An international study led by the University of Copenhagen and involving the Alfred Wegener Institute has discovered an important phenomenon under the Arctic sea ice that was previously thought to be impossible: So-called non-cyanobacteria can also fix nitrogen under Arctic conditions, which in turn could have an impact on the food web and the carbon budget in the cold north, as the researchers now report in the journal Communications Earth and Environment.
The shrinking sea ice in the Arctic Ocean is, overall, a disaster. But paradoxically, the melting of the ice can also fuel the engine of the Arctic food chains: algae. Algae are the main food source for life in the sea, but they need nitrogen to grow. And nitrogen is in short supply in the Arctic Ocean. However, a new international study led by the University of Copenhagen and involving the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) shows that nitrogen is added through a process previously thought not to take place below the Arctic sea ice. This could change the future prospects for marine life in the High North and possibly for the carbon budget.
The researchers are the first to discover that the phenomenon of nitrogen fixation occurs beneath sea ice even in the central Arctic Ocean. Nitrogen fixation is a process in which special bacteria convert nitrogen gas (N2) dissolved in seawater into ammonium. Ammonium helps the bacteria to grow, but it also benefits algae and the rest of the food web in the sea. Until now, the state of science was 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.
Whereas in most other oceans it is cyanobacteria that perform nitrogen fixation, the study shows that in the Central Arctic Ocean it is a completely different type of bacteria that converts nitrogen: the so-called non-cyanobacteria. During an expedition on board the German research icebreaker Polarstern the researchers have measured the highest rates of nitrogen fixation at the ice edge, where the ice melts most actively. Although the bacteria can perform nitrogen fixation under the ice, it is easier for them to do so along the ice edge. So as the sea ice retreats and the area of melting expands, larger amounts of nitrogen are expected to be added through nitrogen fixation.
This would mean that the addition of available nitrogen in the Arctic Ocean has 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. Because algae are the primary food source for small animals such as planktonic crustaceans, which in turn are eaten by small fish, more algae can end up affecting the entire food web.
In addition, the newly discovered source of nitrogen could also be beneficial for the uptake of CO2 – at least regionally. More algae make the ocean better at absorbing 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. But biological systems are very complex, so it is hard to make firm predictions, because other mechanisms may pull in the opposite direction. Nevertheless, the researchers believe that nitrogen fixation should be included in forecasts for the Arctic Ocean.
It is still unclear whether the net effect will be beneficial for the climate. But it is clear that we should include an important process such as nitrogen fixation in the equation when we try to predict what will happen to the Arctic Ocean in the coming decades as sea ice declines, according to the authors of the study.
Original publication:
Lisa W. von Friesen, Hanna Farnelid, Wilken-Jon von Appen, Mar Benavides, Olivier Grosso, Christien P. Laber, Johanna Schüttler, Marcus Sundbom, Sinhué Torres-Valdés, Stefan Bertilsson, Ilka Peeken, Pauline Snoeijs-Leijonmalm, Lasse Riemann: Nitrogen fixation under declining Arctic sea ice. Commun Earth Environ 6, 811 (2025). DOI: https://doi.org/10.1038/s43247-025-02782-4
The original news from the University of Copenhagen can be found here.