Magnesium Promotes Growth–Metabolism Balance in Juvenile Largemouth Bass (Micropterus salmoides) and Modulates Antioxidant–Inflammatory–Apoptotic Responses Under Heat Stress

This study addressed the optimal magnesium (Mg) requirement for juvenile largemouth bass (<i>Micropterus salmoides</i>) and assessed the effects of dietary Mg supplementation on growth performance, nutrient metabolism, and alleviation of heat stress in it. In this study, six diets with varying Mg levels (1.01, 1.26, 1.78, 2.24, 2.35, and 2.51 g/kg), designated as MG1, MG2, MG3, MG4, MG5, and MG6, respectively, were formulated using MgSO₄·7H₂O as the Mg source. These diets were fed to juvenile <i>M. salmoides</i> (initial body weight 2.27 ± 0.02 g) for 8 weeks. The growth performance of the MG4 group was significantly improved. In addition, Plasma GLU, LDL-C, and TG levels were significantly reduced in the MG4 group, while plasma HDL-C levels were increased. In terms of gene expression, <i>glut2</i>, <i>g6pdh</i>, <i>ppar-γ</i>, <i>fas</i>, <i>elovl2</i>, <i>acc</i>, and <i>igf-1</i> were significantly upregulated in the MG4 and MG5 groups, while <i>g6pase</i> and <i>ppar-α</i> were significantly downregulated in the MG5 group. In the heat stress test, MG4 group exhibited enhanced antioxidant capacity, as evidenced by decreased plasma MDA levels and increased CAT activity, coupled with enhanced gill Na⁺/K⁺-ATPase activity. Gene expression results also showed that <i>il-10</i> and <i>bcl-2</i> were significantly upregulated in the MG4 group, while <i>nf-κb</i>, <i>ifn-γ</i>, <i>il-8</i>, <i>tnf-α</i>, <i>casp3</i>, <i>casp8</i>, <i>bax</i>, <i>jnk2</i> and <i>ask1</i> were significantly downregulated. Furthermore, the results of TUNEL immunofluorescence labeling analysis showed that the apoptotic index was significantly decreased in the MG2-MG6 groups. Overall, appropriate dietary Mg levels promoted growth performance, improved glucose metabolism, and induced lipid deposition in juvenile <i>M. salmoides.</i> Notably, Mg reduced oxidative damage by enhancing antioxidant enzyme activity, thereby modulating heat stress-induced Antioxidant-Inflammatory-Apoptotic of juvenile <i>M. salmoides.</i> Based on quadratic regression analysis of SGR and FCR, the optimal Mg requirement for juvenile <i>M. salmoides</i> was 2.04, and 2.15 g/kg, respectively.