Just as marine creatures occupy different depths of the ocean, snow algae species are found at different elevations in the mountains, according to a new study that sheds light on the organisms responsible for the famous but poorly understood phenomena of green and red snow.

In the study, published Monday in Frontiers in Plant Science, researchers analyzed soil samples from different altitudes in the French Alps to look at the distribution of snow microalgae, important members of alpine and polar ecosystems and potential markers of environmental change.

"A lot of people have heard of red snow: The phenomenon is known, but it's not understood," said study co-author Eric Maréchal, a research director at the French National Centre for Scientific Research and the head of the Cell and Plant Physiology Laboratory at the Université Grenoble Alpes. "It's one of the very few ecosystems that is still mysterious for us. It's like an unknown ocean."

Green microalgae are single-celled organisms that photosynthesize, the process that converts carbon dioxide and water into sugars via sunlight. Like plants, these algae are considered primary producers because photosynthesis generates food that forms the basis of ecosystems.  

Microalgae grow in habitats such as streams, lakes, oceans, soils and snow. In chilly ecosystems, beyond the elevational or altitudinal limits of other plants, green algae are the chief primary producers and important carbon sinks. These algae aren't usually visible to the naked eye, but occasional algal blooms stain snow in striking hues.

"It can be green; it can be orange; it can be ochre; it can be red," Maréchal said. "When it becomes red, it's specific algae that protect themselves from the high irradiance of light with a shield of molecules called carotenoids."

According to Maréchal, the red blooms are known as sang de glacier, or glacier blood, in French, or in German, Blutschnee, or blood of the snow. In English, the phenomenon is sometimes called crimson snow or watermelon snow because it can produce a sweet, fruity aroma.

Like dark clothes on a hot day, red or green algae lower the albedo, or ability to reflect solar radiation, of snow, causing it to absorb more heat from the sun. These colorful patches of snow and ice, which can sprawl for hundreds of square meters, melt faster than their pristine white surroundings.

As such, snow algae blooms may be important for understanding environmental change such as glacier melt, but little is known about this phenomenon.

"So far, we don't know why and how and when," Maréchal said. "It's really puzzling. It's a mystery: Why does it bloom, and can we anticipate it?"

Many other aspects of microalgae biology are also unknown, such as the environmental conditions they require, how they survive when there's no snow and how they spread. 

With the goal of solving such mysteries, Maréchal and other researchers launched the ALPALGA project to study snow algae in the French Alps. As a first step, the researchers mapped biodiversity of microalgae across altitudinal gradients.

"Like scientific police, we decided to look at the DNA as a signature of the presence of our suspects," Maréchal said.

Because soil may provide a temporary habitat for the algae or act as a reservoir for dormant life stages, the researchers collected 158 soil samples along an elevation gradient from 1,250 meters (4,100 feet) to about 3,000 meters (9,800 feet). They extracted and sequenced the DNA from these samples.

The researchers characterized the altitudinal distribution of about 50 species of green algae. Different species had different optimal altitudes, suggesting that snow algae exhibit altitudinal zonation, similar to the pattern of trees and plants that can be seen on mountains.  

"For hikers, this is something which is familiar,"  Maréchal explained. As you ascend a mountain, "You start with a forest of oaks or something in the valley, and then you have a mix of coniferous [and deciduous] trees, then purely coniferous forests, and then you have small shrubs and grass."

Some algae were mainly found at low elevations, while others such as the genus Sanguina, which causes red blooms, were found at higher elevations. Other species were found across the altitudinal gradient, suggesting that they have cosmopolitan ranges.

Many environmental parameters change with elevation, including light, temperature, oxygen, carbon dioxide and soil conditions. The researchers found that soil pH, soil nitrogen/carbon ratio and the intensity of freezing events were important factors contributing to the distribution of some algae.

Maréchal said that understanding the habitat requirements of snow algae is important for conservation of these species in a changing climate.

"If something dramatic occurs at the level of the primary producers, like the algae, the ecosystem will be disturbed profoundly," he said. "In our life, we will experience the retreats of the glaciers and the loss of snow at the summit of mountains." 

Because snow acts as insulation, protecting algae from variations in temperature, loss of snow due to climate change could expose sensitive species to more extreme freezing events.

According to Maréchal, snow algae are likely markers of environmental change. In aquatic environments, algal blooms are often connected with excess nitrogen. Similarly, environmental factors likely influence the appearance of red and green snow in mountains and polar regions. 

Increased carbon dioxide in the atmosphere as a result of anthropogenic activities could also increase algal growth and make blooms more common — although this has not yet been demonstrated. 

"Snow algae could be actors of environmental change," Maréchal added. "If global warming increases algae blooms, decreasing the albedo, [snow] melts faster." 

In future work, researchers involved in the ALPALGA project will evaluate the factors that drive algal blooms. The group also wants to learn more about algae communities in different locations and investigate whether there are interactions between these communities. 

"We are at the very beginning of the exploration. It's like as if we were trying to explore an ocean from scratch," Maréchal said. "For me, it's incredibly beautiful and satisfying to work on this because it's like a terra incognita."

But, with glaciers rapidly retreating and alpine areas warming, time may be running out to explore this new world.

"We are at the very beginning and we have no time because it's melting," Maréchal said. "It's disappearing."

The study, "Altitudinal zonation of green algae biodiversity in the French Alps," published June 7 in Frontiers in Plant Science, was authored by Adeline Stewart, Université Grenoble Alpes and Université Savoie Mont Blanc; Delphine Rioux, Fréderic Boyer, Ludovic Gielly, François Pompanon, Amélie Saillard, Wilfried Thuiller and Eric Coissac, Université Savoie Mont Blanc; Jean-Gabriel Valay and Eric Maréchal, Université Grenoble Alpes; and The ORCHAMP Consortium.