Nitrogen stable isotopes in recirculating aquaculture for super-intensive shrimp production: Tracing the effects of water filtration on microbial nitrogen cycling

To understand how filtration affected microbial community nitrogen (N) cycling, natural abundance stable isotope ratios of suspended particles and nitrate (NO3−) were measured in recirculating aquaculture systems (RAS) for the super-intensive culture (425 shrimp m− 2) of Pacific white shrimp, Litopenaeus vannamei. Samples were collected throughout a trial during which triplicate RAS were equipped with either a foam fractionator (FF) or a propeller-washed bead filter (BF) to determine solids removal and inorganic N cycling efficacy of each filtration technique. Shrimp performance was unaffected by filtration technique. The time course of DIN concentration was consistent with rapid full nitrification (NH4+ to NO2− to NO3−) in the BF treatment. Persistent, elevated NO2− suggested a slower onset of NO2− oxidation in the FF treatment. However, NO3− began to accumulate at approximately the same time in both treatments and NO3− accumulated more rapidly in the FF treatment even though feed N input to the FF treatment was slightly lower than in the BF treatment. Consistently depleted δ15N-NO3− in the FF treatment illustrated isotopic fractionation of the NH4+ pool attributable to NH4+ oxidation. After an initial depletion, δ15N-NO3− in the BF treatment remained significantly enriched relative to the FF treatment, consistent with isotopic fractionation due to denitrification where the filtration mechanism promoted sub-oxic conditions. The latter is supported by significantly lower NO3− and N budget analyses which indicate the potential for N loss. Together these results suggest that, despite identical inocula, the microbial community responded differently to the filtration technique and as a result natural abundance isotopes revealed distinct dominant N cycle pathways.