Saltwater intrusion affecting NO2− accumulation in demersal fishery species by bacterially mediated N-cycling

To investigate the underlying effects of saltwater intrusion (SWI) on bottom aquatic ecosystems, a set of environmental parameters and the bacterial community were determined and analyzed by sampling bottom water and surface sediments at the Modaomen waterway of the Pearl River Estuary. Biodiversity of fishery species and their relationship with the environment variables were analyzed together. NO3- and NO2- concentration down-regulation and NH4+ concentration up-regulation in water and sediment were observed along the resulting salinity gradient, indicating that SWI affected N-cycling. Further investigation via 16 s sequencing revealed that taxonomic and functional composition of the bacterial community in the sediment displayed greater discretization than in water, implying that SWI exerted a greater impact on the sedimentary bacterial community. Metagenomic sequencing showed that the sedimentary bacterial community was associated with NO3-, NO2-, and NH4+ transformation under SWI, and that this was driven by salinity and conductivity. Nitrogen metabolism and denitrification related genes were expressed at higher levels in high salinity than in low salinity, consistent with the increased enzymatic activities of NiR and NR. The NO2- concentration in the muscle of six selected fishery species was significantly decreased by 11.15-65.74% (P < 0.05) along the salinity gradient, indicating that SWI reduced NO2- accumulation. The results suggest that SWI alleviates NO2- accumulation in demersal fishery species via bacterial mediation of N-cycling.