Understanding the mechanisms and factors that drive the Sargassum proliferation in the GASB is essential for future efforts to mitigate this phenomenon. Although Sargassum rafts are frequently observed in oligotrophic environments with low nitrogen concentrations [8], it is crucial to determine the various nitrogen sources utilized by these macroalgae and the underlying mechanisms. Our study reveals the significance of N2 fixation compared to nitrification and denitrification in GASB.