Effect of Amorphous -PHB on Growth, Body Composition, Immune-Related Gene Expression and Resistance of Hybrid Grouper ( ♀ × . ♂) Juveniles.

Poly-β-hydroxybutyrate (PHB) is a bacterial metabolite produced by bacteria such as sp. that serves as a carbon and energy storage compound for bacteria under nutrient-limited conditions. Two experiments were conducted to investigate the effects of dietary supplementation with -PHB on hybrid grouper ( ♀ × . ♂). In experiment I, juvenile groupers were fed basal diets supplemented with 3% -PHB (3% HM-PHB) containing 1.4% PHB and 3% (3% HM) without PHB, as well as a control diet, for seven weeks. The results showed no significant difference in survival rate, weight gain, and crude fat content between the 3% HM-PHB group and the control group; however, the crude protein of the 3% HM-PHB group was significantly lower than that of the control group. Furthermore, supplementation with 3% HM-PHB increased the fatty acids content in fish muscles, including long-chain unsaturated fatty acids C18:1n9, EPA, and DHA. In experiment II, groupers were fed a basal diet supplemented with 6.5% -PHB (6.5% HM-PHB) containing 3% PHB and 6.5% (6.5% HM) containing no PHB, as well as a basal diet (Control). After seven weeks of rearing, the fish were challenged with for 48 h. Although no significant difference in survival rate and growth was observed among different groups, the dietary supplement of 6.5% -PHB improved the survival rate of challenged grouper and significantly increased the gene expressions of catalase () and superoxide dismutase () in blood, interleukin 1 () and interleukin 10 () in the liver, spleen, head kidney, and blood ( < 0.05). In conclusion, dietary supplementation of -PHB had no significantly positive effect on fish growth performance but increased the content of fatty acids, including long-chain unsaturated fatty acids C18:1n9, EPA, and DHA in fish muscle; it also improved the resistance, possibly through increasing immune-related gene expression in different tissues and organs. Our findings offer compelling evidence that -PHB can be utilized as a feed additive in intensive grouper farming to enhance the groupers' resistance to .