Brown Seaweed (Padina australis) Extract can Promote Performance, Innate Immune Responses, Digestive Enzyme Activities, Intestinal Gene Expression and Resistance against Aeromonas hydrophila in Common Carp (Cyprinus carpio)

SIMPLE SUMMARY: Fish farming is threatened by various stressors involved in impairing fish health and productivity. Hence, environmentally friendly feed additives such as seaweeds or their extracts could be an effective alternative to antibiotic therapy. Padina australis is a brown seaweed spread on the coasts of Iran and characterized by its abundant supply of several bioactive molecules and acids. Thus, this study was performed to evaluate its functional roles in the rate of growth, immune responses, and disease tolerance of common carp towards Aeromonas hydrophila infection. It could be concluded that dietary P. australis at levels of 100 up to 400 mg/kg diet could promote the growth and general health status of common carp. ABSTRACT: Eight-week oral administration of Padina australis ethyl acetate extract at 100, 200, and 400 mg/kg diets was assessed on the growth performance, tight junction proteins, intestinal immunity, and disease resistance to Aeromonas hydrophila in common carp (Cyprinus carpio). A total of 300 healthy common carp weighing around 14.8 ± 0.03 g were randomly assigned into four equal groups within 12 glass aquariums, each in three replicates (25 fish/tank), for the feeding trial experiment. The first group served as the control group and was fed an un-supplemented diet, whilst the other three groups were offered diets containing graded amounts of Padina australis ethyl acetate extract at 100, 200, and 400 mg/kg, respectively. The growth indices, including final weight, length, weight gain rate, specific growth rate, and feed conversion ratio, were meaningfully improved in fish fed with the algae at 200 and 400 mg/kg compared to the control fish (p < 0.05). Similarly, digestive enzyme activities and serum immune parameters were significantly higher in all treatments, especially 200 and 400 mg/kg fed groups, compared to the control (p < 0.05). In parallel, significant upregulation of genes related to integrity and the immune system was shown in the intestine of these treatment groups compared to control fish (p < 0.05). When fish were challenged with A. hydrophila, the cumulative survival percentages were 53.3% (p = 0.215), 70.0 % (p = 0.009), and 76.7% (p = 0.002) in fish fed 100, 200, and 400 mg/kg diets, respectively, compared to 36.7% survival in control fish (p = 0.134). These data show that the eight-week dietary administration of P. australis extract to common carp can enhance growth performance, digestive enzyme activity, immune response, and disease resistance to A. hydrophila infection.