Distribution Patterns of nirS-Encoding and nirK-Encoding Denitrifiers in the Surface Sediment of the Pearl River Estuary

The abundance and community structure of nir S-encoding and nir K-encoding denitrifiers inhabiting the sediment of the Pearl River Estuary was examined with quantitative PCR and high-throughput MiSeq sequencing. The results indicated that the low-oxygen condition (around 4 mg/L) of the Pearl River Estuary was the suitable environment for the denitrifying bacterial growth, and the abundances of nir S-encoding and nir K-encoding denitrifier were both highest in station P3, while the diversity and evenness were highest in stations P2 and P4, respectively. In addition, gene abundance and diversity of nir S was higher than nir K, which indicated that the denitrifying potential of nir S-encoding denitrifiers were significantly greater than that of nir K-encoding denitrifiers ( p < 0.05). The most dominant nir S-encoding denitrifiers present in the sediment samples belonged to the phylum Proteobacteria, followed by Chloroflexi. However, the dominant classes of Betaproteobacteria and Gammaproteobacteria showed obvious salinity heterogeneity along the Pearl River Estuary. Betaproteobacteria have a strong survival ability in oligohaline environments and Gammaproteobacteria were opposite. Additionally, the distribution of both Betaproteobacteria and Gammaproteobacteria positively correlated with TOC and For nir K-encoding denitrifiers, the distribution of cluster A and cluster C also showed salinity heterogeneity; the former favored oligohaline environments and the latter were opposite.