Dietary manganese requirement and its effects on antioxidant enzyme activities, intestinal morphology and microbiota in oriental river prawn Macrobrachium nipponense (De Haan)
Manganese (Mn) is an essential micronutrient for animals. An 8-week feeding experiment with oriental river prawn Macrobrachium nipponense was conducted to determine the effects of Mn supplementation on growth, antioxidative indices, intestinal morphology, and gut microbiota in juveniles of this species. Manganese sulfate monohydrate was added to a basal semi-purified diet at six levels: 5.4, 13, 22, 34, 73 and 150 mg Mn kg-1 diet. Each diet was fed to juvenile prawns (mean weight 0.104 ± 0.003 g), twice daily to apparent satiation, in four replicates. Prawns fed the 22 mg Mn kg-1 diet achieved the highest weight gain. The Mn concentration in muscle significantly increased with increased dietary Mn supplementation, from 5.4 to 22 mg kg-1 diet, but leveled off in animals fed greater levels. Prawns fed the 34 mg Mn kg-1 diet had the lowest malondialdehyde content. Total superoxide dismutase activity in the hepatopancreas increased significantly with an increasing level of dietary Mn in the groups fed the 5.4 to 73 mg kg-1 diets, but plateaued with higher levels of supplementation. Prawns fed the 13 to 150 mg Mn kg-1 diets had significantly higher Mn superoxide dismutase activity than those fed the 5.4 mg kg-1 diet. Epithelial cells of the intestine were closely arranged, with clear cell definition, in the groups fed the 5.4 or 22 mg Mn kg-1 diet, whereas these cells were completely separated from the basement membrane in prawns fed the 150 mg Mn kg-1 diet. The estimator of community richness (Chao) in the intestine was significantly higher for prawns fed the 22 mg Mn kg-1 diet as compared with animals fed the 5.4 and 150 mg Mn kg-1 diets. The dominant phyla in the gut of M. nipponense were Proteobacteria, followed by Firmicutes and Bacteroidetes, regardless of the level of Mn supplementation. The intestinal microbiota phylotypes significantly differed among the groups fed the 5.4, 22 and 150 mg Mn kg-1 diets. We conclude that Mn supplementation in formulated feeds for this species can improve growth performance and Mn deposition in muscle, whereas diets causing Mn deficiency or excess will induce oxidant stress, and very high levels of dietary Mn could damage the intestinal morphology. The intestinal core microbiota were not affected by the dietary Mn level, although the microbiota richness was somewhat affected. Based on weight gain, the optimal level of Mn supplementation for juvenile M. nipponense is 17.59 mg kg-1 diet.