Introduction
Flathead grey mullet (Mugil cephalus) is a promising omnivorous fish species for a sustainable Mediterranean aquaculture diversification due to the low dietary requirement of nutrients and the high market price of its roe (bottarga) (Solovyev and Gisbert, 2022). Bacterial single cell ingredients produced by fermenting industry by-products represent a sustainable alternative of soybean meal (SBM) and fishmeal (FM) (Glencross et al., 2020). The application of bacterial single cell protein (SCP) ingredient were assessed on the blood biochemistry parameters and the gut microbiota of flathead grey mullet. Gut microbiota structure in farming condition is often associated with nutrients absorption and fish health. To the best of our knowledge this is the first characterisation of grey mullet gut microbiota diversity and structure under captive conditions and fed different diets.
Materials and Methods
Two isonitrogenous and isoenergetic experimental diets containing different SCP and SBM inclusion (SCP0, 0% SCP and 30% SBM; SCP10, 10% SCP and 9% SBM) were tested on triplicated groups of 45 fish (initial weight: 67.9 g) reared in a RAS over a period of 113 days. SCP ingredient consisted of a bulk of dried bacterial (Corynebacterium glutamicum) single cell (73.5 % crude protein). Feed was provided to apparent satiation on a 6 hours meal once a day. Somatometric indices, blood metabolic parameters and gut microbiota (GM) (MiSeq Illumina protocol, targeting V3-V4 regions of 16S rRNA gene) were assessed on 12 fish per treatment. Differences among treatments were considered significant at P < 0.05.
Results and Discussion
At the end of the trial, no significant diet effect were detected on somatometric indices. The levels of glucose, creatine, Ca+2, Fe, K+ and the ratio Na / K were found significantly higher in the fish fed SCP0 compared to SCP10 (P < 0.05). No significant differences between the treatments were observed for the other blood metabolic parameters considered. With regards to the gut microbiota, the most abundant phyla found in grey mullet gut were Fusobacteria and Proteobacteria (shown in Fig. 1). These two taxa represent about 87% of the whole GM in fish fed SCP0 where Fusobacteria were more abundant, while 83% in fish fed SCP10 where Proteobacteria were more abundant, although no significant differences were detected. The phyla Actinobacteria and Firmicutes seems to be higher in fish fed SCP10 (P = 0.1210 and P = 0.0592 respectively). Actinobacteria represented by Corynebacterium genus agreed with the dietary inclusion of bacterial SCP from C. glutamicum, while Firmicutes, especially lactic acid bacteria, have been associated with a beneficial effect and a healthy gut ecosystem. With a focus on the class level, specific compositional differences, although non-significant, where detected in fish fed SCP10 which present high abundance of bacilli (P = 0.1702) and clostridia (P = 0.0621). These two classes of bacteria include some promising probiotics which have been shown to be beneficial for the immune response and disease resistance in cultured fish (Guo et al., 2020; Kuebutornye et al., 2019). Overall in both groups, the more abundant genus was Cetobacterium, which have been found to be dominant in the gut of many freshwater species and have a positive impact on the digestion and metabolism efficiency of the host. Finally (sown in Fig. 2), no difference in the gut ecosystem diversity were detected at phylum and genus level between the two groups, however at species level there is a tendency for higher diversity in fish fed SCP10 (α-diversity; P = 0.1280).
Conclusion
Bacterial SCPs are promising circular ingredients which also present a probiotic effect. As such, further research is needed to better understand their application on grey mullet diet. Moreover, data from the gut microbiota contribute to the deep characterisation of the taxonomic diversity of the grey mullet gut bacterial community which is essential for understanding the dynamics of the diet-host-gut microbiota axis.
References
Glencross, B., Huyben, D., Schrama, J., 2020. The Application of Single-Cell Ingredients in Aquaculture Feeds—A Review. Fishes 5, 22. https://doi.org/10.3390/fishes5030022
Guo, P., Zhang, K., Ma, X., He, P., 2020. Clostridium species as probiotics: potentials and challenges. J. Anim. Sci. Biotechnol. 11, 24. https://doi.org/10.1186/s40104-019-0402-1
Kuebutornye, F.K.A., Abarike, E.D., Lu, Y., 2019. A review on the application of Bacillus as probiotics in aquaculture. Fish Shellfish Immunol. 87, 820–828. https://doi.org/10.1016/j.fsi.2019.02.010
Solovyev, M., Gisbert, E., 2022. Feeding regimes affected the circadian rhythms of pancreatic digestive enzymes and somatic growth in flathead grey mullet (Mugil cephalus) fry. Comp. Biochem. Physiol. A. Mol. Integr. Physiol. 264, 111116. https://doi.org/10.1016/j.cbpa.2021.111116