As Arctic sea ice retreats, scientists have discovered a surprising silver lining buried in an otherwise grim story. Bacteria living beneath the ice are quietly converting nitrogen gas into nutrients that fuel the bottom of the food chain, a process researchers thought was impossible in these frigid, ice-covered waters.
The finding, published today in Communications Earth & Environment, suggests the Arctic Ocean has been harboring an underestimated source of nitrogen, the nutrient that algae desperately need to grow. More algae means more food for the tiny crustaceans that feed fish, potentially reshaping predictions about the future of Arctic marine life.
“Until now, it was believed that nitrogen fixation could not take place under the sea ice because it was assumed that the living conditions for the organisms that perform nitrogen fixation were too poor,” says Lisa von Friesen, who led the study while at the University of Copenhagen.
Ice Edges Buzzing With Bacterial Activity
The researchers measured nitrogen fixation at 13 locations across the central Arctic Ocean, from waters north of Svalbard to the sea off northeast Greenland. They found the highest rates where ice was actively melting along the ice edge, areas where sunlight finally penetrates and meltwater swirls into the darker ocean below.
In these melting zones, special bacteria called non-cyanobacteria are doing the heavy lifting. Unlike their cousins in warmer oceans, Arctic nitrogen fixers are not the familiar photosynthetic cyanobacteria. Instead, they are a different crew entirely, bacteria that feed on dissolved organic matter released by algae and, in return, provide fixed nitrogen that helps algae grow.
The highest nitrogen fixation rates, up to 5.3 nanomoles per liter per day, appeared alongside an ice-edge phytoplankton bloom dominated by diatoms. The researchers found nitrogen fixation even under thick multiyear ice in the central Arctic, though at lower rates.
A Feedback Loop Between Algae and Bacteria
The relationship appears to be reciprocal. Algae release organic carbon as they grow, which feeds the nitrogen-fixing bacteria. The bacteria, in turn, release nitrogen that allows more algae to flourish. When researchers added extra dissolved organic carbon to water samples, nitrogen fixation rates increased at some stations, suggesting the bacteria were carbon-limited.
“In other words, the amount of available nitrogen in the Arctic Ocean has likely been underestimated, both today and for future projections,” von Friesen explains. As sea ice continues its retreat and the area of active melting expands, more nitrogen could become available through this bacterial process.
The team identified two key bacterial groups, dubbed Gamma-Arctic1 and Gamma-Arctic2, that were widespread across the central Arctic Ocean. Another group, Beta-Arctic1, was actively expressing the genes needed for nitrogen fixation even in nitrogen-rich waters, a somewhat counterintuitive finding that hints at the complexity of these microbial ecosystems.
The implications extend beyond the food web. More algae could mean more carbon dioxide absorbed from the atmosphere, as algae act like tiny vacuum cleaners, sucking up CO2 as they grow. Some of that carbon eventually sinks to the deep sea as algal biomass, effectively removing it from the atmosphere.
Lasse Riemann, a professor at the University of Copenhagen and senior author of the study, is cautious about making sweeping predictions. “For the climate and the environment, this is likely good news,” he says, though he notes that biological systems are complex and other mechanisms might pull in opposite directions.
What is clear, Riemann argues, is that nitrogen fixation should be factored into models forecasting the Arctic Ocean’s future. The study covered contrasting ice regimes, from thick multiyear ice to open water, providing a window into how nitrogen dynamics might shift as summers become increasingly ice-free.
The Arctic Ocean is warming up to four times faster than the global average, driving major declines in sea ice coverage, age, and thickness. Between 1998 and 2018, primary production in Arctic surface waters increased by up to 60 percent. This new study suggests those productivity gains might get an extra boost from bacterial nitrogen fixers, especially along the expanding edges where ice meets open water.
The research was based on two expeditions aboard the icebreakers IB Oden and RV Polarstern, with measurements taken from the central Arctic Ocean to the marginal ice zone north of Svalbard and waters off northeast Greenland.
Communications Earth & Environment: 10.1038/s43247-025-02782-4
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