Nitrogen is a vital element for growth, forming the backbone of DNA, proteins, and other biological molecules. However, most nitrogen exists in the atmosphere as di-nitrogen (N₂), a form that's essentially locked away from use to most organisms due to its strong triple bond (with a bond enthalpy of +945.5 kJ/mol). A unique group of microorganisms, known as diazotrophs, has the ability to fix N₂ converting it into ammonia (NH₃) and organic nitrogen forms that other organisms can readily use to grow. This makes diazotrophs the primary source of naturally bioavailable nitrogen in ecosystems, delivering it where it is most needed. On land, they often live in small nodules on the roots of trees, directly supplying nitrogen to the plants. In the ocean they occur in larger numbers where nitrogen sources are particularly scarce, such as the large subtropical gyres, which are considered as the “ocean deserts”.

Yet, in comparison to terrestrial ecosystems, marine ecosystems are  continuously interconnected. Large ocean currents such as the Gulf Stream, continuously connect larger ecosystems. This can cause mixing of different water masses  and organisms from diverse origins. Hotspots for such mixing are mesoscale eddies. Mesoscale eddies occur through shear between two watermasses creating horizontal instabilities that result in rotating structures of around 100 km in diameter that last between a few days up to several months. In the North Atlantic, eddies spinning off from the Gulf Stream travel long distances into the North Atlantic Subtropical Gyre, transporting nutrients and plankton as well as creating unique environments in their core isolated from the surroundings. In our study, we wanted to know whether these eddies would create a beneficial environment for diazotrophs to grow, or whether they could serve as transport vehicles for diazotrophs into the North Atlantic Ocean. Read the full paper here: https://www.nature.com/articles/s41561-024-01567-2

FIGURE-CARING cruise

To explore how mesoscale oceanic eddies influence diazotrophs, we embarked on a field campaign in 2022 to the Gulf Stream and targeted two of its associated eddies. Using metabarcoding and qPCR techniques, we screened the water for diazotroph communities and assessed their nitrogen-fixing activity through ¹⁵N stable isotope incubations. Although nitrogen fixation rates are typically moderate in the North Atlantic, our experiments revealed surprisingly high rates, suggesting that we had identified environments particularly favorable for diazotroph growth. Notably, we observed that the majority of diazotrophs accumulated within the eddies. Moreover, diazotrophs were particularly abundant and active at the eddies’ edges, highlighting the role of  sub-mesoscale structuring within eddies for the distribution and activity of diazotrophs. However, we still don’t fully understand how, or to what extent, these eddy boundaries help move diazotrophs into the nearby Atlantic Ocean. To answer this question we would need much finer-grained resolution sampling, which needs yet to be tackled.

Eddies - a transport vehicle for diazotrophs

Analysis of the microbial community indicates that these abundant and highly active diazotrophs are not just thriving within the eddies—they are also being transported across vast distances, traveling far into the North Atlantic Subtropical Gyre. Here, they likely contribute significantly to the local nitrogen demand sustaining primary productivity. The eddies originated in the mid-Atlantic Bight, a region well known for its elevated nitrogen fixation rates (Mulholland et al. 2012; Selden et al. 2021; Tang et al. 2019; Palter et al. 2020). By "hitchhiking" within these eddies, diazotrophs effectively extend their influence, delivering much-needed nitrogen to regions of the North Atlantic where it is otherwise scarce and vital for supporting marine life.

An eddying ocean

In a second step we used a long-term database of similar eddies that occur in the region. These eddies are actually regularly spinning off the Gulf Stream. By doing so, we estimated the potential nitrogen contribution from these eddy-transported diazotrophs. If each eddy carries a comparable community of diazotrophs, their combined activity could supply approximately 21 µmol N m⁻² yr⁻¹ to the surrounding ocean, providing a significant source of bioavailable nitrogen to the region. This recurring transport mechanism underscores the broader role eddies play in supporting nitrogen cycling in the North Atlantic.