As the Arctic Ocean loses its sea ice due to climate change, sunlight penetrates deeper into the water and encourages the growth of tiny plant-like organisms (phytoplankton). But to thrive, they need nitrogen, a key nutrient. Think of it like fertilizer for ocean life.
Nitrogen enters the Arctic in several ways: through rivers carrying nutrients from the land, through particles falling out of the atmosphere, and through ocean currents flowing in from the Atlantic and Pacific.
Until recently, scientists mostly studied these nitrogen sources using chemical and physical methods. But now, they're paying attention to microbes, invisible workers that help recycle nitrogen. Some microbes have been found to convert ammonia into usable forms through processes such as nitrification.
But even more exciting is the discovery of nitrogen fixation, a process in which certain microbes called diazotrophs convert atmospheric nitrogen gas into ammonium, a form phytoplankton can use. This process wasn't thought to happen in the Arctic, but new evidence suggests it might, offering a fresh supply of nitrogen to fuel life.
To understand how how much life the Arctic Ocean may support in the future, scientists need to study the magnitude and environmental regulation of nitrogen fixation, as well as the distribution, activity, and metabolic functions of key diazotrophs.
A global team of scientists, led by the University of Copenhagen, recently uncovered something remarkable beneath the Arctic sea ice. By studying diazotroph composition and expression at 12 research stations – five in the Central Arctic's thick, multiyear ice and seven in the Eurasian Arctic's thinner, seasonal ice – they measured nitrogen fixation rates and diazotroph composition and expression across different stages of declining sea ice in the Central Arctic Ocean.
What they found was unexpected: nitrogen fixation was happening under Arctic sea ice, something scientists didn't think could happen in such cold, low-nutrient waters. This discovery changes how we understand life and its survival in the Arctic Ocean.
Tiny algae are the ocean's chefs; they cook up the energy that feeds nearly all marine life. But to whip up their meals, they need nitrogen, and in the Arctic Ocean, that ingredient has been in short supply.
This new study brings hopeful news: there may be more nitrogen in the Arctic's future than scientists once believed. That means algae could grow in larger volumes, feeding more creatures and even possibly shifting how carbon moves through the ocean.
"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," said Lisa von Friesen, lead author of the study. "We were wrong."
In warmer oceans closer to the equator, nitrogen is mostly fixed by a group of microbes called cyanobacterial diazotrophs. But in the icy heart of the Central Arctic Ocean, scientists have found a surprising twist: a different kind of bacteria, called non-cyanobacterial diazotrophs (NCDs), are doing the job instead.
The study is based on two scientific cruises in different regions of the Arctic Ocean. Across those two research missions scientists found that the highest levels of nitrogen fixation happen at the ice edge, the zone where sea ice is melting most actively. While some bacteria can perform nitrogen fixation beneath the ice, they work more efficiently along this melting boundary. As climate change causes sea ice to retreat and the melting zone to expand, these microbes are expected to add more nitrogen to the ocean.
In the Central Arctic Ocean, scientists found that greater nitrogen fixation led to greater primary production, in other words more food made by tiny algae. The rates ranged from low to moderate, between 0.4 and 2.5 nanomoles of nitrogen per liter per day.
But at the marginal ice zone, where melting ice meets open water, things got livelier. Nitrogen fixation varied from day to day and across ice conditions, sometimes reaching 5.3 nanomoles per liter per day, especially during phytoplankton blooms near the ice edge.
According to von Friesen this means we have likely underestimated future nitrogen projections.
"This could mean that the potential for algae production has also been underestimated as climate change continues to reduce the sea ice cover," added von Friesen. "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 chain."
In addition, the newly discovered nitrogen source could also be beneficial for CO2 uptake, at least regionally. More algae make the ocean better at absorbing CO2.
"For the climate and the environment, this is likely good news," noted senior author Lasse Riemann. "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."
The study is published in the journal Communications Earth & Environment.
Source: University of Copenhagen
