Toxic effect of chronic nitrite exposure on growth and health in Pacific white shrimp Litopenaeus vannamei

Juveniles of the Pacific white shrimp Litopenaeus vannamei were exposed to 0 (control), 2, 6.67, and 20 mg L⁻¹ nitrite-N for 30 days, and the growth, immunity, and gut microbiota were evaluated. The weight gain of shrimp exposed to 20 mg L⁻¹ nitrite-N was significantly lower than that in the control. Antioxidant capacity, measured as superoxide dismutase and glutathione peroxidase activities, catalase expression, and malondialdehyde content, was not affected by chronic nitrite-N exposure. The expressions of immune related genes (IMD and Toll) in shrimp exposed to 20 mg L⁻¹ nitrite-N were significantly higher than that in shrimp exposed to 2 mg L⁻¹ nitrite-N, but no differences were found with the control. Regardless of nitrite-N exposure concentrations, Bacteroidetes, Proteobacteria, and Actinobacteria were the dominant phyla in the gut of L. vannamei. With the increase of nitrite-N concentration, the diversity of gut microbiota increased in L. vannamei. The relative abundance of opportunistic pathogens, such as Pseudoalteromonadaceae and Vibrionaceae, increased in shrimp exposed to 20 mg L⁻¹ nitrite-N. The relative abundance of denitrifying bacteria increased in shrimp exposed to 2 mg L⁻¹ nitrite-N, but decreased in those exposed to 6.67 or 20 mg L⁻¹ nitrite-N relative to those in the control. The predicted microbial-mediated functions showed that pathways for digestive function and metabolic capacity were significantly inhibited. All findings in this study indicate that L. vannamei can deal with the stress caused by nitrite-N within levels lower than 6.67 mg L⁻¹, but chronic exposure to a high concentration (> 6.67 mg L⁻¹) would reduce shrimp growth and alter its immune system by destroying gut microbiota homeostasis.