Watermelon Snow

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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.

Snowbanks of colorful pink snow are a common sight during summer on alpine slopes of California's Sierra Nevada. Photo taken at Gaylor Lakes, Tuolumne Meadows, Yosemite National Park at about 11,000 feet elevation.

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.

A field of pink snow in the alpine Sierra Nevada of central California. In addition to snow algae (Chamydomonas nivalis), the snow contains leaves (needles) and pollen of the whitebark pine (Pinus albicaulis).

Packs of colorful pink snow are a common sight during summer on alpine slopes of California's Sierra Nevada. Photo taken near the Dana Plateau in Yosemite National Park at about 12,000 feet elevation.

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."

Three days later a follow-up article concluded that the coloration was caused by meteoric iron deposits: "...iron being found to be the colourist of all metallic as well as vegetable matter." It was not until the end of the nineteenth century that the unusual phenomenon was finally attributed to high concentrations or "blooms" of microscopic algae.

Compressing pink snow with your boot increases the density of the red cells and heightens the color.

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.

Walking on pink snow often results in red soles as the red carotenoid algal pigment rubs off on your shoes.

Walking on pink snow often results in red soles as the red carotenoid algal pigment rubs off on your shoes. Well-worn shoes such as these are not very safe for walking on a steep snow bank.

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.

A pink snowball from an alpine snowbank in California's Sierra Nevada. During the summer months, the persistent snowbanks above 10,000 feet elevation are often colored pinkish-red by colonies of algae.

For microscopic examination in the laboratory, pink snow can be collected in a waterproof container. Compressing the snow intensifies the color and density of the algal cells.

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.

Microscopic view (400 X) of the bright red resting cells (aplanospores) of snow algae (Chlamydomonas nivalis). The larger winged structure (lower left) is a pollen grain from the timberline whitebark pine (Pinus albicaulis). The smaller, transparent-green cells (center) with a lipid droplet at each end are Chloromonas, another species of snow algae.

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.

Brian Duval and his associates (Journal of Applied Phycology Volume 11: 559-566, 2000) have shown that snow algae cells (aplanospores) exposed to ultaviolet light produce antioxidant compounds called flavonoids. Flavonoids are 3-ring phenolic compounds consisting of a double ring attached by a single bond to a third ring. In leaves they block far ultraviolet (UV) light (which is highly destructive to nucleic acids and proteins), while selectively admitting light of blue and red wavelengths which is crucial for photosynthesis. Flavonoids include water soluble pigments (such as anthocyanins) that are found in cell vacuoles. [Note: Carotenoids are fat soluble pigments found in plastids.] The increased production of flavonoids in snow algae apparently reduces the level of cellular free radicals that damage chlorophyll molecules in thylakoid membranes of chloroplasts. The defensive value of flavonoid production in snow algae may be explained by UV-stimulated phenolic compounds in other plants, including the highly-acclaimed resveratrol. Future research on antioxidant production in snow algae may have valuable pharmacological implications in the treatment of certain cancers in humans.

Another pharmacological spinoff from Duval's research on snow algae is the effect of UV-induced phenolic compounds on red blood cell coagulation and atherosclerosis in humans. For years it has been known that people in France who consume red wines on a regular basis have a reduced risk of coronary heart disease compared with the United States. This data is paradoxical considering that the French also consume a lot of fatty foods, such as pastries. A phenolic compound in the grape skins called resveratrol was discovered that seems to inhibit the plaque build-up or clogging of arteries (atherosclerosis) by increasing the level of high density lipoproteins (HDLs) in the blood. Beneficial HDLs carry cholesterol away from the arteries so that it doesn't form plaque deposits in the arterial walls. Resveratrol also reduces blood platelet aggregation or clotting (thromboses) within blood vessels. Resveratrol belongs to a class of plant chemicals called phytoalexins. They are used by plants as a defense mechanism in response to attacks by fungi and insects (and possibly UV radiation). One interesting phytoalexin called psoralen comes from the leguminous herb Psoralea. It has a chemical structure similar to coumarin. Psoralen has been used in the treatment of certain cancers, including T-cell lymphomas in AIDS patients. Another potentially valuable herbal medicine from grapes Vitis vinifera is grape seed extract, a mixture rich in bioflavonoids, specifically proanthocyanidins. The proanthocyanidins appear to enhance the activity of vitamin C through some unknown synergistic mechanism. Vitamin C protects cells from the damaging oxidation of free radicals, thus preventing mutations and tumor formation. The bioflavonoids in grape seed extract may also reduce the painful inflammation of swollen joints and prevent the oxidation of cholesterol in arteries which leads to fatty deposition (plaque) in the arterial walls.

Read About Phenolic Compounds
Grapes: Good Source Of Bioflavonoids

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.

Snow Fleas Were Formerly Placed In The Arthropod Class Insecta Order Collembola. Cladistical
Analysis Has Now Shown That They Belong In A Separate Class Along With Protura & Diplura.

Snowfleas (Achorutes nivicolus) often swarm in dense, dark masses on snow fields. Their tiny dark bodies (less than a millimeter long) absorb solar radiation and heat. These minute wingless insects belong to the primitive order Collembola (springtails). They hop by means of a springing device or furcula (red arrow) on the lower side of their abdomen. Springtail species at lower elevations are much lighter in color.

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.

See Bacteria & Algae That Cause Pink Salt Lakes
See Article About Desert Varnish And Lichen Crust

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.


  1. Armstrong, W.P. 1987. "Watermelon Snow." Environment Southwest Number 517: 20-23.

  2. Duval, B., Shetty, K. and W.H. Thomas. 2000. "Phenolic Compounds and Antioxidant Properties in the Snow Alga Chlamydomonas nivalis After Exposure to UV Light." Journal of Aplied Phycology 11: 559-566.

  3. Duval, B., Duval, E. and R.W. Hoham. 1999. "Snow Algae of the Sierra Nevada, Spain, and High Atlas Mountains of Morocco." International Microbiology 2: 39-42.

  4. Kawecka, B. and B.G. Drake. 1978. "Biology and Ecology of Snow Algae." Acta Hydrobiologica 20: 111-116.

  5. Thomas, W.H. 1995. "Sierra Nevada, California, U.S.A., Snow Algae: Snow Albedo Changes, Algal-Bacterial Interrelationships, and Ultraviolet Radiation Effects." Arctic and Alpine Research 27 (4): 389-399.

  6. Thomas, W.H. 1972. "Observations of Snow Algae in California." Journal of Phycology 8: 1-9.

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