Phosphate availability affects fixed nitrogen transfer from diazotrophs to their epibionts

Dinitrogen (N 2 ) fixation is a major source of external nitrogen (N) to aquatic ecosystems and therefore exerts control over productivity. Studies have shown that N 2 -fixers release freshly fixed N into the environment, but the causes for this N release are largely unclear. Here, we show that the availability of phosphate can directly affect the transfer of freshly fixed N to epibionts in filamentous, diazotrophic cyanobacteria. Stable-isotope incubations coupled to single-cell analyses showed that <1% and ~15% of freshly fixed N was transferred to epibionts of Aphanizomenon and Nodularia , respectively, at phosphate scarcity during a summer bloom in the Baltic Sea. When phosphate was added, the transfer of freshly fixed N to epibionts dropped to about half for Nodularia , whereas the release from Aphanizomenon increased slightly. At the same time, the growth rate of Nodularia roughly doubled, indicating that less freshly fixed N was released and was used for biomass production instead. Phosphate scarcity and the resulting release of freshly fixed N could explain the heavy colonization of Nodularia filaments by microorganisms during summer blooms. As such, the availability of phosphate may directly affect the partitioning of fixed N 2 in colonies of diazotrophic cyanobacteria and may impact the interactions with their microbiome.