Arctic microbes fixing nitrogen beneath sea ice could reshape climate models

Beneath the Arctic sea ice, in conditions once thought utterly hostile to life, scientists have found microbes quietly doing work that could reshape how we model the planet's climate. The discovery challenges a foundational assumption in marine biology: that nitrogen-fixing bacteria—organisms capable of converting atmospheric nitrogen into forms other life can use—belonged exclusively to warm tropical oceans. They don't.

Nitrogen makes up roughly 78 percent of Earth's atmosphere, yet most living things cannot access it directly. Nitrogen-fixing microbes solve this problem for entire ecosystems by transforming the gas into ammonia or ammonium, creating a resource that cascades through food webs. For decades, scientists assumed these crucial organisms could only thrive in warm water. The Arctic, with its darkness and cold, seemed an impossible place for them to survive.

Lisa von Friesen, a biologist at the University of Copenhagen and lead author of the research, put it plainly: "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." Her team collected samples from the Central Arctic Ocean and the Eurasian Arctic and found thriving communities of bacteria called non-cyanobacterial diazotrophs—NCDs for short. These are nitrogen-fixers that don't photosynthesize, organisms that had never before been documented beneath Arctic ice.

The researchers have not yet proven that these microbes are actively fixing nitrogen in the Arctic, only that they possess the genetic machinery to do so. But their distribution and abundance suggest they are deeply involved in the region's nitrogen-fixing activity. The finding matters because of what happens next. The fringes of Arctic sea ice host higher concentrations of these bacteria and greater nitrogen-fixing activity. As climate change melts Arctic ice, these microbes may proliferate, setting off a chain of consequences that ripples through ocean and atmosphere alike.

NCDs feed algae, and more algae means more food for small crustaceans, which feed small fish, which feed larger animals. A richer Arctic food web could emerge. But there is a second, climate-relevant consequence. Algae absorb carbon dioxide from the atmosphere as they grow. Lasse Riemann, a marine microbial ecologist, explained the mechanism: "If algae production increases, the Arctic Ocean will absorb more CO2 because more CO2 will be bound in algae biomass." Yet he was careful to note the limits of prediction. "Biological systems are very complex, so it is hard to make firm predictions, because other mechanisms may pull in the opposite direction."

What is certain, Riemann argues, is that Arctic nitrogen-fixers have been missing from climate models. The research team's conclusion is direct: "Sea ice melt may, directly or indirectly, stimulate nitrogen fixation." They call for future modeling efforts to measure the magnitude and dynamics of nitrogen fixation in the Arctic Ocean. The discovery opens a door to understanding a process that was invisible until now—one that could alter how the Arctic responds to warming, how its waters feed life, and how much carbon the region absorbs from the air.