Nitrite contamination in environments, largely driven by human activity, can disrupt multiple physiological functions in fish. In freshwater species, nitrite is readily absorbed across the gills, accumulating internally and interfering with the oxygen-carrying capacity of blood. Here we evaluated the effects of 72-hour exposure to a moderate environmental nitrite concentration (10 μM) on zebrafish (Danio rerio), a well-established model to study the physiology of freshwater fish. Specifically, the study examined changes in routine metabolism (rMO₂), the oxygen consumption of fish at rest and in a fasted state but with spontaneous basal activity, spontaneous activity, and in the percentages of blood oxyhemoglobin (HbO₂), metHb, and nitrosyl hemoglobin (HbNO, an index of NO level in the blood), as well as the effects on mitochondrial respiratory chain activity (ETC), oxidative metabolism, ROS content, lipid peroxidation levels and antioxidant defense responses of the muscle tissue. The amount of nitrite accumulated within the muscle was also measured.

The moderate, environmentally relevant concentration of nitrite reduced rMO₂ and spontaneous activity, while increasing HbNO, indicating enhanced NO formation. Nitrite also accumulated in muscle, where it promoted oxidative stress and altered aerobic metabolism, including an increase in mitochondrial efficiency, antioxidant capacity. These changes reflected coordinated metabolic adjustments.

In conclusion, acute exposure to a moderate environmental concentration of nitrite induces nitrite accumulation in skeletal muscle and alters muscular metabolism and the redox state of adult zebrafish. The results underscore the sensitivity of fish to nitrite levels commonly encountered in impacted freshwater habitats.