Eddy-driven diazotroph distribution: horizontal variability prevails over particle sinking speed

Abstract

Mesoscale eddies influence the distribution of diazotroph (nitrogen fixing) cyanobacteria, impacting marine primary productivity and carbon export. Recently, non-cyanobacterial diazotrophs (NCDs) have emerged as potentially important contributors to the marine nitrogen fixation. Lacking a photosynthetic apparatus, NCDs are thought to rely on organic matter particles to obtain carbon and nutrient resources, simultaneously impacting nitrogen and carbon cycling. However, their diversity and biogeochemical importance remain poorly understood. Combining amplicon sequencing with hydrographic and biogeochemical data, allowed the investigation of the horizontal and vertical spatial variability of NCDs associated with suspended, slow-sinking, and fast-sinking particles collected with a marine snow catcher. Despite cyanobacterial diazotrophs and NCDs were equally abundant, their diversity was explained by the structure of the eddy, with communities in the core more similar than in the periphery. The unicellular symbiotic cyanobacterium UCYN-A was widespread across the eddy, whereas Trichodesmium and Crocosphaera accumulated at the external eddy fronts. Particle-associated NCDs dominated the fast-sinking fractions, mainly comprising Alphaproteobacteria. Gammaproteobacteria and Betaproteobacteria exhibited inverse distributions, influenced by the physicochemical characteristics of water intrusions at the eddy periphery. Niche differentiation across the anticyclonic eddy underscored NCDs’ particle associations and mesoscale dynamics, deepening our understanding of their ecological role and impact on ocean biogeochemistry.