Integrating Microbial Protein Production and Harvest Systems into Pilot-Scale Recirculating Aquaculture Systems for Sustainable Resource Recovery: Linking Nitrogen Recovery to Microbial Communities

In aquaculture, it is important to raise the nitrogen recovery efficiency (NRE) to improve sustainability. To achieve this, recovery of microbial protein (RMP), instead of nitrification/denitrification in conventional wastewater treatment, is a promising approach whose microbiological mechanisms must be characterized. Here, periodic RMP was conducted in an in situ biofloc-based aquaculture system (IBAS) and a separating assimilation reactor-based recirculating aquaculture system (SRAS). Kinetic analysis indicated that a microbial biomass level of 3 g L-1 was optimal for inorganic N removal, and excess biomass was harvested to improve the NRE. Unlike the IBAS, the SRAS eliminated the fluctuation in water quality caused by the RMP. Periodic RMP significantly increased the NRE to 44-57% by promoting the filamentous bacterium Herpetosiphon and suppressing anaerobic denitrifiers. Aerobic chemoheterotrophy was the main microbial metabolic process for energy. After RMP, nitrate reductase-encoded functional genes (napA and narG) significantly decreased, while nitrite reductase-encoded functional genes, especially nirK, significantly increased. Co-occurrence networks analysis indicated that the cooperation and competition among organic matter degraders, filamentous bacteria, nitrifiers, and denitrifiers determined the microbial protein yield. These results provide fundamental insights into the influence of the RMP on microbial communities and functions, which is important for realizing sustainable aquaculture.