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.
Some sea ice algae species secrete ice-binding proteins (IBP) as a gelatinous extracellular polymeric substance (EPS) to protect cell membranes from damage from ice crystal growth and freeze thaw cycles. EPS alters the microstructure of the ice and creates further habitat for future blooms. Surface-dwelling algae produce special pigments to prevent damage from harsh ultraviolet radiation. Higher concentrations of xanthophyll pigments act as a sunscreen that protects ice algae from photodamage when they are exposed to damaging levels of ultraviolet radiation upon transition from ice to the water column during the spring. Algae under thick ice have been reported to show some of the most extreme low light adaptations ever observed. Extreme efficiency in light utilization allows sea ice algae to build up biomass rapidly when light conditions improve at the onset of spring.
Ice algae play a critical role in primary production and serve as part of the base of the polar food web by converting carbon dioxide and inorganic nutrients to oxygen and organic matter through photosynthesis in the upper ocean of both the Arctic and Antarctic. Within the Arctic, estimates of the contribution of sea ice algae to total primary production ranges from 3-25%, up to 50-57% in high Arctic regions. Sea ice algae accumulate biomass rapidly, often at the base of sea ice, and grow to form algal mats that are consumed by amphipods such as krill and copepods. Ultimately, these organisms are eaten by fish, whales, penguins, and dolphins. When sea ice algal communities detach from the sea ice they are consumed by pelagic grazers, such as zooplankton, as they sink through the water column and by benthic invertebrates as they settle on the seafloor. Sea ice algae as food are rich in polyunsaturated and other essential fatty acids, and are the exclusive producer of certain essential omega-3 fatty acids that are important for copepod egg production, egg hatching, and zooplankton growth and function.
The timing of sea ice algae blooms has a significant impact on the entire ecosystem. Initiation of the bloom is primarily controlled by the return of the sun in the spring (i.e. the solar angle). Because of this, the ice algae bloom usually occurs before the blooms of pelagic phytoplankton, which require higher light levels and warmer water. Early in the season, prior to the ice melt, sea ice algae constitute an important food source for higher trophic levels. However, the total percentage that sea ice algae contribute to the primary production of a given ecosystem depends strongly on the extent of ice cover. The thickness of snow on the sea ice also affects the timing and size of the ice algae bloom by altering light transmission. This sensitivity to ice and snow cover has the potential to cause a mismatch between predators and their food-source, sea ice algae, within the ecosystem. This so called match/mismatch has been applied to a variety of systems. Examples have been seen in the relationship between zooplankton species, which rely on sea ice algae and phytoplankton for food, and juvenile walleye pollock in the Bering Sea.
Climate change and warming of Arctic and Antarctic regions have the potential to greatly alter ecosystem functioning. Decreasing ice cover in polar regions is expected to lessen the relative proportion of sea ice algae production to measures of annual primary production. Thinning ice allows for greater production early in the season but early ice melting shortens the overall growing season of the sea ice algae. This melting also contributes to stratification of the water column that alters the availability of nutrients for algae growth by decreasing the depth of the surface mixed layer and inhibiting the upwelling of nutrients from deep waters. This is expected to cause an overall shift towards pelagic phytoplankton production. Because sea ice algae are often the base of the food web, these alterations have implications for species of higher trophic levels. The reproduction and migration cycles of many polar primary consumers are timed with the bloom of sea ice algae, meaning that a change in the timing or location of primary production could shift the distribution of prey populations necessary for significant keystone species.
The production of DMSP by sea ice algae also plays an important role in the carbon cycle. DMSP is oxidized by other plankton to dimethylsulfide (DMS), a compound which is linked to cloud formation. Because clouds impact precipitation and the amount of solar radiation reflected back to space (albedo), this process could create a positive feedback loop. Cloud cover would increase the insolation reflected back to space by the atmosphere, potentially helping to cool the planet and support more polar habitats for sea ice algae. As of 1987, research has suggested that a doubling of cloud-condensation nuclei, of which DMS is one type, would be required to counteract warming due to increased atmospheric CO2 concentrations. 
Sea ice plays a major role in the global climate. Satellite observations of sea ice extent date back only until the late 1970s, and longer term observational records are sporadic and of uncertain reliability. While terrestrial ice paleoclimatology can be measured directly through ice cores, historical models of sea ice must rely on proxies.
Organisms dwelling on the sea ice eventually detach from the ice and fall through the water column, particularly when the sea ice melts. A portion of the material that reaches the seafloor is buried before it is consumed and is thus preserved in the sedimentary record.
There are a number of organisms whose value as proxies for the presence of sea ice has been investigated, including particular species of diatoms, dinoflagellate cysts, ostracods, and foraminifers. Variation in carbon and oxygen isotopes in a sediment core can also be used to make inferences about sea ice extent. Each proxy has advantages and disadvantages; for example, some diatom species that are unique to sea ice are very abundant in the sediment record, however, preservation efficiency can vary.
Algae also occur on terrestrial ice sheets and glaciers. The species found in these habitats are distinct from those associated with sea ice because the system is freshwater. Even within these habitats, there is a wide diversity of habitat types and algal assemblages. For example, cryosestic communities are specifically found on the surface of glaciers where the snow periodically melts during the day. On enduring ice sheets and snow pack, terrestrial ice algae often color the ice due to accessory pigments, popularly known as "watermelon snow".
Recent research, known as the Black and Bloom project, has shown the impact of ice algae on the rate of melting of ice sheets. Because of the dark colors of the algae, sunlight absorption by the ice is enhanced, leading to an increase in the melting rate. The goal of the Black and Bloom project is to determine how much the algae are contributing to the darkening of the ice sheets.