If you are up at high elevations this summer and see a pink snowbank, you're not hallucinating. It's watermelon snow, a tint produced by a living creature, a single-celled alga with a tongue-twisting name, Chlamydomonas nivalis.

This tiny creature, about four times the diameter of a human red blood cell spends its entire life in snowbanks (hence nivalis, Latin for "snowy").

C. nivalis thrives in the abundant light of high altitudes, producing its own food using sunlight.

But too much light--specifically ultraviolet light--can damage its cells. So the little alga shields itself with a layer of dark red pigments.

When spring sunlight melts the top layer of winter snowbanks, the algae multiplies rapidly, resulting in as many as one million individuals in a single teaspoon of meltwater. These dense crowds tint the snow reddish-pink.

A stroll across a snowbank colored by C. nivalis leaves brilliant red or pink tracks and releases a faint watermelon-like fragrance, hence the common name, watermelon snow. (In Scandinavia, it is called "blood snow.")

When the arctic explorer Sir John Ross brought back samples of meltwater from snowbanks off the coast of Greenland in 1818, the London Times reported that the liquid was "of so dark red as to resemble port wine" and concluded that the tint came from meteoric iron.

More than a century passed before scientists recognized the living organism that produced the unusual color. But not until recently has the life of this snow-loving alga come to light.

And an odd life it is: C nivalis thrives in conditions fatal to most other organisms, toxic levels of ultraviolet light, months of sub-freezing temperatures, and a summer "thaw" to water barely above freezing.

During the long dark winters, the alga survives in a thick-walled "resting" cell buried under layers of accumulating snow. As snowbanks warm in spring, meltwater and windblown nutrients stimulate the cell to split, releasing the next generation.

These green cells swim to the surface of the snow propelled by two whiplike flagella (the algal equivalent of tails). After reaching the brilliant light and abundant water of the top few inches of the snowbank, the algae lose their flagella and grow a thick cell coat and that protective layer of red pigment.

There they float, photosynthesizing and storing sugars, until the snow either vanishes or refreezes, signaling the little algae to form protective resting cells for winter.

The ability to thrive in the harsh environment of snowbanks is unusual, but not unique. C. nivalis is simply the most visible member of a snowbank-loving community including sixty other species of microscopic algae and an array of tiny herbivores: protozoans, ciliates, rotifers, nematodes, and springtails.

These minute grazers eat the algae, plus pollen and other wind-blown detritus that accumulates atop the snow. Alpine birds, including rock wrens and rosy finches, eat the grazers, plucking them from the snow with tweezer-like bills.

The microscopic algae that produces what we call watermelon snow may be too tiny to see with the naked eye, but new research shows the multitudes of C. nivalis soak up large amounts of carbon dioxide in photosynthesis.

Because the algae is so widespread, found on every continent except perhaps Africa, and because its populations explode in summer, researchers speculate the microscopic organisms could play a significant part in mitigating the affects of global climate change.

As a warming world means snow banks in arctic and alpine areas diminish, however, so too will populations of these miniature carbon-dioxide fixers, reducing their ability to remedy a serious problem.

Still, watermelon snow, produced by a brightly colored algae barely four times the size of a human blood cell and which remains dormant for much of the year, could have a huge impact on our global climate.

Copyright 2008 Susan J. Tweit
First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.