Improvement of fish production and water quality in a recirculating aquaculture pond enhanced with bacteria-microalgae association

Intensive aquaculture plays an essential role in meeting human beings' growing demand for aquatic products. Recirculating pond has been considered as an effective approach for water and energy saving, but its wastewater also causes severely impair to aquatic animal health in some extent due to residual noxious nitrogen compounds (such as ammonia and nitrite) which are derived from mineralisation and break-up of uneaten feed and fish faeces. In order to purify water efficiently, biological processes are combined with recirculating pond such as using biological filtration, adding probiotics and so on. In this study, a field experiment was conducted to evaluate the potential of a closed recirculating aquaculture pond enhanced with bacteria-microalgae association in improving fish production and water quality. The results showed that the concentrations of total nitrogen (TN), total ammonia-N (TAN) and nitrite-N in the group of bacteria-microalgae with biofilm carriers were reduced by 29.15%, 51.28% and 33.48%, respectively, compared with treatment without any bioaugmentation. Meanwhile, the fish production was significantly improved (13.3%). Microbial community analysis (qPCR and high-throughput sequencing of 16 s and 18 s rRNA genes) suggested that the application of bacteria-microalgae with biofilm carries reduced the abundance of chiA gene (83.19%) which convert organic nitrogen into ammonia-N and increased the abundance of ammonia-monooxygenase (amoA) gene carried by ammonia-oxidizing bacteria (330.75%) and copper-containing nitrite reductase (nirK) gene (464.44%), indicating that the conversion of organic nitrogen to inorganic nitrogen was reduced and the denitrification process was increased. At the initial period, most of nitrogenous compounds were assimilated by microalgae. Along with the experiment, the bacteria community relative to nitrification and denitrification was also increased in bacteria-microalgae with biofilm carriers group compared with CK, such as Delftia (500%) and Acidovorax (450%) in ammoxidation process.