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 nifH gene amplicon sequencing, we show that NCDs dominate the diazotrophic community, accounting for 81%-100% of all sequences, whereas filamentous cyanobacteria contribute up to 19% and are largely restricted to the post-productive phase. Productivity phases emerged as a descriptor for structuring the diazotroph community composition, with Thermodesulfobacteriota dominating during the productive phase (up to 64.8%) and Gammaproteobacteria during the post-productive phase (up to 77.1%). Suspended and sinking particles were broadly similar, yet each exhibited distinct taxon-specific enrichments between phases, which might suggest different ecological strategies within the major NCD groups. This suggests potential roles in aggregate-associated nitrogen fixation; such mechanisms could include polysaccharide degradation and anaerobic metabolism. Phytoplankton composition (18S rRNA) varied between productivity phases and co-occurred with shifts in the diazotrophic community, suggesting potential cross-domain interactions. Comparing across basins, we did not observe significant differences in diazotroph community composition, except for a higher relative abundance of cyanobacterial diazotrophs in the Gulf of Finland. Taken together, our findings show diverse NCD lineages inhabiting particle-associated niches, highlighting the need to further investigate their ecological significance in the Baltic Sea.

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.