Watermelon Snow A Strange Phenomenon Caused by Algal Cells of The Chlorophyta

Have you ever hiked across an alpine meadow or traversed a steep talus slope in high mountain ranges and noticed pinkish patches, or streaks of red, on the snowbanks? This phenomenon is especially common during the summer months in the Sierra Nevada of California where snow has lingered from winter storms, mainly at altitudes of 10,000 to 12,000 feet. Compressing the snow with your boot leaves a distinct footprint the color of watermelon pulp. The snow even has a fresh watermelon scent and is sometimes called "watermelon snow." Walking in pink snow often results in a temporary discoloration of your clothing, such as bright red soles and pinkish pant cuffs. There are unconfirmed reports that consuming "generous quantities" of pink snow may cause diarrhea, a rather distressing situation above timberline.

For thousands of years the mysterious pink snow puzzled mountain climbers, explorers and naturalists alike. Some people thought it was caused by mineral deposits or oxidation products leached from rocks. Colored snow was mentioned in the writings of Aristotle, several centuries before the Christian Era.

In May 1818 four ships sailed from England to search for the Northwest Passage and chart the unknown Arctic coastline, with stalwart plans to rendezvous on the other side of the globe. Although fate and severe weather conditions eventually turned the ships back, the expedition made some valuable scientific contributions. Rounding Kap York (Cape York), off the northwest coast of Greenland, Captain John Ross noticed crimson snow that streaked the white cliffs like streams of blood. A landing party stopped to examine the spectacular display and brought samples back to England. This unusual discovery appeared in the London Times, Dec. 4, 1818:

"Captain Sir John Ross has brought from Baffin's Bay a quantity of red snow, or rather snow-water, which has been submitted to chymical analysis in this country, in order to the discovery of the nature of its colouring matter. Our credulity is put to an extreme test upon this occasion, but we cannot learn that there is any reason to doubt the fact as stated. Sir John Ross did not see any red snow fall; but he saw large tracts overspread with it. The colour of the fields of snow was not uniform; but, on the contrary, there were patches or streaks more or less red, and of various depths of tint. The liquor, or dissolved snow, is of so dark a red as to resemble red port wine. It is stated, that the liquor deposits a sediment; and that the question is not answered, whether that sediment is of an animal or vegetable nature. It is suggested that the colour is derived from the soil on which the snow falls: in this case, no red snow can have been seen on the ice."

In the high mountain ranges of the western United States at least 60 different species of snow algae have been identified, but only a few kinds have been reported from the Sierra Nevada. One of the most common species of snow algae in California, and the one responsible for pink snow, is Chlamydomonas nivalis. This unicellular organism is a member of the diverse green algae Division Chlorophyta (Order Volvocales), and contains a bright red carotenoid pigment in addition to chlorophyll. Unlike most species of fresh-water algae, it is cryophilic (cold-loving) and thrives in freezing water. Its scientific surname, nivalis, is from Latin and refers to snow.

During late spring and summer, alpine snowbanks are often colored beautiful shades of red by a myriad of algal cells. The concentrations or "blooms" may extend to a depth of 25 centimeters (10 inches). Each spherical cell is approximately 30 micrometers in diameter, about four times the diameter of a human red blood cell. It has been estimated that one teaspoon of melted snow may contain more than a million cells of snow algae. Compacting the snow increases the density of the red cells and heightens the color. Sometimes the algae accumulate in shallow depressions in the snow called sun cups. Because the dark red pigment absorbs heat, the cells melt their way into the snow, thus deepening the sun cups and accelerating the melting rate of snowbanks and glaciers. Snow algae are not always confined to regions of perpetual snow, particularly in the high mountains of southern California where there is a complete melting of snow every summer.

To really appreciate these organisms you must view them under high magnification, preferably 400 power. Through a microscope a drop of melted snow contains literally thousands of brilliant red cells of Chlamydomonas nivalis that resemble globular hard candies. Critical focusing reveals a thickened wall with a warty or minutely bumpy ornamentation. Sometimes the cells are mixed with the distinctive winged pollen grains of timberline pines, such as whitebark pine (Pinus albicaulis). In the Sierra Nevada I can usually find a few cells of another kind of snow algae in drops of pink snow. The other species, Chloromonas, has oval cells with a greenish center and a distinctive orange-yellow lipid droplet at each end.

