Microbial Surprise Under the Ice May Change How We Model the Arctic
A new study led by researchers at the University of Copenhagen has uncovered unexpected nitrogen fixation occurring beneath melting Arctic sea ice — a process that could increase algal growth, strengthen local food webs and alter how much CO2 the Arctic Ocean can absorb.
Published in Communications Earth & Environment, the study examined nitrogen fixation, the microbial conversion of atmospheric nitrogen gas into forms usable by algae. The research team — led by postdoctoral marine ecologist Lisa W. von Friesen with senior author Lasse Riemann — found that declining ice cover is linked with elevated nitrogen-fixation activity in areas beneath and along the edges of sea ice.
What the Researchers Found
Contrary to prior assumptions that conditions under sea ice were too harsh for nitrogen-fixing microbes, the team detected active fixation by non-cyanobacterial diazotrophs. These microbes appear to consume dissolved organic matter produced by expanding algal communities and, in turn, release fixed nitrogen that can support further algal production — especially near ice margins.
"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," said Lisa W. von Friesen.
Potential Impacts and Uncertainty
An increase in nitrogen availability could stimulate more frequent or larger algal blooms. More algal biomass can strengthen the Arctic marine food web and potentially increase biological uptake of CO2, temporarily drawing more carbon into the ocean ecosystem.
However, the authors caution that biological systems are complex. As Professor Lasse Riemann noted, "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." Factors such as remineralization, food-web transfers, and changing physical conditions could offset any carbon gain.
What This Means for Climate Modeling
The study suggests that current Arctic biogeochemical and climate models may underestimate available nitrogen and, therefore, primary production under future reduced ice cover. The authors recommend including nitrogen-fixation processes in projections of Arctic ecosystem responses as sea ice declines.
Further research is needed to quantify how widespread and persistent sub-ice nitrogen fixation will be, how it scales seasonally and regionally, and whether the net effect on atmospheric CO2 is beneficial or neutral over longer timescales.
Bottom line: The discovery changes assumptions about life under Arctic ice and highlights an important, previously underappreciated biogeochemical pathway that could influence future ecosystem productivity and carbon cycling in a warming Arctic.
