The understanding of ocean mechanisms in the Arctic is currently experiencing a real turning point. Research conducted by the University of Copenhagen reveals that biological processes believed to be reserved for tropical waters are also operating in the icy conditions of the Far North.
The study published in
Communications Earth & Environment demonstrates that nitrogen fixation, a vital phenomenon for the marine food chain, is actively occurring under the sea ice. This fundamental reassessment implies a complete revision of polar marine productivity models and their role in the planetary carbon cycle.
The little-known biological mechanism
Nitrogen fixation represents a fundamental process where certain bacteria transform dissolved gaseous nitrogen in water into ammonium that can be assimilated by marine life. In the Arctic Ocean, this function is performed by non-cyanobacterial microorganisms, distinct from those found in warmer waters. Their particularity lies in their ability to thrive in conditions of low light and freezing temperatures, contrary to previous assumed limitations for this type of biological activity. Their specialized metabolism functions effectively despite the extreme conditions that characterize Arctic depths.
Oceanographic campaigns conducted aboard the research vessels Polarstern and Oden have allowed for the first quantification of the scale of this phenomenon. Scientists measured fixation rates reaching 5.3 nanomoles of nitrogen per liter daily, values comparable to those observed in some temperate zones. These measurements were taken from the Wandel Sea to the Eurasian Basin, indicating a widespread distribution of microbial activity. The study combines molecular biology approaches and isotopic tracing techniques to validate these observations.
The spatial distribution of these microorganisms follows a particular gradient, with maximum activity observed at the edge of melting ice. This transition zone benefits from increased light and organic matter, creating favorable conditions for bacterial development. Researchers note that dissolved organic matter appears to play a key role in activating the fixation process. This symbiotic relationship between glacial melting and microbial activity suggests a potential amplification of the phenomenon with the acceleration of sea ice retreat.
Implications for the Arctic ecosystem
The additional supply of available nitrogen substantially modifies the productivity dynamics of the Arctic Ocean. Marine algae, limited in their growth by nutrient deficiency, directly benefit from this new source of ammonium. This stimulation of phytoplankton production could lead to an increase in algal biomass (relative to algae) in areas previously considered biologically poor.
Algal proliferation directly influences the Arctic food web from its base. Planktonic crustaceans, the main consumers of phytoplankton, could potentially see their abundance increase, with repercussions throughout the entire food chain. Small fish, seabirds, and higher mammals could thus benefit from this increased productivity. This trophic cascade would modify the ecological structure of entire Arctic regions, with consequences still difficult to predict accurately.
The impact on the carbon cycle represents the other major dimension of this discovery. The increase in algal population intensifies the ocean carbon sink through the fixation of atmospheric carbon dioxide. However, scientists emphasize the nature of the interactions at play, where several opposing mechanisms could counterbalance this effect. Accurate modeling of these processes becomes essential to anticipate the evolution of the Arctic Ocean's regulatory role in the global climate, requiring the integration of these new biological data.
To go further: What is biological nitrogen fixation?
Biological nitrogen fixation refers to the transformation of atmospheric gaseous nitrogen, although abundant but unusable by most living beings, into assimilable chemical forms such as ammonium. This conversion is made possible by the action of specialized enzymes, mainly nitrogenase, which only certain bacteria and archaea possess. This natural process represents a key step in the planetary biogeochemical cycle of nitrogen.
In the marine environment, this transformation is mainly carried out by microorganisms called diazotrophs. Their activity produces nitrogen compounds that literally fertilize the ocean by serving as essential nutrients for phytoplankton growth. These fixing organisms thus constitute the cornerstone of ocean food webs by initiating the transfer of nitrogen to higher links.
While cyanobacteria were considered the main actors of this fixation in warm waters, recent research in the Arctic has revealed the importance of non-cyanobacterial diazotrophs. The latter operate in radically different environmental conditions, considerably expanding the habitats where this vital process was assumed to be active and challenging established paradigms.
What is the role of phytoplankton in the carbon cycle?
Phytoplankton plays a fundamental role in the planetary carbon cycle by acting as a natural biological sink. Through the process of photosynthesis, these marine microorganisms absorb significant amounts of atmospheric carbon dioxide dissolved in water. This fixation converts inorganic carbon into living organic matter, forming the base of ocean food webs and directly influencing the chemical composition of the atmosphere.
A significant portion of the sequestered carbon is transferred to ocean depths through what scientists call the "biological pump." When phytoplankton dies or is consumed by zooplankton, organic particles rich in carbon gradually sediment toward the seafloor. This natural mechanism allows for long-term carbon storage, which can persist for centuries in sedimentary layers.
The potential increase in phytoplankton biomass in the Arctic, stimulated by nitrogen fixation, could amplify this carbon sequestration process. However, scientists emphasize that this dynamic remains complicated to grasp, as it interacts with other factors such as ocean acidification and changes in marine currents, making global predictions still difficult to establish with certainty.
Article author: Cédric DEPOND