Eddy-driven diazotroph distribution in the subtropical North Atlantic: horizontal variability prevails over particle sinking speed.

ABSTRACT

Mesoscale eddies influence the distribution of diazotrophic (nitrogen-fixing) cyanobacteria, impacting marine productivity and carbon export. Non-cyanobacterial diazotrophs (NCDs) are emerging as potential contributors to marine nitrogen fixation, relying on organic matter particles for resources, impacting nitrogen and carbon cycling. However, their diversity and biogeochemical importance remain poorly understood. In the subtropical North Atlantic along a single transect, this study explored the horizontal and vertical spatial variability of NCDs associated with suspended, slow-sinking, and fast-sinking particles collected with a marine snow catcher. The investigation combined amplicon sequencing with hydrographic and biogeochemical data. Cyanobacterial diazotrophs and NCDs were equally abundant, and their diversity was explained by the structure of the eddy. The unicellular symbiotic cyanobacterium UCYN-A was widespread across the eddy, whereas Trichodesmium and Crocosphaera accumulated at outer fronts. The diversity of particle-associated NCDs varied more horizontally than vertically. NCDs constituted most reads in the fast-sinking fractions, mainly comprising Alphaproteobacteria, whose abundance significantly differed from the suspended and slow-sinking fractions. Horizontally, Gammaproteobacteria and Betaproteobacteria exhibited inverse distributions, influenced by physicochemical characteristics of water intrusions at the eddy periphery. Niche differentiations across the anticyclonic eddy underscored NCD-particle associations and mesoscale dynamics, deepening our understanding of their ecological role and impact on ocean biogeochemistry.