What started off as an unexpected find in the 1800s could now have big implications for climate.
Captain John Ross, a British Royal Navy officer and Polar explorer, embarked on his first polar expedition in 1818. The goal? To find the Northwest Passage, a way to the Pacific Ocean from the Atlantic by crossing the Arctic Ocean.
While Ross and his crew had to turn back early due to severe weather, he found something significant off the shores of Greenland in Baffin’s Bay: pink snow known as “watermelon snow” strewn across the ground, streaky and of various shades.
(This wasn’t the first time humans have encountered watermelon snow, but it was probably what led to the identification of the life behind the red tinged snow.)
For the next century, scientists debated what the tiny organisms behind watermelon snow were: were they lichen, plant, alga, or animal? It wasn’t until the early 20th century when scientists finally came to consensus that the organism was of an algal nature and named it Chlamydomonas nivalis, Latin for “found growing in or near snow.”
Today C. nivalis, which has been found in snowfields in the Alps and polar regions, has become one of the most well known and well studied snow algae. But there are many other microorganisms that dwell in snowfields.
In 2016, a team of scientists led by Stefanie Lutz, from the University of Leeds and the GFZ German Research Center for Geosciences, studied 40 red snow sites and 15 glaciers and snow fields acquiring the first large-scale biogeographical data set for red snow. They found snow communities contained a wealth of different microbes.
Bacterial species from the snow varied dramatically from location to location. When the team looked at snow algae however, they saw that over 99% of the algal community came from six taxa in their environments tested.
The colors of snow algae
So how does snow algae influence climate? To understand this, let’s first look at the algae’s three stage life cycle using C. nivalis as an example.
C. nivalis has a three stage life cycle that’s somewhat reflected by the color of the cell: green in the spring and early summer, orange in summer, and red in the fall and winter. These colors are a result of changes in carotenoid composition within the cell as seasons change.
Spring and summer are times of maximal growth for C. nivalis. And as a photosynthetic algae, what does C. nivalis need to achieve this? Lots of chlorophyll. Thus, during these warmer times, the green pigment of chlorophyll dominates the other pigments.
As winter approaches, C. nivalis begins to prepare for the harsh temperatures and increased UV radiation that’s to come. They begin to produce and accumulate sugars, lipids, and secondary carotenoids which absorb UV light as a protective mechanism. These carotenoids happen to be orange or red in color with thickened cell walls.
From the snow samples from Lutz’s study, one milliliter of melted snow contained between 103 and 104 red pigmented cells and all samples contained an abundance of secondary carotenoids (~70-90% of total pigment).
How snow algae affect climate
It is precisely these secondary carotenoids that affect climate because of something known as albedo. Albedo is the measure of sunlight that is reflected by a surface. Clean white snow has high albedo, reflecting sunlight away and dissipating heat. Dirty snow, on the other hand, has low albedo and absorbs more sunlight, thus trapping heat and encouraging snow melt.
By comparing albedo measurements on red snow versus that of algae-free snow, Lutz and colleagues estimated that microbial darkening of snow reduced albedo by about 13% over a 100 day period. They also found a significant negative correlation between albedo and algal biomass: more algae meant lower albedo.
A year later, another group of researchers reported similar results: microbial communities in snow were responsible for 17% of the snowmelt in a 1,900 km2 icefield in southern Alaska.
This team also analyzed the effects of meltwater on algal blooms. Glacial meltwater often contain nitrogen and phosphorus, which serves as nutrients for the algae. Adding nitrogen-phsphorous-potassium fertilizer to the red snow resulted in a fourfold increase in alga cell counts while the addition of water alone increased counts by a half. In a 100 day study, they also found that sites enriched with algae growth melted much faster than sites without algae.
As the planet warms, melting snow can trigger a self-perpetuating feedback loop. As temperature rises, snowmelt occurs more quickly, which leads to more algae growth. That growth lowers albedo even further, melts even more snow, and puts these environments at risk of accelerated melting.
While snow microbes speed up melting of glaciers and snowfields, they cannot be blamed as a cause of climate change. They’re only another symptom.
Further reading:
The Living Snow Project. Kodner Lab.
Why the Last Snow on Earth May Be Red. The New Yorker. 2017.
