All children know that snow, at least until it falls on the ground, is white. Then it possibly takes on various other colors, from yellow through shades of gray to black slush. These colors come from different types of pollution, and no one is surprised by such a change. It’s a different matter when a stain of red develops on the snow. And then you can be concerned. If this is not the result of paint spills or other more drastic criminal acts, it is very possible that we are dealing with the “red snow” phenomenon.

Seemingly white, yet colorful

The term “red snow” refers to a snow bloom caused by freshwater microorganisms, known as “red snow. Snow algae, taking on a characteristic red color. In the United States, it is also called “watermelon snow” because not only is it the color of watermelon flesh, but it can also emit a delicate watermelon scent.

The red coloration of the snow is most often caused by species belonging to the green algae (Chlorophyta), which are commonly included in the genus lungwort, including the most common snow lungwort Chlamydomonas nivalis, as well as the Ch. sanguinea, Scottiella nivalis or Smithsonimonas abbotii. The red color is given by astaxanthin, a carotenoid pigment used to protect against ultraviolet radiation, which is particularly strong on the reflective surface of snow. Depending on the species, life cycle stage and dominant pigments, algae are also known to dye snow in other colors: yellow, green, blue, various shades of orange or pink. Purple-gray ice caused by Mesotaenium berggrenii or Ancylonema nordenskiöldii adhesives has also been described. However, the most common is precisely the red bloom.

The first samples of red snow were taken for study near Baffin Bay, between northern Greenland and Canada, during an Arctic expedition under the command of Captain John Ross in August 1818. Since then, researchers have been documenting an increasing number of algal species in the snow environment, and increasingly accurate and sophisticated research techniques have made it possible to revise their systematics. In recent years (2019), research by a team of scientists from the Czech Republic and Germany, led by Procházkova, using molecular analysis and electron microscopy, has made it possible to separate a new genus, Sanguina, within the order Sanguina, which includes two species, S. nivaloides causing the red color of snow and S. aurantia causing the orange color. Given the development of analytical techniques and the increasingly widespread use of molecular sequencing to analyze relatedness between species, the phylogenetic tree of snowy algae will certainly be revised many more times.

Of course, the phenomenon of red snow will not be observed in the Bieszczady Mountains or on Szczesliwice Hill in Warsaw. These organisms are found in the extreme conditions of permanent snow and glacial fields in the Earth’s polar and high mountain regions. In a review of the issue published in 2020. by Ronald Hoham and Daniel Remias in the Journal of Phycology, the authors list 28 countries and regions of the world where snow algae have been identified since 2000. In addition to the more obvious regions, such as Antarctica, Svalbard or Greenland, the higher elevations in Slovakia, the Czech Republic or Poland also have conditions favorable to the occurrence of these organisms. In our country, their presence has been recorded in the High Tatras, in the Valley behind the Monk. The use of various types of imaging information, including hyperspectral imaging, satellite imagery or drone observations, combined with field surveys and taxonomic laboratory analyses, is significantly expanding our knowledge of the distribution of snowy algae in different areas.

Cold, hungry, but how sunny

A distinguishing feature of snow algae is their adaptation to low temperatures. True snow algae, so called. Psychrophilic algae (cryophiles), are obligatorily adapted to low temperatures, i.e. they undergo their life cycle at temperatures close to 0°C with an optimum for growth below 15°C and cannot withstand temperatures above 20°C. In fact, only certain strains, belonging to the genera Chlamydomonas or Chloromonas and some closely related taxa, show such strong adaptation to low temperatures. Other species have wider ranges of tolerance to thermal conditions, and these are called psychrotrophs (obligate cryophiles), although the definition of what are true psychrotrophs and what are psychrotrophs is not strict and unambiguous.

