Olivia: If it snows where you live, you’ve probably been told not to eat the yellow snow, and for good reason. But what about pink snow that smells vaguely of watermelon, aka watermelon snow?

It’s not pink because it’s contaminated with blood or anything — watermelon snow has algae in it. But you probably still wouldn’t want to eat it.

People have been trying to explain pink snow for a while. Even Aristotle wrote about it, back in the day. But we’ve only known why it’s pink for the past couple of centuries.

In May of 1818, British explorer Captain John Ross was sailing past Greenland, looking for a convenient path between the Atlantic and Pacific oceans. That’s when he spotted patches of pink and red snow on the white cliffs of Cape York.

He took some samples and brought them back home to England, and when the London Times reported on his find, they said that the color came from iron deposits in the underlying soil. But Scottish botanist Robert Brown disagreed. He thought that the pink color might be coming from algae in the snow.

And these days, we know that he was right. It turns out that the color comes from Chlamydomonas nivalis, which is actually a type of green algae. If you look closely enough, the algae’s cells are green — on the inside. But they’re covered in a red outer layer to protect them from solar radiation, which is what turns the snow pink.

And the algae smell kind of sweet, which is why it’s sometimes called watermelon snow. C. nivalis is one of over 60 kinds of snow algae, all of which can withstand cold temperatures, high levels of solar radiation, and low levels of nutrients.

It lives at pretty high altitudes — mostly between 3000 and 3600 meters up. And that means it needs extra protection. At such high elevations, there’s a lot less atmosphere to protect the algae from the sun’s radiation, and the snow reflects lots of light, too.

All that light can lead to photoinhibition, where an organism has a much harder time photosynthesizing. That’s because the light damages a protein that’s critical to photosynthesis, and it can’t repair itself quickly enough. So most plants and algae have pigments called carotenoids that help absorb just enough light for photosynthesis to work properly, but protect them from photoinhibition.

C. nivalis has a carotenoid called astaxanthin, which is red enough to mask the green underneath, and turn the snow pink or red. The algae are dormant during the winter, so you won’t usually see pink snow then. But as the snow melts during the spring, nutrients from things like pollen and plant debris reach the dormant cells underneath the layer of snow.

That’s when the algae wake up from their long winter’s nap and start the germination process, releasing cells that move toward the sunlight. These cells have flagella, small whip-like arms that they use to swim up until they’re close to the snow’s surface. Then, they shed their flagella because they’re just going to stay where they are.

The cells form thick walls with that trademark red pigment, then spend most of their lives feeding off of a combination of photosynthesis and stored nutrient reserves. The reddish color does help protect the algae from some of the sun’s rays, but they still don’t live right on the surface — they’re usually one or two centimeters below, which gives them even more protection.

And as the snow melts throughout the spring and summer, the algae naturally reposition themselves by flowing along with the snow-melt. They help support even more life in the cold, snowy fields, including some single-celled organisms like protozoa and microscopic animals like rotifers.

If you ever come across watermelon snow, you’ll probably notice small divots in the snow where the color is an especially intense red. These indents are called sun cups, and they’re in a kind of positive feedback loop where the darker color absorbs more heat, melting the snow into deeper indentations. That lets the cells move closer together, which intensifies the color even more and leads to more melting.

So, planning your next vacation and on the hunt for some watermelon snow? In the United States, you can find C. nivalis in California’s Sierra Nevada mountain range. But it’s also in other snowy, elevated places all over the world, from Australia to Europe. You probably still don’t want to eat it, though.

Some report that eating too much of the stuff leads to diarrhea, which could derail your whole algae sightseeing adventure. Researchers have actually tested this, because of course they have. They gave seven people half a kilogram each of watermelon snow, and none of them had any digestive issues. But that’s only seven people, so it wasn’t exactly a conclusive study. If you want a natural watermelon snow-cone, eat it at your own risk.

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