World of Science | Ecosystem snow cover

Energy, carbon and survival in extremes

In general, there are three components that are essential for every known form of life: an energy source (also known as an electron donor), an electron acceptor (i.e. a molecule that accepts electrons) and a carbon source (building material for all the important structures of an organism) (Pascal 2012).

In humans, energy and carbon come mainly from glucose, while oxygen serves as an electron acceptor. However, there are many other molecules that can take over these functions. As long as there are any chemical compounds in a place that fulfill the respective purpose and the physical conditions allow stable biomolecules, it can be assumed that life has conquered these spots on earth.

The archaea (organisms that resemble bacteria) Geogemma barossii, for example, live at a hydrothermal spring in the Pacific. The high pressure prevailing there ensures that water remains liquid even at temperatures of around 120 °C. To date, the highest temperature at which life forms can grow has been measured under these conditions (Clarke 2014).

Even at temperatures below freezing, organisms can thrive under certain conditions. The bacterium Planococcus halocryophilus Or1 was isolated from small inclusions of liquid water in a core sample of sea ice. The high salt content in the liquid prevents freezing and thus enables growth at extremely low temperatures. The minimum growth temperature detected to date is -10 °C. Laboratory tests have even reached -15°C (Pascal 2012, Maccario et al. 2015, Merino et al. 2019).

Adaptations of organisms to the snow cover as a habitat

However, this article is about organisms that have found the snow cover as their ecological niche and can help shape its composition.

Compared to hot springs or salt water bubbles, the snowpack is a fairly temperate habitat. Nevertheless, this habitat has some characteristics that require special molecular changes in psychrophilic (cold-loving) organisms.

Firstly, light is reflected and scattered in the snow. The result is increased UV radiation, which can cause damage to DNA. A classic protective mechanism is the production of pigments such as astaxanthin. This molecule acts like a UV-absorbing shield: it protects cell structures by filtering out harmful wavelengths and thus reducing the penetration of UV radiation into sensitive organelles such as chloroplasts, the site of photosynthesis. Astaxanthin is produced by the snow algae Chlamydomonas nivalis, among others, and is responsible for the intense red coloration that creates the so-called "blood snow" effect (Fig. 3). The special properties of this molecule also make it a useful component of sunscreen (Remias et al. 2005).

The low temperatures also pose a major challenge. Water is essential for vital processes of all organisms; however, if it is solid and therefore less mobile, it is less available for biological purposes. In addition, the formation of ice crystals can cause physical damage to organisms, as unicellular organisms can be destroyed by the crystals. To protect against this, the composition of the fatty acids in the cell membrane is altered to make it more dynamic and flexible. There are also proteins that prevent the formation of large ice crystals in the immediate vicinity of the cell.

The so-called anti-frost proteins bind to the surface of developing ice crystals and, thanks to their special shape, create a concave bend that inhibits the growth of large ice crystals. These proteins are produced by many plants, fungi, algae and animals (Arrigo 2014, Maccario et al. 2015) and are used in the food industry, for example to preserve the structure of frozen vegetables or meat or in the production of antifreeze.

Ice nucleating proteins have the opposite effect. These are usually produced by bacteria, such as Pseudomonas syringae, and can initiate the formation of ice crystals at temperatures as low as +2 °C. In doing so, they orientate the water molecules. They orient the water molecules at suitable angles to promote the formation of the crystal structure. This controls the formation of ice crystals and reduces their size. These proteins are also used in industry, for example in the production of ice cream or artificial snow. These bacteria often live in symbiosis with other organisms such as snow algae and thus protect their structures (Roeters et al. 2021).

Nutrient cycles and microbial communities in the snow cover

As you can imagine, important nutrients are also rather scarce in snow. Although nitrogen and phosphorus compounds are washed up by melting periods or extracted from the atmosphere during precipitation events, there is little exchange between the biomass of the soil and the snow cover. The organisms therefore had to find new sources of nutrients and use them effectively. The yeast Phenoliferia psychrophenolica, for example, is a true expert in efficient food procurement.

It has been found in blood snow fields around the world and obtains its daily dose of carbon mainly from the remains of dead snow algae. Among other things, it can decompose the red UV protective pigments and is an important link in the carbon cycle between the atmosphere, snow cover and soil. Its impressive capacity to break down organic matter makes the fungus an interesting candidate in bioremediation, i.e. the cleaning of polluted areas with the help of microorganisms (Ezzedine et al. 2025, Maccario et al. 2015).

Life on Earth is extremely adaptable and therefore widespread. It is therefore not surprising that the snowpack is also full of organisms that have specialized to the physical conditions there and are an important link in global geochemical cycles. In fact, studies show that about 10⁵ bacterial cells can be detected in 1 ml of fresh snow, comparable to the number in about 1 mg of soil in the Central European climate. In firn, the number can even be significantly higher, depending on the composition.

How and where exactly the organisms are within the snow cover is rather difficult to determine. It is assumed that they live both in cavities and enclosed in the ice crystals, surrounded by a tiny film of liquid water (≈ 1 nm) (Fig. 4).

Bibliography:

Arrigo, K. R. (2014). Sea ice ecosystems. Annual Review of Marine Science, 6(1), 439-467. https://doi.org/10.1146/annurev-marine-010213-135103

Clarke, A. (2014). The thermal limits to life on Earth. International Journal of Astrobiology, 13(2), 141-154. https://doi.org/10.1017/S1473550413000438

Ezzedine, J. A., et al. (2025). Snow- and ice-ecosystem cleaning capability of the pucciniomycotinous yeast Phenoliferia psychrophenolica. Communications Biology, 8(1), Article 1084. https://doi.org/10.1038/s42003-025-1084-x

Maccario, L., et al. (2015). Snow and ice ecosystems: Not so extreme. Research in Microbiology, 166(10), 782-795. https://doi.org/10.1016/j.resmic.2015.10.002

Merino, N., et al. (2019). Living at the extremes: Extremophiles and the limits of life in a planetary context. Frontiers in Microbiology, 10, Article 780. https://doi.org/10.3389/fmicb.2019.00780

Mykytczuk, N. C. S., Wilhelm, R. C., & Whyte, L. G. (2012). Planococcus halocryophilus sp. nov., an extreme sub-zero species from high Arctic permafrost. International Journal of Systematic and Evolutionary Microbiology, 62(Pt 8), 1937-1944. https://doi.org/10.1099/ijs.0.035097-0

Pascal, R. (2012). Life, metabolism and energy. In Astrochemistry and Astrobiology (pp. 243-269). Springer. https://doi.org/10.1007/978-3-642-27585-4_10

Remias, D., Lütz-Meindl, U., & Leya, C. (2005). Photosynthesis, pigments and ultrastructure of the alpine snow alga Chlamydomonas nivalis. European Journal of Phycology, 40(3), 259-268. https://doi.org/10.1080/09670260500202148

Roeters, S. J., et al. (2021). Ice-nucleating proteins are activated by low temperatures to control the structure of interfacial water. Nature Communications, 12(1), Article 1183. https://doi.org/10.1038/s41467-021-21310-3