In addition to temperature, the second stress factor in the habitat of snowy algae is strong sunlight and high UV radiation. It is from such high sunlight, typical of snow-covered high mountain areas, that the algae are protected by the mentioned pigment, astaxanthin. It acts as a powerful antioxidant and protective filter against ultraviolet radiation. Snow algae thrive in both shady and strong sunlight, with the species that cause red blooms belonging to the photophilic ones. In very high light situations, they have the ability to migrate deeper into the snow, making the color of the bloom paler. So the intensity of the color of the bloom depends on the intensity of the light and the insolation time.

The habitat of snowy algae is poor in nutrients. In snow and ice fields, biogen production is essentially non-existent, and the supply of minerals is mainly carried out by downwind transport of industrial dust, conifer particles and weathering products of the bedrock. Studies of the chemical composition of snow on Spitzbergen and Greenland showed a significant content of micro- and macro-elements, and snow in the Polish and Slovakian parts of the High Tatras contained amounts of nitrogen and phosphorus comparable to those found in eutrophic waters. Besides, the algae themselves, by assimilating and accumulating nutrients and then releasing them into the environment, significantly alter the chemical composition of the habitat in which they live. As a result, they play a key role in colonizing these unfavorable environments and creating the conditions for the development of more complex biocoenoses.

Victims or beneficiaries

Snow algae do not actually live in the ice or snow crystals, but in the layer of water covering them. Thanks to their ability to absorb light energy, they blend into the surface of the ice, where they form microhabitats. In these niches, the temperature rises and stabilizes, and the water washing over the surface of the snow crystals provides a supply of mineral salts and gases. The abundant bloom of pigmented algae significantly reduces albedo and accelerates melting. Thus, snow and glacier algae have a huge impact on snow and glacier melt, and as climate change continues and temperatures rise, this impact will increase. Thus, algae are both “markers” and “actors” of climate change, on the one hand, benefiting from the environmental changes taking place, on the other, contributing to them.

This phenomenon was looked at by a group of scientists from France, who studied the zonation of algal species from the green algae cluster in the French Alps. The results of their study were released in June 2021. in the pages of the journal Frontiers in Plant Science, in an issue devoted entirely to the topic of snow algae. Scientists have found that different species of algae thrive at different altitudes due to the variation in their thermal requirements. For example, species of the genus Sanguina have been associated with high elevations, above 2,000 meters above sea level, with an optimum of occurrence at ca. 2400 meters above sea level, while the optimum occurrence of Chloromonas nivalis was found at an altitude of approx. 1800 m above sea level. However, climate warming may cause this zonation to be disrupted, changing the life cycle of glacier organisms and transforming mountain ecosystems.

Given the direction of climate change in our country and the fact that winters are becoming milder rather than harsher, perhaps an attack of Arctic red algae on the snow is not threatening us. But since last year we lived to see saltwater golden algae in a freshwater river, and piranha catches happen in the Warta, what else will amaze us? So if you ever, on a winter walk in the Tatras or the Giant Mountains, see blood on the snow, check first to see if it smells like watermelons.

In the article, I used, among others. From the works:

  • Hoham R.W., Remias D., 2020. Snow and Glacial Algae: a review. Journal of Phycology, 56, 264-282
  • Kawecka B., 1986. Ecology of snow algae. Polish Polar Research. 7(4), 407-415
  • Leya T., 2013. Snow algae: adaptation strategies to survive on snow and ice. w: J. Seckbach et al. (eds.), Polyextremophiles: Life under multiple forms of stress, Cellular Origin, Life in Extreme Habitats and Astrobiology 27, 401-423
  • Procházková L., Leya T., Křižková H., Nedbalová L., 2019. Sanguina nivaloides and Sanguina aurantia gen. et spp. nov. (Chlorophyta): the taxonomy, phylogeny, biogeography and ecology of two newly recognized algae causing red and orange snow. FEMS Microbiology Ecology, 95, fiz064
  • Stewart A., Rioux D., Boyer F., Gielly L., Pompanon F., Saillard A., Thuiller W., Valay J-G., Maréchal E., Coissac E., 2021. Altitudinal zonation of green algae biodiversity in the French Alps. Front. Plant Sci. 12:679428

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