An Evaluation of Laminarin Additive in the Diets of Juvenile Largemouth Bass (<i>Micropterus salmoides</i>): Growth, Antioxidant Capacity, Immune Response and Intestinal Microbiota

A 28 day feeding trial was conducted to investigate the growth performance, immune response and intestinal microbiota of laminarin (LAM) supplemented diets in juvenile largemouth bass (<i>Micropterus salmoides</i>). Four hundred and eighty fish (initial average weight: 0.72 ± 0.04 g) were randomly divided into four groups (40 fish per tank with three replicates in each group) Four diets were prepared with LAM supplementation at the doses of 0 (control), 5 g Kg⁻¹ (LL), 10 g Kg⁻¹ (ML) and 15 g Kg⁻¹ (HL), respectively. No significant difference in the specific growth rate (SGR) and hepatosomatic index (HSI) was observed in fish among the four groups, or in the lipid and ash content of fish flesh. In addition, fish in the LL group exhibited much higher antioxidant capacity (<i>p</i> < 0.05), while the diets with the inclusion of 5 and 10 g Kg⁻¹ LAM remarkably decreased the antioxidant capacity of fish (<i>p</i> > 0.05). Dietary LAM at the dose of 5 g Kg⁻¹ inhibited the transcription of interleukin-1β (<i>il-1β</i>) and tumor necrosis factor-α (<i>tnf-α</i>), while promoting the expression of transforming growth factor-β (<i>tgf-β</i>) in fish intestine. Moreover, the beneficial intestinal bacteria <i>Bacteroide</i>, <i>Comamonas</i> and <i>Mycoplasma</i> abundance significantly increased in fish from the LL group, while the content of opportunistic pathogens <i>Plesiomonas</i>, <i>Aeromonas</i> and <i>Brevinema</i> in fish of the HL group was substantially higher than the control group. Overall, the appropriate dose of supplemented LAM in the diet was 5 g Kg⁻¹, while an excessive supplementation of LAM in the diet led to microbial community instability in largemouth bass.