Effects of C/N ratio control combined with probiotics on the immune response, disease resistance, intestinal microbiota and morphology of giant freshwater prawn (Macrobrachium rosenbergii)

A 90-day growth trial was performed to evaluate the effects of biofloc technology in combination with probiotics based on a rearing system on the growth and immunity of giant freshwater prawn (Macrobrachium rosenbergii). Two experimental groups (B-10 and B-20, with the addition of carbohydrates and without water change) and one control group (with water change and without the addition of carbohydrates) were designed. At the beginning of this study, the C/N ratios were adjusted to 10: 1 and 20: 1 through the addition of molasses to promote the formation of bioflocs in the two experimental groups. Each group comprised triplicate tanks, and each tank was stocked with 225 prawns (0.26 +/- 0.02 g). On the fourth day post-stocking, Bacillus subtilis and Lactobacillus were dosed into all tanks. The results indicated that no significant difference existed in the survival rate and growth performance among the three groups (P > 0.05). However, biofloc technology combined with the addition of probiotics could improve the humoral and hepatopancreatic immunity of the prawns by increasing the total hemocyte count, phagocytic activity, respiratory burst activity, and serum activity of superoxide dismutase (SOD) and lysozyme (LZM) (P < 0.05). The activities of total nitric oxide synthase, SOD and LZM in the hepatopancreas were also improved in the two biofloc groups (P < 0.05). After being challenged with Aeromonas hydrophila, the prawns in the control group had significantly higher cumulative mortality than did those in the B-10 and B-20 groups within 7 days (P < 0.05). Moreover, a relatively high proportion of Bacillus subtilis and Lactococcus was detected in the rearing water and intestines of the prawn reared in the biofloc systems (P < 0.05). Meanwhile, biofloc technology combined with the addition of probiotics produced a higher fold height (HF) of the distal intestine than in the control group (P < 0.05). The study suggested that rearing prawns using in situ biofloc technology combined with the addition of probiotics could enhance nonspecific immune responses and disease resistance under zero-water-exchange systems, sustain the homeostasis of intestinal microbiota and improve the absorption area of the intestine by increasing intestinal HF, which ensures the sustainability of M. rosenbergii culture. Statement of Relevance: Giant freshwater prawn, Macrobrachium rosenbergii reared using in situ biofloc technology in combination with probiotics could enhance non-specific immune responses, disease resistance under zero water exchange system, sustain the intestinal microbiota homeostasis and improve the absorption area of the intestine by increasing intestinal fold height, which ensured the sustainability of M. rosenbergii culture.