Carbohydrate structural complexity modulates the formation, composition and functionality of microbial flocs and its impact on gut microbiota and growth of Pacific white shrimp

The addition of carbohydrate into the culture water to facilitate the growth of heterotrophic bacteria is a common practice in shrimp grow out culture. The present study aimed to evaluate the effect of carbohydrate structural complexity on the formation, composition and functionality of microbial flocs, and how it impacts the gut microbiome and growth of Pacific white shrimp (Litopenaeus vannamei). Pacific white shrimp juveniles (1.07 ± 0.01 g) were randomly distributed in 12 indoor fiber tanks (50 cm × 50 cm × 100 cm) and maintained for 30 days at stocking density 107 ind.m−3 (20 ind per tank). Three different carbohydrate sources were added according to the treatments, that is dextrose (Dx) representing simple carbohydrate, potato starch (St) representing starch polysaccharide, and cellulose (Cl) representing complex non-starch polysaccharide. Settleable solids level and particle size of microbial flocs in Dx treatment were consistently higher than those of St and Cl treatments (P < 0.05). Amino acids concentrations were not significantly different amongst the treatments (P > 0.05), however essential amino acid index relative to shrimp requirement was the lowest in the Dx treatment (P < 0.05). Shannon diversity index level in the microbial flocs with Cl as the carbohydrate sources was higher compared to Dx and St. T6SS2 and ExsA gene expressions in the co-culture of Vibrio parahaemolyticus and the microbial floc suspension was highly upregulated in Dx treatment. Accordingly, differences in the shrimp gut microbiome was also observed amongst the treatments. The highest growth performance was demonstrated by the shrimp cultured with cellulose as the carbohydrate source. In conclusion, carbohydrate structural complexity could significantly affect the characteristics of microbial flocs suspended in the culture water and could ultimately affect the shrimp growth performance.