Phosphate availability affects fixed nitrogen transfer from diazotrophs to their epibionts

Area of Science:

• Aquatic microbiology
• Biogeochemistry
• Cyanobacterial physiology

Background:

• Dinitrogen (N₂) fixation is a key nitrogen source in aquatic ecosystems, influencing productivity.
• Nitrogen (N) release by N₂-fixing cyanobacteria is observed, but its drivers are poorly understood.

Purpose of the Study:

• To investigate the role of phosphate availability in regulating the transfer of freshly fixed nitrogen to epibionts in filamentous diazotrophic cyanobacteria.
• To understand how phosphate influences nitrogen partitioning and microbial interactions within cyanobacterial colonies.

Main Methods:

• Stable-isotope incubations were used to trace nitrogen transfer.
• Single-cell analyses quantified nitrogen transfer to epibionts.
• Experiments were conducted during a Baltic Sea summer bloom under varying phosphate conditions.

Main Results:

• Phosphate scarcity led to <1% (Aphanizomenon) and ~15% (Nodularia) of fixed N transferred to epibionts.
• Phosphate addition halved N transfer to Nodularia epibionts, while Aphanizomenon showed a slight increase.
• Nodularia growth rate doubled with phosphate addition, indicating increased N assimilation for biomass.

Conclusions:

• Phosphate availability directly impacts the partitioning of freshly fixed N₂ in diazotrophic cyanobacteria.
• Phosphate scarcity and subsequent N release may explain heavy epibiont colonization on Nodularia during blooms.
• Phosphate levels can mediate cyanobacteria-microbiome interactions and influence nutrient cycling in aquatic environments.