Effects of dietary organic acids and nature identical compounds on growth, immune parameters and gut microbiota of European sea bass

A 71-day study was conducted to explore the effect of increasing dietary levels (0, 250, 500, 1000 mg kg feed⁻¹; D0, D250, D500 and D1000, respectively) of a blend of microencapsulated organic acids (citric and sorbic acid) and nature identical compounds (thymol and vanillin) on growth, intestinal immune parameters and gut microbiota of European sea bass juveniles reared under normal and subsequently suboptimal environmental conditions (high temperature, 30.0 ± 0.4 °C and low oxygen, 4.6 ± 0.6 mg L⁻¹). OA and NIC did not promote growth, feed utilisation or feed intake at the inclusions tested but induced significant upregulation of IL-8, IL-10 and TGFβ. Next-generation sequencing showed prebiotic-like effects, stimulating beneficial taxa such as Lactobacillus, Leuconostoc and Bacillus sp. PICRUSt analyses indicated a functional reconfiguration of the gut microbiome with decreases in inflammation-promoting and homeostatic functions at increasing OA and NIC administration. For the first time in this species, exposure to suboptimal rearing conditions modified gut microbiota structure by reducing lactic acid bacteria and increasing Proteobacteria, consistent with inflammatory processes observed at mRNA level.