Have you ever seen red snow, like the crystals themselves are bleeding? Then you might have seen algae known as "snow blood," a phenomenon that accelerates Alpine thaw and that scientists worry is spreading.
Standing on a snowy mountainside about 2.5 kilometers (1.5 miles) above sea level, Eric Marechal holds up a crimson test tube containing "snow blood."
"These algae are green. But when it's in the snow, it accumulates a little pigment like sunscreen to protect itself," said Marechal, research director at Grenoble's Scientific Research National Center, who was collecting laboratory samples on Le Brevent mountain with teammates.
Around his feet, patches of red snow can be seen gleaming in the sunlight.
The algae were first described by Aristotle in the third century B.C. But it was only formally identified and given its Latin name Sanguina nivaloides in 2019.
Scientists are now racing to understand it better before it's too late, with snow volumes falling due to rising global temperatures which are hitting the Alps disproportionately hard.
"There's a double reason" for studying the algae, Marechal explained. "The first is that it is an area that is little explored and the second is that this little explored area is melting before our eyes, so it's urgent," he said.
Some scientists, including Alberto Amato, a genetic engineering researcher at CEA Centre de Grenoble, say the volumes of algae appear to be growing due to climate change, with higher carbon dioxide concentrations in the atmosphere favoring blooms.
Research is ongoing and what is certain is that the presence of the algae accelerates snow-melt, since algae's pigment reduces its ability to reflect the sun's heat.
Other algae, including a purple variety and soot from forest fires, have the same effect. If the algae spreads, snow and glaciers melt around the world could speed up.
"The warmer it is, the more algae there are and the more the snow melts quickly," said Amato. "It's a vicious circle and we are trying to understand all the mechanisms to understand this circle so we can try to do something about it."