Comparision of nitrogen removal characteristic and microbial community in freshwater and marine recirculating...

Recirculating aquaculture system (RAS) has a good prospect in aquaculture, but its nitrogen removal characteristics and microbial community changes in freshwater and marine water remain unclear. In this study, six RAS were designed and divided into freshwater group and marine water group with salinity of 0‰ and 32‰, respectively, and operated for 54 days to test changes in nitrogen (NH₄⁺-N, NO₂^--N, NO₃^--N), extracellular polymeric substances and microbial communities. The results showed that ammonia nitrogen was rapidly reduced and almost converted to nitrate nitrogen in the freshwater RAS but to nitrite nitrogen in marine RAS. Compared with freshwater RAS, marine RAS had lower tightly bound extracellular polymeric substances and worse stability and settleability condition. 16S rRNA amplicon sequencing reflected significantly lower bacterial diversity and richness in marine RAS. Microbial community structure at phylum level showed lower relative abundance of Proteobacteria, Actinobacteria, Firmicutes, Nitrospirae, but higher abundance of Bacteroidetes under a salinity of 32‰. High salinity decreased the abundance of funtional genera (Nitrosospira, Nitrospira, Pseudomonas, Rhodococcus, Comamonas, Acidovorax, f_Comamonadaceae), which may account for nitrite accumulation and low nitrogen removal capacity in marine RAS. These findings could provide theoretical and practical basis for improving the start-up speed of high-salinity nitrification biofilm.