Comparison of Diazotrophic Composition and Distribution in the South China Sea and the Western Pacific Ocean

SIMPLE SUMMARY: Diazotrophs are important contributors of bioavailable nitrogen that is essential to maintaining biological productivity in marine ecosystems. In this study, surface water samples were analyzed to explore the spatial variation of the diazotrophic community in the upper seawater of the South China Sea (SCS) and the Western Pacific Ocean (WPO). The well-conserved gene nifH, which is considered an important marker gene, was used for analyzing the phylogeny, diversity, and abundance of diazotrophs in this study. Our results showed that Proteobacteria was the main diazotroph in the SCS, while Cyanobacteria accounted for the largest proportion in the diazotroph community in the WPO. In addition, high abundances of diazotrophs in the subequatorial WPO indicated the occurrence of diazotrophs blooming. Variation in the composition of diazotrophs was significantly correlated with temperature, dissolved inorganic nitrogen, dissolved inorganic phosphate, and spatial variables between these regions. Our results provide insights into the ecological success and biogeochemical importance of diazotrophic communities in changing environments. ABSTRACT: The variation of diazotrophs has been elusive in multiple SCS and WPO regions due to insufficient data. Therefore, the dynamics of diazotrophic composition and distribution were investigated in this study, based on high-throughput sequencing and quantitative PCR of the nifH gene. We found that Proteobacteria dominated the diazotrophic community in the river-impacted SCS and cyanobacteria and Proteobacteria were more abundant in the ocean-dominated SCS and WPO. The qPCR analysis showed that cyanobacterial Trichodesmium was abundant in the Pearl River plume and in the SCS basin influenced by the Kuroshio intrusion, and it also thrived in the subequatorial region of the WPO. Unicellular cyanobacteria UCYN-A were mainly detected in the river-impacted area, UCYN-B was abundant in the WPO, UCYN-C had a relatively high abundance in the ocean-dominated area, and a preponderance of γ-Proteobacteria γ-24774A11 was observed in the ocean-dominated SCS and pelagic WPO. Diazotrophic communities had significant distance–decay relationships, reflecting clear biogeographic patterns in the study area. The variations of diazotrophic community structure were well explained by dissolved inorganic nitrogen, dissolved inorganic phosphate by an eigenvector spatial variable PCNM1. These results provide further information to help determine the ecological mechanism of elusive diazotrophic communities in different ocean ecosystems.