Blood snow. Image: © Jean-Gabriel/ VALAY/ JARDIN DU LAUTARET/UGA/CNRS
Phrases like “glacier blood” and “blood snow” sound like something you’d read in a supernatural horror novel. But despite the morbid nicknames, this crimson phenomenon is a colorful sign of blossoming life, produced by tiny organisms that have colonized snowy mountain habitats. The odd reddish hue found in mountain snow is caused by green algae. Algae is one of the most diverse and ubiquitous forms of life on Earth, so it’s not particularly surprising that it has managed to find its way to some of the highest peaks in the world. However, the climate crisis is accelerating snowmelt and glacial retreat in these habitats, a trend that may upend many algal colonies and the larger ecosystems that rely on them.
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That’s why European scientists have banded together to form ALPALGA, a collaboration that aims to better understand these alpine algae, which can act as primary producers, pioneer species, and “potential markers of climate change,” according to a study published on Monday in Frontiers in Plant Science. “I've seen, in my lifetime, the disappearance of ecosystems on a scale not of 100 years, but decades,” said study author Eric Maréchal, director of the Cell and Plant Physiology Laboratory, a joint unit of CNRS, CEA, INRA and University Grenoble Alpes, in a call. “This ecosystem is super fragile.”In a way, the fragility of these systems is counter-intuitive, because the algal blooms that produce these alpine stains of red, purple, and orange are actually a sign of flourishing life. The eerie biological dye goes by many names—watermelon snow or red snow, for instance—and it is common on mountain tops and glaciers around the world, from the Sierra Nevada, to the Himalayas, to Greenland’s ice sheets, sparking speculation among mountaineers, naturalists, and polar explorers for centuries.Scientists now know the effect is a microalgal adaptation strategy to the onset of summer, which brings with it snowmelt and intense sunshine and UV radiation. Green algae are green, as the name implies, but warmer weather triggers them to produce a red carotenoid pigment as a kind of sunscreen.
The pigment also lowers the reflectivity of snow by darkening its color, which speeds up snowmelt and provides the aquatic algae with more water to thrive in. In this way, microalgae are “actors” of environmental change because these organisms exacerbate snow loss at high elevations, according to the study. Anthropogenic air pollution has also reached mountain tops and stimulated the growth of alpine blooms in much the same way that pollution in freshwater and marine environments have produced bigger algal blooms.For these reasons, algal colonies may actually benefit from warmer temperatures, air pollution, and increased atmospheric carbon dioxide in the short term, though the long-term impact of climate change on their habitats could be devastating for some of these ecosystems. Maréchal noted that there is plenty of anecdotal evidence to suggest that alpine blooms have become more common in recent decades, which is why the ALPALGA collaboration is eager to constrain the complex roles of green algae as possible drivers, beneficiaries, and casualties of climate change“I suspect there is an increase [in blooms], because I’ve seen it,” Maréchal said. “But we need to provide the figures.”To that end, Maréchal and his colleagues provided a comprehensive overview of alpine algae in the new study by meticulously analyzing 158 soil specimens collected in 2016 from five sites at elevations between 1,250 and 3,000 meters in the French Alps: Chamrousse, Loriaz, Anterne, Ristolas, and Vieux Chaillol. The team sampled the soil for environmental DNA, which is made up of genetic fragments shed by organisms, enabling them to reconstruct the range of dozens of microalgae species at different altitudes, pH levels, and other environmental conditions.
The results revealed that some algal groups, such as Sanguina, were more abundant at high alpine elevations above 2,000 meters, while others, like Symbiochloris, lived at exclusively lower elevations under 1,500 meters. One of the key environmental factors that emerged for the high-altitude Sanguina was this group’s reliance on the stable thermal environment provided by the snowpack throughout the winter. As the snow season shortens over time, it could seriously disrupt the life cycles of these organisms, which is especially troubling considering that they are an ecological bedrock that provides nutrients, directly and indirectly, to many other lifeforms.As the photosynthetic powerhouses of these ecosystems, these algae are the “basis of the trophic network,” Maréchal said. “Detaching not only the primary producers, but also some other species, will have an impact.” “It may not be deleterious for the ecosystem,” he noted. “Maybe a new ecosystem will organize into new communities. But it will be different.”The new research provides a baseline census of microalgae in the Alps, along with some clues about their distribution and behavior, that the ALPALGA collaboration intends to build upon in its future work. Their efforts can help to predict how these key species, and the ecosystems they support, will react to warming global temperatures in the coming decades in all of the environments they inhabit, ranging from the North to the South pole.“We are leaving the old world,” Maréchal said. “For me, it’s sad, because I love the mountains with the snow. It’s like I'm nostalgic in advance, because it's disappearing—in my lifetime, I will see it disappearing. So, I’m trying to capture, in my memory, everything that I can.”“This is also a fascinating time for a researcher, because you will also see the new world coming,” he concluded. “We don't exactly know what it will be. But we know that life will survive, so there will be new communities, new arrangements, and new organizational structures.”