Fine-scale diazotroph community structure in the continental slope of the northern South China sea

Abstract

Diazotrophs have made significant contributions to marine nitrogen cycles. However, their distribution patterns and determined mechanisms have not been fully understood, particularly at the small regional scales. Here, the diazotrophic community structure by different sample sizes (0.2–10 μm and >10 μm), evaluated using high-throughput sequencing of the nifH gene, and the abundance of four typical diazotroph phylotypes, estimated by quantitative PCR of the nifH gene, were investigated on the continental slope of the northern South China Sea at a horizontal spatial resolution of 47.19–71.63 km. The results showed that Proteobacteria and Cyanobacteria were the dominant diazotrophic groups, which mainly contributed by Gamma-proteobacteria and Trichodesmium, respectively. Trichodesmium occupied the large-particle samples in the surface water, while other diazotrophs were nonuniformly distributed across water depths, particle sizes, and stations, indicating the heterogeneous distribution of diazotrophs at fine scales. Vertical profiles of environmental factors, especially the profile data of Fe concentration, were investigated, and the results indicated that temperature, dissolved inorganic nitrogen, and N:Fe were strongly correlated to diazotroph abundance and distribution of Trichodesmium and γ-24774A11. In the transect influenced by the Pearl River plume, γ-24774A11 nifH gene abundance significantly decreased than in other stations, indicating the important role of Pearl River plume in shaping the diazotrophic community structure. These findings advance our understanding of diazotrophic biogeography at the smaller regional scales.