Nitrogen removal performance, microbiological characteristics, and functions of bioreactor based lignocellulosic substrate in recirculating aquaculture system

In this study, to remove nitrate from the recirculating aquaculture system (RAS) for Litopenaeus vannamei, lignocellulose was utilized to construct solid-phase denitrification reactors (SPDRs) outside or inside the culture tanks. Continuous water quality monitoring showed that lignocellulose could remove approximately 90 % of total nitrogen (TN) in the aquaculture water. According to the nitrogen budget of the culture system, 52.82–56.68 % of the input nitrogen was removed by lignocellulose. The denitrification rate of the SPDR was 35.63–36.21 mg/(L medium day), as determined by the stable isotope method. Metagenomic analysis revealed that the aquaculture systems possessed nitrogen removal functions such as nitrification, complete nitrification, and denitrification, while also exhibiting functions that were detrimental to nitrogen removal, such as dissimilatory nitrate reduction to ammonium (DNRA) and nitrogen fixation. In the SPDR, Ruegeria, Draconibacterium, etc., were the dominant denitrification bacteria according to their high contributions to nirS and norB, respectively; Clostridium and Spirochaeta were the dominant DNRA bacteria according to their high contributions to the nirB and hcp, respectively. In addition, SPDR also had a lot of nitrifiers including Nitrospina, Nitrospira, Nitrosococcus, and Nitrosomonas. In terms of substrate degradation, the functional bacterial community analysis of cellulose-degrading enzymes showed that the SPDR contained a rich variety of cellulose-degrading bacteria, including Ruminiclostridium, Paenibacillus, Draconibacterium, Spirochaeta, Bacteroides, etc. These results demonstrated that, lignocellulose coupled with the synergistic action of cellulose-degrading bacteria and nitrogen-cycling functional bacteria, was an effective means for nitrogen regulation in this RAS, regardless of whether they are applied in situ or ex situ.