N₂ fixation by diazotrophic bacteria plays a crucial role in supporting phytoplankton productivity, which in turn supports carbon export to the deeper ocean layers in the N-limted regions. The North Pacific Subtropical Gyre (NPSG) is recognized as a hotspot for N₂ fixation in the global ocean. However, much of the research in this area has been focused on the Hawaii Ocean Time-series station, leaving the broader biogeography of diazotrophs and the facotors regulating them across the basin largely unexplored. In this study, we conducted an extensive sampling campaign, spanning approximately 11,000 km across the tropical western North Pacific. Our founding reveal that the unicellular diazotrophic cyanobacterium UCYN-B dominates the diazotroph community and is responsible for the elevated N₂ fixation rates in this region. Nutrient amendment bioassays using surface water demonstrated a transition from iron to phosphate limiation of community N₂ fixation rate across the study area. However, gene expression analyses of iron and phosphate stress biomaker in UCYN-B from deeper water samples suggest the potential for co-limiation by both nutrients at depth. This study provides valuable new insights into N₂ fixation and its nutrient regulation in the western North Pacific.
 

N2 fixation, UCYN-B, biomarker, nutrient limitation