Ice algae

Ice algae are any of the various types of algal communities found in annual and multi-year sea or terrestrial ice. On sea ice in polar regions of the oceans, ice algae communities play an important role in primary production. The timing of blooms of the algae is especially important for supporting higher trophic levels at times of the year when light is low and ice cover still exists. Sea ice algal communities are mostly concentrated in the bottom layer of the ice, but can also occur in brine channels within the ice, in melt ponds, and on the surface.

Because terrestrial ice algae occur in freshwater systems, the species composition differs greatly from that of sea ice algae. These communities are significant in that they often change the color of glaciers and ice sheets, impacting the reflectivity of the ice itself.

Microbial life in sea ice is extremely diverse. Dominant species vary based on location, ice type, and irradiance. In general, pennate diatoms such as Nitschia frigida (in the Arctic) and Fragilariopsis (in the Antarctic) tend to dominate. Melosira arctica, which forms up to meter-long filaments attached to the bottom of the ice, are also widespread in the Arctic and are an important food source for marine species. Sea ice algae communities are found throughout the column of sea ice. Algae make their way into the sea ice from the ocean water during the formation of frazil ice, the first stage of sea ice formation, when newly formed ice crystals rise to the surface, bringing with them micro-algae, protists, and bacteria. Algae can be found within brine channels that form when seawater freezes and creates a matrix of tiny veins and pores containing concentrated brine and air bubbles. Sea ice algal communities can also thrive at the surface of the ice, in surface melt ponds, and in layers where rafting has occurred. In melt ponds, dominant algal types can vary with pond salinity, with higher concentrations of diatoms being found in melt ponds with higher salinity. Because of their adaption to low light conditions, the presence of ice algae (in particular, vertical position in the ice pack) is primarily limited by nutrient availability. The highest concentrations are found at the base of the ice because the porosity of that ice enables nutrient infiltration from seawater. To survive in the harsh sea ice environment, organisms must be able to endure extreme variations in salinity, temperature, and solar radiation. Algae living in brine channels can secrete osmolytes, such as dimethylsulfoniopropionate (DMSP), which allows them to survive the high salinities in the channels after ice formation in the winter, as well as low salinities when the relatively fresh meltwater flushes the channels in the spring and summer.

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