Unicellular nitrogen-fixing cyanobacteria group B (UCYN-B) is a key diazotrophic bacteria that converts dinitrogen (N²) gas into ammonia (NH₄⁺) through biological nitrogen fixation in the marine biological pump, enhancing new nitrogen input into the ocean and boosting phytoplankton productivity, which indirectly impacts the global carbon cycle. Its nitrogen fixation function is crucial in oligotrophic environments, influencing ecosystem stability and global climate change. Studying UCYN-B is essential for understanding marine nitrogen cycling and carbon sequestration mechanisms. To further comprehend UCYN-B’s distribution in natural marine environments, this study investigated the in situ distribution and regulatory factors of UCYN-B in the Northwest Pacific. The results show that UCYN-B primarily occurs in the mixed layer of the North Pacific Subtropical Gyre (NPSG) and is mainly distributed between latitudes 10-21°N. Analysis of the regulatory factors revealed that UCYN-B distribution under natural conditions is controlled by specific temperatures, almost exclusively found at sea surface temperatures between 28-31°C. Furthermore, seasonal variation significantly affects UCYN-B distribution, with a narrower range during the cold season and a shift towards the equator compared to the warm season. These findings suggest that predicting the global distribution of UCYN-B requires more stringent conditions. Therefore, projections of UCYN-B’s distribution and nitrogen fixation under future climate change must be approached with caution to more accurately predict its role in global carbon and nitrogen cycles.
 

nitrogen fixation, unicellular nitrogen-fixing cyanobacterium, UCYN-B, northwestern Pacific Ocean, temperature regulation