Ice algae are microscopic organisms that inhabit frozen environments, from sea ice to glaciers. These tiny life forms are capable of photosynthesis, transforming light and inorganic nutrients into organic matter. Their presence highlights life’s adaptability in some of the planet’s harshest conditions, and they form a foundational component of polar ecosystems.
Defining Ice Algae
Ice algae encompass various algal communities found within sea ice, lake ice, or glacier ice. These single-celled microalgae are primary producers, meaning they generate their own food through photosynthesis, converting carbon dioxide and inorganic nutrients into oxygen and organic matter. They are generally autotrophic, but some can also be mixotrophic, obtaining energy by consuming other algae and bacteria, especially when light or nutrients are scarce.
Diatoms constitute the vast majority of all ice algae. Other groups include flagellates and dinoflagellates. These diverse communities can lend a characteristic brownish-green hue to the sea ice.
Life in the Ice
Ice algae primarily thrive in the bottom layer of sea ice, where the ice interacts with the ocean below. They also inhabit brine channels, which are super-salty pockets of liquid trapped within the ice, as well as melt ponds and the ice surface. These environments present extreme challenges, including very low temperatures, high salinity, and limited light availability.
Ice algae have developed remarkable adaptations to survive these conditions. They can produce specialized pigments, which act like a sunscreen to protect them from high ultraviolet radiation when exposed to light during spring melt. Some species secrete ice-binding proteins (IBP), which shield cell membranes from damage caused by ice crystal growth and freeze-thaw cycles.
Algae can also adjust their internal pigment content, increasing chlorophyll a in winter to absorb more light, and some diatoms can enter a sleep-like state, forming thick-walled cysts to survive months or even years of darkness. They also produce osmolytes to manage extreme shifts in salinity. These adaptations allow them to perform photosynthesis even with as little as 0.02% of surface light.
Important Role in Polar Ecosystems
Ice algae form the base of the polar food web, converting carbon dioxide and inorganic nutrients into organic matter through photosynthesis. Their contribution to total primary production in sea-ice-covered marine areas can range from 2% to 24%, and in high Arctic regions, it can be as high as 50% to 57%. This rapid accumulation of biomass, often forming algal mats at the base of the sea ice, provides an important early-season food source for various marine organisms.
Organisms like amphipods, krill, and copepods consume these algal mats, transferring the energy up the food chain. Traces of ice algae have been found in 96% of species studied across the Arctic food web, from tiny filter-feeding shrimp to large predators like whales, highlighting their widespread importance. Ice algae are rich in polyunsaturated and other fatty acids, including omega-3 fatty acids, which are important for zooplankton growth and reproduction. Beyond their role as food, ice algae also contribute to nutrient cycling by accumulating macronutrients and iron, which are released when the ice melts, stimulating further production in the ecosystem.
Responding to a Warming World
Climate change is impacting ice algae populations, with implications for polar ecosystems. The rapid thinning and decline of sea ice coverage reduces the habitat available for ice algae to grow. While initial thinning of ice and snow might allow more light to penetrate, potentially enhancing under-ice algal production in some areas, the overall loss of ice coverage is a major concern.
The melting of sea ice and glaciers also introduces more freshwater into the ocean, which can create a layer of less dense, nutrient-depleted water at the surface. This stratification hinders the mixing of nutrients from deeper layers, limiting the growth of plankton algae that rely on these nutrients. The reduction in ice algae due to these changes could have cascading effects throughout the polar food web, affecting the reproduction and migration cycles of many species that rely on this early-season food source. Ice algae also contribute to the ocean’s ability to absorb carbon dioxide, and their decline could impact the global carbon cycle. Some research suggests that certain ice algae can darken the ice as it melts, potentially accelerating further melting, creating a feedback loop in climate change.