Algae, commonly recognized as green growths in ponds or aquariums, are simple organisms fundamental to aquatic ecosystems. While often perceived to thrive only in warm, sunlit conditions, certain types of algae can grow in extremely cold environments, even under ice and snow. These specialized algae possess adaptations that allow them to survive temperatures well below freezing.
Algae That Thrive in the Cold
Organisms capable of growing and reproducing in low temperatures are known as psychrophiles, and a diverse group of algae falls into this category. These cold-loving algae inhabit Earth’s most frigid regions, from snowfields to polar oceans. Snow algae, such as Chlamydomonas nivalis, are found on snow and ice surfaces in alpine and polar areas. They create “watermelon snow” due to the red pigments they produce, coloring vast expanses of snow.
Sea ice algae thrive within and beneath the sea ice in the Arctic and Antarctic. These communities are concentrated in the bottom layers of the ice, and also exist within tiny, super-salty brine channels and melt ponds. Polar ocean algae, primarily phytoplankton, also flourish in the frigid waters of the Arctic and Antarctic. These microscopic organisms, including various diatoms, form the base of the food web in these icy marine environments, growing even under 100% ice cover.
How Algae Survive Frigid Conditions
Cold-adapted algae employ biological mechanisms to survive in sub-zero temperatures. One adaptation involves producing antifreeze proteins (AFPs) or similar compounds. These substances bind to ice crystals, preventing their growth and recrystallization. This keeps the cell’s internal fluids from freezing solid, protecting cellular structures and ensuring the algae’s cellular machinery remains functional.
Pigment adaptations also play a role in their survival. Many cold-water algae, particularly snow algae, produce secondary pigments like red carotenoids, such as astaxanthin. These pigments act as a natural sunscreen, shielding the algae from intense ultraviolet (UV) radiation reflected by snow and ice. They also absorb different wavelengths of light, optimizing photosynthesis in low-light conditions under ice or snow. The dark pigmentation can absorb solar energy, which helps melt surrounding ice to create liquid water necessary for growth.
To maintain cellular function in the cold, algae adjust the fluidity of their cell membranes. At low temperatures, cell membranes can become rigid, impairing cellular processes. Cold-adapted algae modify the lipid composition of their membranes, often increasing unsaturated fatty acids, which helps maintain a flexible and permeable membrane structure. These algae also exhibit metabolic adjustments, including enzymes that remain efficient at low temperatures, sometimes by increasing their abundance to compensate for reduced activity.
The Widespread Impact of Cold-Water Algae
Cold-water algae are components of polar and high-altitude ecosystems, serving as primary producers at the base of food webs. In the Arctic and Antarctic, these microscopic organisms are the initial energy source, sustaining grazers like zooplankton and krill, which support fish, seals, and large marine predators such as polar bears. Their productivity is important for the survival of wildlife in these harsh environments.
These algae also contribute to the global carbon cycle. Through photosynthesis, they absorb carbon dioxide from the atmosphere, even in extreme cold, converting it into organic matter. This carbon sequestration helps regulate Earth’s climate system. The presence of dark-pigmented algae on snow and ice surfaces can also influence the ice-albedo effect.
When algae darken snow or ice, they reduce its reflectivity, causing more solar radiation to be absorbed. This absorption can accelerate the melting of snow and ice, creating a positive feedback loop where increased melting promotes further algal growth and more darkening. Understanding these dynamics is important for climate models, as melting glaciers and ice sheets contribute to sea level rise. Cold-water algae also play a role in nutrient cycling, helping to process and recycle nutrients within these unique, often nutrient-limited, environments.