Increasing the C/N ratio of input feed has been reported as a practical approach for improving water quality and enhancing shrimp growth through changing the bacterial community of rearing water. However, little is known about the effects of different C/N ratios of feed input on the intestinal microbiota and metabolome of shrimp. In the present study, the effects of three different C/N ratio levels (CN6, CN10, and CN15) maintained by adding sucrose on the growth, intestinal microbiota and metabolome of Litopenaeus vannamei, and bioflocs formation were investigated after 17 days of feeding. The results indicated that higher C/N ratio (10 and 15), especially CN15, of feed input significantly enhance the length and weight of shrimp individuals accompanied by a significant accumulation of bioflocs, compared to that of CN6. The increase of C/N ratio input decreased the α-diversity of the intestinal microbiota and changed the microbial community structure through increasing the relative abundance of Actinobacteria, Rhodobacteraceae (mainly consist of Roseobacter and Paracoccus groups), Alteromonadaceae, and inhibiting the growth of Cyanobacteria, certain Rhodobacteraceae, Mycoplasmataceae and Vibrio. The change of microbial community caused by increasing C/N ratio input was closely associated with various bioactive metabolites of flavonoids, benzenoids, prenol lipids, and indole derivatives, which are benefit for shrimp growth either as an antimicrobial agent or as a nutrient component. Overall, this study demonstrated that manipulating high C/N ratio of feed input helps to the growth of shrimp through increasing the relative abundance of potential beneficial bacteria and the accumulation of various bioactive metabolites to suppress the growth of detrimental bacteria.

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Sucrose has been reported as an effective carbon source for improving water quality in the biofloc culture system. However, the microbiological mechanisms of adding sucrose on water quality improvement remain to be fully clarified. This study was conducted to evaluate the effects of three C/N ratio levels (CK, CN10, and CN15) on shrimp growth, water quality and bacterioplankton community in shrimp (Litopenaeus vannamei) culture system. The results showed that the growth performances were significantly improved in CN10 and CN15 groups, compared with those in CK group. The contents of nitrite‐nitrogen (NO2‐‐N), ammonium‐nitrogen (NH4+‐N), nitrate‐nitrogen (NO3‐‐N) and phosphate‐phosphorus (PO43‐‐P) in the CN10 and CN15 groups were significantly reduced, compared with those in CK group. High input of C/N ratio enhanced the relative abundances of Rhodobacteraceae, Flavobacteriaceae Demequinaceae and Saprospiaceae, and decreased the relative abundances of Vibrionaceae and Mycoplasmataceae. The discriminatory OTUs induced by adding sucrose showed significant correlations with water quality parameters. Additionally, by predicting functional annotation of bacterioplankton community, we found that the addition of sucrose enhanced the function of denitrification and nitrite‐respiration, and reduced the accumulation of pathogens‐related expression. In conclusion, sucrose addition may provide a healthy water environment for shrimp growth by regulating the changes of bacterioplankton community.

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Supplementing exogenous carbon sources is a practical approach to improving shrimp health by manipulating the microbial communities of aquaculture systems. However, little is known about the microbiological processes and mechanisms of these systems. Here, the effects of glucose addition on shrimp growth performance and bacterial communities of the rearing water and the shrimp gut were investigated to address this knowledge gap. The results showed that glucose addition significantly improved the growth and survival of shrimp. Although the α-diversity indices of both bacterioplankton communities and gut microbiota were significantly decreased by adding glucose, both bacterial communities exhibited divergent response patterns to glucose addition. Glucose addition induced a dispersive bacterioplankton community but a more stable gut bacterial community. Bacterial taxa belonging to Ruegeria were significantly enriched by glucose in the guts, especially the operational taxonomic unit 2575 (OTU2575), which showed the highest relative importance to the survival rate and individual weight of shrimp, with the values of 43.8 and 40.6%, respectively. In addition, glucose addition increased the complexity of interspecies interactions within gut bacterial communities and the network nodes from Rhodobacteraceae accounted for higher proportions and linked more with the nodes from other taxa in the glucose addition group than that in control. These findings suggest that glucose addition may provide a more stable gut microbiota for shrimp by increasing the abundance of certain bacterial taxa, such as Ruegeria.