Endorsed by the Second International Indian Ocean Expedition (IIOE-2), our project aims to reveal the diversity and contribution of cyanobacterial vs NCDs to N2 fixation fluxes in the IO under the varying monsoon system. This work will lead to further exploration and insight into the N2 fixation of IO in the contrasted sub-basins and contrasted seasons to solve the biogeochemical mystery of IO. In this study, we compare the contributions of nifH and nifD amplicon sequencing approaches to elucidate diazotroph community composition.