A Hidden Driver of Ocean Productivity
As the Arctic Ocean warms and sea ice retreats, scientists are uncovering surprising biological processes that could reshape our understanding of marine ecosystems. A recent study published in Communications Earth & Environment reveals that nitrogen fixation, a process once thought absent in the Arctic, is actively occurring beneath sea ice and in marginal ice zones, potentially fuelling primary production in ways previously overlooked.
What Is Nitrogen Fixation and Why Does It Matter?
Nitrogen fixation is the conversion of atmospheric nitrogen (N₂) into bioavailable forms like ammonium (NH₄⁺), carried out by specialized microorganisms called diazotrophs. In nutrient-limited environments like the Arctic, this process can provide a crucial source of nitrogen for phytoplankton, which form the base of the marine food web and drive carbon sequestration.
Key Findings
• Active Nitrogen Fixation Across Ice Regimes: The researchers measured nitrogen fixation rates across 13 stations in the Central Arctic Ocean (CAO), the Eurasian marginal ice zone (MIZ), and land-fast ice near Greenland. Rates ranged from 0.4 to 5.3 nmol N L⁻¹ d⁻¹, with the highest values observed near melting sea ice and ice-edge phytoplankton blooms.
• Non-Cyanobacterial Diazotrophs (NCDs) Dominate: Unlike tropical oceans where cyanobacteria are the main nitrogen fixers, the Arctic is dominated by heterotrophic or mixotrophic NCDs, particularly Gammaproteobacteria and Betaproteobacteria. These microbes rely on organic carbon and thrive in environments rich in dissolved organic matter (DOM) and particulate organic matter (POM).
• Sea Ice Melt Stimulates Fixation: Melting sea ice appears to enhance nitrogen fixation indirectly by promoting phytoplankton blooms, which release labile DOM. This creates favourable conditions for NCDs, especially in regions like the Wandel Sea and the Yermak Plateau.
• DOC Amendments Boost Fixation: Experimental additions of dissolved organic carbon (DOC) increased nitrogen fixation rates at several stations, confirming that carbon availability is a key regulator of microbial nitrogen fixation in the Arctic.
• Limited but Crucial Contribution to Primary Production: Although nitrogen fixation contributed less than 1% to primary production in most MIZ stations, it reached up to 8.6% in decaying multiyear ice zones, suggesting that under certain conditions, it plays a significant role in supporting Arctic productivity.
Implications for a Changing Arctic
As climate change accelerates the loss of sea ice, the Arctic Ocean is transitioning toward a more seasonally ice-free state. This shift alters light penetration, stratification, and nutrient dynamics, potentially expanding the niche for nitrogen-fixing microbes. The study suggests that future Arctic productivity may increasingly depend on microbial nitrogen fixation, especially by NCDs associated with phytoplankton blooms.
Moreover, the presence of alternative nitrogenases (e.g., iron-only and vanadium-based enzymes) in some diazotrophs hints at adaptive strategies for nitrogen fixation under cold and variable conditions, warranting further investigation.
This research challenges long-held assumptions about nitrogen cycling in polar oceans. By documenting active nitrogen fixation under sea ice and identifying key microbial players, it opens new avenues for understanding Arctic biogeochemistry and forecasting ecosystem responses to climate change.
Sources and Further Reading
• Nature Communications Earth & Environment
• PANGAEA Data Repository
• NCBI Sequence Archive: PRJNA995422
• Figshare Supplementary Data
• University of Copenhagen
Communications Earth & Environment
University of Copenhagen – University of Copenhagen
