An international team of researchers has discovered that nitrogen-fixing bacteria thrive beneath Arctic sea ice, challenging previous assumptions about nutrient availability in the region. This process, driven by non-cyanobacteria, could boost algae growth and enhance the ocean's CO2 absorption as ice melts. The findings suggest a need to revise models of Arctic ecological changes amid climate warming.
The rapid decline of Arctic sea ice, often viewed as an ecological disaster, may unexpectedly support marine life through newly identified nutrient processes. Researchers, led by the University of Copenhagen, confirmed for the first time that nitrogen fixation occurs beneath the ice, even in remote central areas. This involves bacteria converting nitrogen gas in seawater into ammonium, a nutrient essential for algae growth at the base of the food web.
"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. We were wrong," says Lisa W. von Friesen, lead author and former PhD student at the Department of Biology.
Unlike other oceans where cyanobacteria dominate this process, the Arctic relies on non-cyanobacteria bacteria. The highest fixation rates were observed along the ice edge, where melting is most intense. These bacteria consume dissolved organic matter from algae and produce fixed nitrogen in return, creating a nutrient loop. As climate change reduces ice cover, this expanding melt zone could increase nitrogen availability, potentially leading to more algae production.
"In other words, the amount of available nitrogen in the Arctic Ocean has likely been underestimated, both today and for future projections. This could mean that the potential for algae production has also been underestimated as climate change continues to reduce the sea ice cover," von Friesen explains. She adds that greater algae could ripple through the food chain, benefiting plankton, fish, and larger marine species.
The discovery also holds implications for carbon cycling. More algae would enhance photosynthesis, allowing the ocean to absorb additional CO2. "For the climate and the environment, this is likely good news. If algae production increases, the Arctic Ocean will absorb more CO2 because more CO2 will be bound in algae biomass," says Lasse Riemann, senior author and professor at the Department of Biology. However, he cautions that biological complexities make firm predictions difficult, and models must now incorporate nitrogen fixation to forecast Arctic changes accurately.
The study draws from two expeditions on icebreakers IB Oden and RV Polarstern, collecting samples at 13 sites across the central Arctic Ocean, including areas off northeast Greenland and north of Svalbard. It involved collaborators from institutions in Denmark, Sweden, Germany, France, the UK, and elsewhere, and was published in Communications Earth & Environment in 2025.