Sea ice algae are microscopic, plant-like organisms that flourish within and under polar sea ice. Often appearing as a brownish-green layer, these organisms are a form of phytoplankton adapted to one of Earth’s most challenging environments. As autotrophs, they produce their own food through photosynthesis, making them a foundational component of polar ecosystems, with their presence most noticeable during extensive spring blooms triggered by increasing sunlight.
The Unique Habitat of Sea Ice Algae
Sea ice algae colonize various microhabitats within the structure of sea ice. The most populous communities are found in the bottom 20 centimeters of the ice where it meets the ocean, benefiting from stable conditions and access to seawater nutrients. The physical architecture of the ice, whether free-floating pack ice or land-fast ice, provides a scaffold for life. The algal community’s composition can differ depending on the ice type and its location.
A network of brine channels and pockets forms as the ice freezes. These interconnected liquid veins contain highly saline water and serve as protected niches for the algae. Algae can also establish themselves in internal layers of the ice, within loose frazil ice, or on the ice surface during the spring and summer melt. The availability of light and nutrients varies between these microhabitats, influencing the distribution and density of algal populations.
Algae become incorporated into the ice as it forms. As seawater freezes, ice crystals sieve algal cells from the water column into the growing ice sheet. This initial “seeding community” can originate from organisms in the water, on the seafloor in shallow areas, or from older multi-year ice. The makeup of the algal community in new ice is influenced by the species present during the autumn freeze-up.
Survival in Extreme Conditions
To survive the polar environment, sea ice algae have developed several adaptations. A primary challenge is the low temperature, so many species produce cryoprotectant compounds like extracellular polymeric substances (EPS) to prevent freezing in sub-zero brine channels. These biopolymers act as an antifreeze, inhibiting the formation of large ice crystals and keeping the brine channels liquid.
Surviving inside the ice means contending with low light, especially during the polar winter or under thick snow. Sea ice algae possess efficient light-harvesting systems with pigments tailored to capture the minimal light that penetrates the ice and snow. They can photosynthesize at very low light intensities, allowing them to grow early in the spring. Some species can also enter a dormant state or consume organic matter to survive prolonged darkness.
The brine channels that the algae inhabit are hypersaline, as ice formation leaves behind concentrated salt solutions. To cope with this high salinity, sea ice algae use osmoregulation. They manage the concentration of solutes inside their cells to balance the osmotic pressure with their external environment, preventing water loss. This adaptation allows them to thrive in conditions that are lethal to many other forms of life.
Ecological Significance in Polar Ecosystems
Sea ice algae function as primary producers, and their production is especially important in early spring when ice cover still limits phytoplankton growth in the water below. The algae create concentrated blooms, providing an early-season food source that fuels the entire ecosystem after the dark winter months. This makes them a foundational component of polar marine food webs.
This concentrated algal biomass is a direct food source for primary consumers. Zooplankton, such as copepods and krill, graze heavily on the algae on the underside of the ice. These small crustaceans are then consumed by larger animals like Arctic cod and Antarctic silverfish, which in turn become prey for seals, penguins, and whales. The timing of the algal bloom is a major event for the survival of many polar species.
Beyond the food web, sea ice algae contribute to broader biogeochemical cycles. Their photosynthetic activity draws down carbon dioxide from the atmosphere, incorporating it into organic matter. When the algae die and sink, this carbon is transported to the deep ocean, helping to regulate the global climate. Some sea ice algae also produce dimethyl sulfide (DMS), a compound that can act as a nucleus for cloud condensation, influencing regional weather patterns.
Sea Ice Algae and Climate Change
Climate change threatens sea ice algae by altering their habitat. The primary impact comes from the rapid decline in the extent, thickness, and duration of sea ice in polar regions. Less ice means a direct loss of the habitat these algae depend on for growth. Thinner ice may allow more light to penetrate, but it is also less stable and melts earlier in the season.
Changes in snow cover also have a profound effect. While less snow can increase light availability, an increase in rain-on-snow events can create icy layers that block light. The timing of the spring melt is another concern. An earlier melt can cause a mismatch between the algal bloom and the life cycles of zooplankton that depend on them for food, altering the polar food web.
The cascading consequences of these changes extend throughout the ecosystem. A reduction in the food source provided by ice algae can lead to declines in populations of zooplankton, fish, seabirds, and marine mammals. Alterations in the timing and magnitude of the algal bloom also affect carbon cycling and the production of compounds like DMS. As polar regions continue to warm, these communities of sea ice algae face an uncertain future, with significant implications for the entire polar marine environment.