The dietary presence of feed additives is crucial for boosting fish growth and immunity. Accordingly, this feeding trial aimed to investigate the effects of the separate and concurrent dietary supplementation of Spirulina platensis (SP) and bitter lemon (Citrus limon) peel essential oil (LEO) on the growth, immunity, antioxidant capacity, and intestinal health of Nile tilapia (Oreochromis niloticus). Four groups of male Nile tilapia were employed. The first group (control) was given the basal diet, while the second and third groups received the basal diet supplemented with LEO extract (1%) and SP (1 g/kg diet), respectively. The fourth group received the basal diet supplemented with a mix of LEO (1%) and SP at 1 g/kg. After two months of feeding, using LEO or/and SP improved the overall growth and immunological parameters, with their combination yielding the best outcomes. The supplementation of LEO or/and SP improved the Nile tilapia's growth metrics and transcriptomic levels of growth-regulating genes such as (oligo-peptide transporter 1 (Pep1), growth hormone receptors 1 (GHR1), and insulin-like growth factor (IGF1). The improved growth performance was linked to significant increases in the expression levels of mucin and fat metabolism-related genes. Moreover, fish supplemented with LEO, SP, or their combination showed enhanced non-specific immunological measures, including phagocytic and lysozyme activities and the mRNA copies of its regulating genes. Additionally, remarkable increases in the antioxidant enzyme activities and the mRNA levels of their related genes were detected. The complement (C3) gene's transcriptomic level was also significantly increased. Furthermore, the dietary supplementation of LEO, SP, or their combination improved the histological structures of the spleen, hepatopancreas, and intestine. The enhanced effects of LEO, SP, or their combination on fish immunity and growth are suggested to be due to their contents of bioactive compounds with anti-inflammatory, antioxidant, and antimicrobial properties. Thus, using the LOE and SP blends as feed additives is recommended for better growth and immunity of Nile tilapia.
Keywords: Spirulina platensis; antioxidant; fat metabolism genes; growth; immunity; lemon peel essential oil.