Dietary fishmeal replacement by Clostridium autoethanogenum protein meal influences the nutritional and sensory quality of turbot (Scophthalmus maximus) via the TOR/AAR/AMPK pathways

Abstract

Clostridium autoethanogenum protein (CAP) is a promising protein source for aquaculture; however, how CAP influences fish quality is worth extensive research. We randomly allocated 630 turbot with initial body weights of about 180 g into 6 groups, with fishmeal-based control diet or diet with CAP replacing 15% (CAP15), 30% (CAP30), 45% (CAP45), 60% (CAP60), or 75% (CAP75) of fishmeal protein. After a 70-d feeding trial, the fillet yield (P = 0.015) and content of protein (P = 0.017), collagen (P < 0.001), hydroxyproline (P < 0.001), C20:5n-3 (P = 0.007), and ∑n-3/∑n-6 polyunsaturated fatty acids ratio (P < 0.001) in turbot muscle was found to decrease linearly with increasing CAP. However, turbot fed CAP15 diet maintained these parameters (P > 0.05). By contrast, the muscle hardness increased linearly with increasing CAP (P = 0.004), accompanied by linear reduction of muscle fiber volume (P = 0.015) and expression of myogenesis-related genes, including ctsd (P < 0.001), ctsl (P = 0.093), and murfl (P < 0.001). Phosphorylation of Akt (P < 0.001), TOR (P = 0.001), 4EBP-1 (P < 0.001), and S6 (P < 0.001) decreased linearly; however, phosphorylation of AMPK (P < 0.001), eIF2α (P < 0.001), and the abundance of ATF4 (P < 0.001) increased with increasing CAP, suggesting that the TOR signaling pathway was inhibited, and the AAR and AMPK pathways were activated. Additionally, expression of genes related to protein degradation, including myf5 (P < 0.001), myod (P < 0.001), pax7 (P < 0.001), and ctsd (P < 0.001), decreased linearly with increasing CAP. In conclusion, CAP could be used to replace up to 15% of fishmeal without negatively impacting turbot quality. However, higher levels of CAP decreased fillet yield, muscle protein content, and muscle fiber diameter while increasing muscle hardness, which could be attributed to the inhibition of the TOR pathway and activation of the AAR and AMPK pathways.