Composition of non-cyanobacterial diazotrophs in suspended and sinking particles is driven by productivity phase.

Biological nitrogen fixation is a key process impacting productivity in the Baltic Sea. Most studies of this process have focused on filamentous cyanobacterial diazotrophs, leaving the ecological roles of non-cyanobacterial diazotrophs (NCDs) poorly resolved. Here, we investigate the diazotrophic community across different primary productivity phases (productive and post-productive) and within distinct particle fractions (suspended and sinking) across the Northern Baltic Proper, Gulf of Finland, and Gulf of Bothnia. Using IMPORTANCE: Nitrogen is a key nutrient that supports life in marine ecosystems. Traditionally, nitrogen fixation in the Baltic Sea has been attributed mainly to filamentous cyanobacteria. In this study, we show that non-cyanobacterial diazotrophs dominate the nitrogen-fixing community across productivity phases. We also demonstrate that suspended and sinking particles provide habitats for these microorganisms, allowing different groups to thrive under varying conditions. We further show that shifts in the phytoplankton community co-occur with changes in the diazotrophic composition. Together, our findings reveal that nitrogen fixation in the Baltic Sea extends beyond filamentous cyanobacteria and involves a diverse particle-associated diazotrophic community. Understanding the role of these overlooked microorganisms is important for predicting how nitrogen cycling will respond to ongoing changes, such as warming, eutrophication, and shifts in plankton communities.