Introduction
A potential solution to the increasing demand of fish meal (FM) in aquafeeds would be the replacement of FM in the diets by alternative protein sources, but they do not normally meet with the nutritional requirements of shrimp and can lead to nutritional imbalances (Malcorps et al., 2019). Moreover, shrimp farming is usually affected by bacterial diseases, such as these known as “Bright-red syndrome” and “bacterial white tail disease” in whiteleg shrimp (Penaeus vannamei) caused by Vibrio harveyi, which is one of its most pathogenic agents in terms of mortality rates (Soto-Rodriguez et al., 2012; Zhou et al., 2012). Under this backdrop of threats to shrimp aquaculture, the use of single cell proteins (SCPs) in shrimp diets may be a potential nutritional alternative to FM. In particular, bacterial single cell proteins (BSCP) are composed of up to 80% of protein content and have a high proportion of essential amino acids, vitamins, phospholipids, and other functional molecules (Pereira et al., 2022). Further, some studies testing the replacement of FM by BCSP coming from Methylococcus capsulatus in whiteleg shrimp have reported promising results for improvement of immune status, disease resistance and gut microbiota (Chen et al., 2021; Jintasataporn et al., 2021; Felix et al., 2023). Thus, this work aimed to evaluate the suitability of a BSCP from M. capsulatus as an alternative to FM in Pennaus vannamei.
Materials and Methods
A 140 days-nutritional trial was carried out in whiteleg shrimp fed five isonitrogenous (36% crude protein) and isolipidic (8% crude fat) experimental diets. One was formulated based on a commercial feed for whiteleg shrimp and was used as a control diet (D1), and the others were the same diet but with graded levels of FM replacement by the BSCP (Uniprotein® Aqua, Unibio, Roskilde, Denmark): 75% FM, 25% BSCP (D2); 50% FM, 50% BSCP (D3); 25% FM, 75% BSCP (D4); and 0% FM, 100% BSCP (D5). At the end of the trial, survival, growth in terms of final body weight (BWf) and specific growth rate (SGR), and feed conversion ratio (FCR) were measured. Body proximate composition was also evaluated, as well as the body profile of fatty acids and of amino acids. The overall condition of the intestine was histologically evaluated. In addition, DNA was extracted from the shrimp intestines, and 16S rRNA gene library sequencing (Illumina - MiSeq platform) was conducted using specific primers for the V3-V4 hypervariable regions. After the trial, a bacterial challenge of 10 days was also performed, in which whiteleg shrimp was injected with the pathogen V. harveyi in order to measure the survival rates for each dietary group.
Results and Discussion
Whiteleg shrimp fed the control diet showed a survival rate of 47.7 ± 2.9%, while substituting all the FM by the BSCP it increased to 65.3 ± 2.5%, an improvement of 75% (P < 0.05). On the other hand, the BWf decreased in shrimp fed D4 and D5 (13.1 ± 0.72%; 11.5 ± 0.72%, respectively) with respect to those fed D1 (16.0 ± 0.34%; P < 0.05), while the rest of diets did not compromise the BW (P > 0.05). Similarly, D4 and D5 reduced the SGR from 4.89 ± 0.02% (D1) to 4.74 ± 0.04% and 4.65 ± 0.05% BW/day, respectively (P < 0.05). Considering that there were not differences in apparent FCR values among any dietary group (P > 0.05), probably the decreased BWf and SGR with the higher inclusions of BSCP were attributed to the higher stocking density, in line with the observed negative correlation between survival and BW (Pearson Product Correlation; r = -0.90, P < 0.001). In fact, many aquaculture studies in shrimps and fishes have reported a dependency between growth and stocking density, which are usually inversely correlated (Schram et al., 2006; Rodríguez-Olague et al., 2021). Regarding gut microbial communities, shrimp fed D2 displayed a reduction in richness (ACE index = 156.81 ± 26.39) with respect to those fed with the control diet (201.02 ± 68.24; P < 0.05). Similarly, shrimp fed D2 showed a different beta diversity based on Weighted UniFrac distances with respect to the control group (as well as with respect to those fed the D4 and D5) (PERMANOVA; P < 0.05). However, all the dietary groups showed differential phylogenetic features among them (Unweighted UniFrac distances; P < 0.05). These results were reflected in the differential relative abundances of the genera Albimonas, Nautella, Aurantivirga, Tropicibacter, Pseudoalteromonas, Aliiroseovarius and Alkalimarinus among others (P < 0.05), as will be discussed. There were no significant differences in survival when whiteleg shrimp was challenged with V. harveyi (P > 0.05). To sum up, the use of Uniprotein® as a protein source in whiteleg shrimp diets is a sustainable and feasible strategy for improving its production.
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