Watermelon snow, also known as snow algae, pink snow, red snow, or blood snow, is a fascinating natural phenomenon where snow appears pink or red and emits a distinctive watermelon scent. This captivating occurrence is primarily caused by a species of green algae called Chlamydomonas nivalis, which contains a secondary red carotenoid pigment, astaxanthin, in addition to chlorophyll.
Watermelon snow in the Sierra Nevada. Source: Wikipedia
The Science Behind the Color
The pink snow is caused by an algae called Chlamydomonas nivalis which thrives in very cold temperatures. The cells of the algae have a gelatinous sheath that protect them from the strong ultra-violet radiation of the sun at high altitudes, and it is this sheath that produces the pink color and odor!
All snow algae producing red or orange snow are actually green alga that owe their red color to a bright red carotenoid pigment, which protects the chloroplast from intense visible and also ultraviolet radiation, as well as absorbing heat, which provides the alga with liquid water as the snow melts around it.
Algal blooms may extend to a depth of 25 cm (10 inches), with each cell measuring about 20 to 30 micrometers in diameter, about four times the diameter of a human red blood cell. It has been calculated that a teaspoon of melted snow contains a million or more cells. The algae sometimes accumulate in "sun cups", which are shallow depressions in the snow.
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Where Does Watermelon Snow Occur?
This type of snow is common during the summer in alpine and coastal polar regions worldwide, such as the Sierra Nevada of California. Here, at altitudes of 10,000 to 12,000 feet (3,000-3,600 m), the temperature is cold throughout the year, and so the snow has lingered from winter storms.
The colored ice - known as "watermelon snow" - has been spotted at the Presena Glacier, a popular winter sports area in Italy's northern Trentino region, which is already feeling the effects of climate change.
In May 1818, Captain John Ross noticed crimson snow that streaked the white cliffs like streams of blood as they were rounding Cape York on the northwest coast of Greenland. A landing party stopped and brought back samples to England.
Life Cycle and Growth
During the winter months, when snow covers them, the algae become dormant. In spring, nutrients, increased levels of light and meltwater, stimulate germination. Once they germinate, the resting cells release smaller green flagellate cells which travel towards the surface of the snow.
Snow algae undergoes a variety of distinct life-stage forms, which is one reason why they prosper in the cold, sun-blasted snowfields they colonize. From the immobile algal spores to colonizing motile cells and vegetative cells, the different life stages of snow algae may feature different pigments that partly account for the variety of colors produced.
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Impact on Glaciers and Climate Change
Snow algae dominates glacial biomass immediately after the onset of melting, and its pigmentation can significantly darken the surface of a glacier. The bright, white surface of a typical glacier generally has a high albedo, meaning it reflects around 80% of the sun's radiation back into the atmosphere.
Glaciers in the European Alps have shrunk by about half since 1900, according to the European Environment Agency. Di Mauro says the ice, darkened by the algae, absorbs the sun's rays and melts faster, eating away at the glacier.
A 2020 study on green snow algae on the Antarctic Peninsula found their distribution broadly reflected areas with average summer temperatures a little above freezing. Using satellite imagery and on-the-ground field methods, the researchers counted almost 1,700 blooms on the Antarctic Peninsula that collectively accounted for about 1.9 square kilometers.
That 2020 study, published in Nature Communications, suggests that snow algae-which, like green plants, produce their own energy from sunlight via photosynthesis-may be the most important primary producers on the Antarctic Peninsula, where only a small percentage of bare, ice-free ground support vegetation.
There’s also evidence that-despite the potential loss of green snow algae on low-lying Antarctic islands vulnerable to sea-level rise-algal blooms may increase on the Antarctic Peninsula, the most rapidly warming part of the White Continent, as climate change continues. Slightly warmer temperatures and slushier snowpacks (favored by the algae) might allow for an expansion of blooms on the Peninsula, “greening up” more snow there.
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Cultivation of Snow Algae
The most studied and cultivated microalgae have a temperature optimum between 20 and 35°C. This temperature range hampers sustainable microalgae growth in countries with colder periods. To overcome this problem, psychrotolerant microalgae, such as the snow alga Chloromonas typhlos, can be cultivated during these colder periods.
One snow alga with a potential commercial value is the red snow alga Chloromonas typhlos (Chlamydomonas nivalis). It is one of the most studied snow algae and is known to cause the phenomenon of red patches of snow (Mosser et al., 1977; Remias et al., 2005; Cvetkovska et al., 2017). This phenomenon of red snow, often called “watermelon snow”, is caused by the red pigment astaxanthin and its acid ester derivatives (Remias et al., 2005; Cvetkovska et al., 2017).
Photobioreactor used for growth experiments with C. typhlos. Source: MDPI
Chloromonas typhlos (SAG 26.68) was purchased from SAG (Department Experimental Phycology and Culture Collection of Algae, University of Göttingen, Germany). The culture was maintained in the laboratory in a sterilized (autoclaved at 121°C for 20 min) freshwater medium based on the SAG basal medium (version 10.2008). It was kept in a 250-ml Erlenmeyer flask on an orbital shaker at 90 rpm with 70 μmol m−2 s−1 light exposure (cool-white fluorescent) in a climate-controlled room at 22°C (±0.2 SD) under a 16/8-h day/night regime.
Growth and Productivity
Table 1 summarizes these results. The highest growth rate was achieved during the 10-19 December period, which was five times higher than the 17-26 April period. The highest total volumetric biomass productivity (0.067 g L−1 d−1) was obtained during the period 27 January-5 February 2020.
Productivities and growth rates for the Chloromonas species were lower than the literature values: over the nine batches average volumetric and areal productivities of 0.043 ± 0.021 g L−1 d−1 and 1.072 ± 0.514 g m−2 d−1, respectively, were obtained, while growth rates varied between 0.020 and 0.105 d−1.
| Growth Period | Specific Growth Rate (µ) | Total Volumetric Productivity (Pv) (g L−1 d−1) | Areal Productivity (Pa) (g m−2 d−1) |
|---|---|---|---|
| 1-10 April 2019 | 0.058 | 0.031 | 0.776 |
| 17-26 April 2019 | 0.020 | 0.010 | 0.245 |
| 7-16 May 2019 | 0.061 | 0.047 | 1.176 |
| 10-19 December 2019 | 0.105 | 0.057 | 1.436 |
| 17-26 January 2020 | 0.047 | 0.062 | 1.546 |
| 27 January-5 February 2020 | 0.058 | 0.067 | 1.673 |
| 10-19 February 2020 | 0.056 | 0.030 | 0.750 |
Table 1: Volumetric and areal biomass productivities in dry weight of C. typhlos grown in batch for 9-day periods in a 350-L horizontal tubular reactor and the specific growth rates are shown.
A potential explanation for the lower total volumetric productivity during the 1-10 April period could be that a maximum ambient temperature of 30.5°C was reached inside the greenhouse on 7 April 2019, causing temperature stress on the cells and leading to a decline in growth. During this period, the lowest total volumetric biomass productivity (0.010 g L−1 d−1) and growth rate (0.020 d−1) were obtained. This growth period had the highest average temperature (22.5°C), the highest maximum temperature (35.2°C), and several (5) consecutive days with temperatures above 30°C.
Aside from temperature stress, algae, in general, react to light stress, although C. typhlos has mechanisms; for example, the production of astaxanthin to cope with high light intensities.
