Long-term hypoxia or hyperoxia alter organ morphology and function of juvenile turbot (Scophthalmus maximus) in a recirculating aquaculture system

In turbot (Scophthalmus maximus), the effects of hypoxia and hyperoxia on organ morphology and function are lacking. To address this issue, 135 S. maximus (mean dry weight 6 ± 0.05 g) were subjected to a 42-day experiment in three RAS setups with different DO levels: hypoxia (LF, 4.0 ± 0.5 mg/L), normoxia (NF, 8.0 ± 0.5 mg/L), and hyperoxia (HF, 24.0 ± 0.5 mg/L). DO was regulated using nitrogen–oxygen (N₂-O₂), air aeration (Air-O₂), and oxygen nanobubbles (NB-O₂). This study evaluated organ morphology, including the skin, skeleton, gills, and intestine, alongside assessments of organ function such as intestinal digestive enzyme activity, liver antioxidant capacity, and muscle metabolism. The results showed that the LF group had the worst growth performance and the HF group had the best growth performance. The HF group showed swelling in the gill lamellae, suggesting respiratory organ abnormalities. Digestive enzyme activity was lowest in the LF group; in contrast, the HF group showed the highest activity. Both LF and HF groups experienced oxidative stress, as indicated by increased antioxidant enzyme activity, including peroxidase (POD), total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), and catalase (CAT). Muscle metabolism differed: the HF group had higher glycogen (Gn) and total free amino acids (T-AA) but lower adenosine triphosphate enzyme (ATP) activity, while the LF group had increased ATP activity and reduced crude protein and fat. These results indicate that S. maximus can adapt to mild hypoxia and hyperoxia. However, this comes at the cost of altering organ morphology and function.

Graphical Abstract