The bright red carotenoid pigment inside the cells of snow algae is similar to that found in tomatoes, red peppers and in many colorful flowers and autumn leaves. Carotenoids may also be orange, yellow or yellow-green as in carrots and the fleshy meat of avocados. They are also found in a variety of animals, including the exoskeletons of shrimp, crab and lobsters, brightly colored corals, skins of fish and amphibians, egg yolks, and pink plumage of flamingos. Since flamingos cannot synthesize carotenoids, they are often fed shrimp in captivity to intensify the color.

Carotenoid pigments presumably help to protect the delicate cells of snow algae from intense solar radiation at the surface of the snow. Because of the thin layer of atmosphere for filtration, alpine snowbanks are subjected to more damaging ultraviolet radiation than at lower elevations. Cells of snow algae (and other particulate matter in snow fields) may also concentrate airborne radiation. This phenomenon was apparently discovered by a uranium prospector who inadvertently let his coffee pot go dry after melting snow in it and heard his Geiger counter nearby begin to click.

There are several explanations for what happens to snow algae when they are covered by deep layers of snow during winter. Experiments by several researchers indicate that algae lie dormant during the winter months under drifts of snow. The following spring, meltwater and nutrients reach the dormant cells and stimulate germination. Upon germination, the resting cells release smaller, green swimming cells with two whiplike flagella that propel them through the snow pack to the surface and daylight. Exactly what triggers this remarkable migration of biflagellate cells to the surface has been the subject of extensive research. It may be related to a combination of factors, such as melting snow and dissolved nutrients, light intensity, and possibly the length of daylight.

Once at the surface, the swimming cells loose their flagella and form thick-walled resting cells (aplanospores) containing the protective red pigment and reserve food. Some of the swimming cells may function as gametes (sex cells) and fuse in pairs to form zygotes. The nutrient source for snow algae comes from minerals leached from boulders and underlying soil, and from detrital material (especially pollen) that blows onto the snow from nearby timberline trees and shrubs. The plant debris, along with dead snow algae and small insect life, is broken down by decay bacteria and fungi, thus making the essential nutrients available to the algae. The powerful mountain winds that bring nutrients to the snowbank may also serve to disperse the dormant cells to distant snow-capped mountains. Since microorganisms of snow and ice fields are dependent on air-transported nutrients, scholars of arctic-alpine ecosystems refer to these habitats as "aeolian regions."

As photosynthetic plants (protists), snow algae represent the "primary producers" of aeolian regions and form the beginning of a unique food chain. During the summer months, blooms of snow algae are often associated with a variety of animal life, including many species of protozoans, ciliates, rotifers, nematodes, snow worms (Phylum Annelida) and springtails. The minute snow fauna represents herbivore "grazers" in a snow ecosystem. There are reports of snow worm populations literally covering snow fields and glaciers of Alaska and British Columbia. Springtails or "snowfleas" (Achorutes nivicolus) are minute wingless insects (order Collembola) less than a millimeter long, often swarming in enormous numbers on debris-laden snow. Their tiny dark gray or black bodies absorb solar radiation and heat. By means of a unique springing device (furcula), extending from the tip of the abdomen and folded forward along the underside, they hop around on the snow, browsing on pollen, snow algae and other microscopic debris. The minute herbivore fauna provides food for diminutive carnivores, such as mites, spiders and insects, culminating in birds (including the Rock Wren and Rosy Finch) who hunt the snow surface with tweezerlike bills.

There are many other striking examples of colorful microscopic algae in our environment. Algal cells color the trunks of trees velvety green, and the trunks of Monterey cypress on the Monterey Peninsula of California a brilliant orange. The colorful crusted growth on rocks and boulders is caused by an intimate association between algae and fungi called lichen. In the Sierra Nevada and high desert ranges to the east there are many colors of lichen, including black, orange, green, yellow and chartreuse. The unicellular alga Dunaliella, related to snow algae, and salt-loving bacteria grow in salt lakes and brine pools throughout arid regions of the world and often color the water vermilion red.

Cells of microscopic algae are able to survive in a variety of unusual places, from the dry salt crust and brine pools of blistering desert playas to boiling hot springs and windswept lichen-covered peaks of high mountains. Algal cells even live inside the hollow cores of polar bear hairs, producing peculiar green coats. But one of the most remarkable habitats of all is the freezing water of alpine snowbanks. Here they live and multiply through countless centuries in a sea of fallen snowflakes, efficiently utilizing the sun's energy in a world that never gets above freezing.

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