata_03_-BH-_S%C3%A8ve_bleue.jpg
[INTRO ♪].

The South Pacific island of New Caledonia has one of the weirdest plants on Earth. It's a tree whose sap is this teal-ish, blue-green color that you don't usually see in nature—especially not in living things.

But as alien as it looks, the green color is totally natural, and it comes from the fact that more than a quarter of the sap… is metal. The tree is called Pycnandra acuminata, and it grows in the rainforests in what are called ultramafic soils. Ultramafic soils are sort of unusual because they form from rocks with large concentrations of nickel.

And rocks like that are rare—you typically only see them in places where material from the Earth's mantle has been pushed to the surface. In general, that type of soil is also pretty hard for plants to grow in. Even though they need a little bit of nickel to survive, too much of it is typically toxic.

That's because nickel has a similar chemical structure to other nutrients plants need to grow, like calcium and magnesium, so plants can end up absorbing nickel instead of some of the other nutrients they need. All that nickel can mess with their metabolism and stunt their growth—if it doesn't just straight-up kill them. But the roots of Pycnandra acuminata guzzle up that metal, and the plant has about 250 grams of nickel per kilogram of sap.

That's around 250 thousand times more than your average plant. It also happens to be enough to turn its sap green. But this tree doesn't seem to mind.

That makes it a metal hyperaccumulator—in other words, a plant that can deal with a lot of metal. These plants deal with nickel by capturing it with molecules called ligands, which bind to nickel atoms and prevent them from reacting with other chemicals. That helps keep the plant's chemical structure intact despite all that extra metal.

Pycnandra acuminata also basically quarantines the nickel in cells that are separate from other parts of the plant. And those solutions seem to work out for this tree. The question is why plants like these have evolved to store so much metal, and scientists haven't figured out the exact reason.

One possibility is that it evolved as a way of surviving extremely metallic soils. But scientists think it's now what's called an exaptation—a trait evolved for one purpose but now serves a different one. For example, one hypothesis suggests that plants build up all this nickel to protect themselves from predators, since nickel is toxic to animals as well plants.

But the case isn't closed. While there's evidence that some insects are sensitive to high levels of nickel in plants, larger plant-eaters don't seem to be put off by it at all. So there may be a different reason plants have this trait, and another hypothesis suggests that the nickel is a type of chemical attack on other plants—something called allelopathy.

The idea is that when a hyperaccumulator's leaves drop to the ground, they enrich the nickel in the soil and make it harder for other plants to thrive. Its young offshoots may have a better chance of surviving if they don't have to compete with other plants for resources. But while there's some evidence for it, this idea isn't proven yet either.

Whatever its reason for being the way it is, this bizarre tree could offer us more than its weirdness. Metal hyper-accumulators like it could make it possible to remove metal pollutants from soils, which could be valuable here on Earth or even in future civilizations on Mars. But we have to protect this tree because there are only a few hundred left, and we still have plenty more to learn from it. [OUTRO ♪].