The Effects of Dietary Protein Level on the Growth Performance, Body Composition, Intestinal Digestion and Microbiota of Litopenaeus vannamei Fed Chlorella sorokiniana as the Main Protein Source

SIMPLE SUMMARY: The shortage of fishmeal is currently a pressing issue for the aquaculture industry, as it increases the cost of aquatic product production and contributes to overfishing in the oceans. To solve this problem, Chlorella sorokiniana has started to be used in recent years as a major protein source in aquatic animal diets. However, differences in dietary protein sources will result in different optimal protein requirements. Accordingly, this study determined the growth performance, body composition, intestinal digestion and microbiota of Litopenaeus vannamei fed with five isolipid diets using C. sorokiniana as the main protein source but in different protein levels. The results showed that a dietary protein level of 40.81% resulted in the best weight gain rate when C. sorokiniana was used as the main protein source. Additionally, the optimal nutritional composition, digestibility and intestinal microbiota stability of L. vannamei occurred at a 40% dietary protein level. Higher dietary protein levels increased the colonisation of beneficial bacteria and inhibited the colonisation of pathogenic bacteria. These data inform the scientific application of C. sorokiniana in L. vannamei diets and promote the sustainable development of L. vannamei farming. ABSTRACT: This study investigated the effect of dietary protein levels on Litopenaeus vannamei. Five isolipid diets with protein levels of 32%, 36%, 40%, 44% and 48% were prepared using C. sorokiniana as the main protein source. L. vannamei (initial body weight 0.83 ± 0.02 g) were fed these five diets for 8 weeks and referred to as the CHL32, CHL36, CHL40, CHL44 and CHL48 groups, respectively. When the feeding trial was finished, the growth performance, body composition, intestinal digestion and microbiota of L. vannamei were studied. The results showed that the maximum weight gain rate (WGR) of L. vannamei was in the CHL40 group while the lowest feed conversion ratio (FCR) was in the CHL48 group. According to the regression analysis using WGR as the evaluation index, the best growth performance of L. vannamei was obtained when the dietary protein level was 40.81%. The crude protein content of whole shrimp showed an increasing and then decreasing trend with increasing dietary protein levels. Furthermore, the L. vannamei muscle amino acid composition was relatively stable and, to some extent, independent of dietary protein levels. Trypsin, lipase and amylase (AMS) activity increased and then decreased with increasing dietary protein levels and, significantly, peaked in the CHL44 group. Analysis of the alpha diversity of the intestinal microbiota showed that the Chao1 index peaked in the CHL40 group and was significantly lower in the CHL48 group. Additionally, the relative abundance of pathogenic bacteria decreased significantly while the relative abundance of beneficial bacteria increased significantly in the intestine of L. vannamei as the dietary protein levels increased. The functional prediction of the intestinal microbiota revealed that dietary protein levels may influence the growth of L. vannamei by regulating various metabolic activities, and the highest WGR in the CHL40 group may have been related to the significant enrichment of nicotinate and nicotinamide metabolism and biotin metabolism functions. In summary, the optimal protein requirement for L. vannamei was around 40% when C. sorokiniana was used as the primary protein source. Too high or too low dietary protein levels could adversely affect shrimp body composition, intestinal digestion and microbiota.