Effects of nitrate on aquaculture production, blood and histological markers and liver transcriptome of Oplegnathus punctatus

Nitrate, the end-product of nitrification, tends to accumulate in recirculating aquaculture systems (RASs). We investigated the 96-h acute and 28-d chronic toxicity of nitrate-N to spotted knifejaw, Oplegnathus punctatus. We exposed juveniles (1.3 ± 0.6 g, 2.3 ± 0.7 cm TL) to seven different nitrate-N concentrations (11.3 (control), 673.1, 873.9, 1052.9, 1471.1, 1887.1, or 2368.3 mg/L) in a triplicated experimental design for 96 h; the estimated LC50 values for nitrate-N at 24, 48, 72, and 96 h were 774.4, 678.8, 586.3, and 484.5 mg/L nitrate-N − N, respectively. In an examination of chronic effects of elevated nitrate-N, juvenile O. punctatus (44.9 ± 7.0 g; 7.3 ± 0.6 cm) were exposed to 21.4 ± 4.4 mg/L (treatment 1), 165.5 ± 7.8 mg/L (treatment 2), or 299.4 ± 5.8 mg/L (treatment 3) nitrate-N in triplicated RASs, and the impact of nitrate-N on performance, blood, histological, and gene expression markers was evaluated. After 28 days, final length, specific growth rate and feed conversion ratio did not differ (p > .05) among treatments. Final weight and mortality did not differ significantly between treatments 1 and 2, but was significantly (p