Nitrogen (N₂) fixation is critical for the biological productivity of the ocean, yet clear mechanistic controls on this process and its response to expected ocean acidification remain elusive. In this presentation, a comprehensive case study from the tropical and subtropical western North Pacific will be introduced to discuss the nutrient regulation of N₂ fixation and diaozotroph communities. Our findings reveal a basin scale coherence of diazotroph abundances and N₂ fixation rates with the supply ratio of iron: nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N₂ fixation rates increased, phosphate concentrations decreased and bioassay experiments demonstrated evidence for N₂ fixation switching from iron to phosphate limitation. Furthermore, the complex interaction between nutrients and ocean acidification in regulating N₂ fixation within the diazotroph communities, particularly those dominated by the filamentous Trichodesmium and the unicellular UCYN-B, will be discussed. Our results highlight the iron:nitrogen supply ratio as the primary factor regulating N₂ fixation distribution across the tropical ocean and demonstrate a negative impact of ocean acidification on N₂ fixation.

Nitrogen fixation;Nutrient regulation;Western North Pacific;Ocean acidification