But regardless of whether or not the disappearing ice will mean new challenges for the polar cod, the most central role for survival in the Arctic and Antarctic is played by the “ice algae” – types of algae that live in sea ice or on its underside. What makes them so special? According to AWI biologist Ilka Peeken: “Like everywhere else in the world, in the waters of the Arctic and Antarctic there are algae that float freely in the water as phytoplankton, but as long as the water is covered by a layer of ice, they lack the light they need to grow. In contrast, ice algae can thrive in the twilight under the ice.”
Like forest ferns, which can flourish in the shadows of trees, the single-celled ice algae can get by with only minimal light. And each one can divide every two days, which means a mass of algae weighing 200 grams can grow to a kilogram in just four days. In large numbers they can form veritable carpets on the underside of the ice, offering ideal feeding grounds for amphipods, copepods, krill and other animals.
Living in the half-light under the ice also has its advantages for the ice algae: though there may be less light under the ice floes of the Arctic and Antarctic, they enjoy relatively constant access to what light there is, because, unlike the phytoplankton in open water, there are no currents or waves to pull the algae into deeper (and darker) waters.
Further, ice algae aren’t exclusively found on the underside of the ice. AWI researchers have also discovered them in the sea ice itself, in the minute, porous passages and channels that form when seawater freezes. Once the first ice crystals form, they no longer absorb any salt from the water. Instead, the salt gathers between the crystals and forms the passages and channels mentioned above, before ultimately trickling out the underside and back to the sea.
But how do the ice algae get in these brine channels to begin with? With the first freeze, they cling to the channels, braving both the extreme cold and the high salinity. For comparison: sea-ice brine can be up to six times saltier than North Sea water. But that doesn’t seem to bother the algae: AWI biologist Ilka Peeken estimates that more than a thousand different species of single-cell algae and related single-cell organisms inhabit this extreme environment.