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In the Pink (Snow)

By Cheryl Dorschner Article published February 25, 2003

Pink Snow
It's slushy, it's pink: The strange, hardy algae cells shown here give some spring snow a decidedly festive cast. (Photo courtesy of Tom Vogelmann)

Maple syrup, newborn lambs, tiny white flowers called snowdrops, pink snow — these are the harbingers of early spring in cold climates.

Pink snow?

In high altitudes on all continents except Africa, the heavy, wet snows of spring sometimes come with large patches tinged bright pink. "In the U.S., it's known as 'watermelon snow' or 'snow algae‚' while Scandinavians call it 'blood snow,'" says Tom Vogelmann, professor and chair of botany, who has been studying the phenomena for more than four years.

The source of the red tint is an unusual kind of microscopic, spherical, freshwater algae, Chlamydomonas nivalis. The microorganism has a fascinating and mysterious life cycle, and some scientists are even debating whether pink snow helps counter global warming by sucking carbon dioxide out of the air. Vogelmann doesn't place much stock in that theory, but he's found plenty in the algae to hold his interest.

"It appears magically each spring, and anywhere the snow pack persists longer. It grows well in slushy snow, which is why we don't find it throughout the winter. But we know little or nothing about what it does at other times of the year," says Vogelmann.

Researchers can't culture this particular algae in a lab, but they know the cells are more complicated than they might initially seem. The pink-snow effect might come from algae swimming up from below in the slushy water or blowing in on the wind.

Even more intriguing than how they arrive is how they survive at all.

"It's a bizarre creature," Vogelmann says. "It's hostile where they live. Most of the time it's near the freezing point and bathed in very high ultraviolet radiation. Photosynthesis in almost every green plant that I can think of would be near zero under these conditions. Somehow this creature not only grows, but thrives — it has to have some interesting structures. Studying how they survive will give us ideas about cold hardiness."

One thing Vogelmann and colleagues do know through measurements is that pink algae takes up more carbon dioxide than would normally be expected — "high from the standpoint that this alga is growing in ice slush," he clarifies. Vogelmann was among a team of researchers who spent four summers at 11,000 feet in the Snowy Range of the Rocky Mountains, 50 miles west of Laramie, Wyoming measuring how much carbon dioxide the algae soaked up in response to variable sunlight levels by placing a transparent plastic box fitted with gas sensors over patches of algae.

Their results: "Surface Gas-Exchange Processes of Snow Algae" by William E. Williams, Holly L. Gorton, and Thomas C. Vogelmann, published Jan. 6 in the Proceedings of the National Academy of Sciences, found the snow microbes absorbed relatively high levels of carbon dioxide from both the soil and air in bright light.

After the paper speculated that "red algae may be acting as significant carbon sinks‚" articles quickly started appearing concluding as one United Press International report phrased it, "that these microbes might help combat global warming by guzzling up greenhouse gases." Other scientists extrapolated that even if the amount of carbon dioxide uptake wasn't high, the large areas the algae cover might be a significant contributor.

Vogelmann disagrees. "Personally, I am rather skeptical about the idea, but it shouldn't be all that hard to come up with an estimate of the total amount of global carbon that they may be fixing. Not my area of expertise though," he says.

This study is just the tip of the pink iceberg when it comes to understanding watermelon snow. Vogelmann says he'll be out looking for pink algae in the snows of Vermont's Mount Mansfield, probably in early April, so he can study it a bit closer to home.