In fed aquaculture, fishes are commonly fed with a fish-oil based diet mostly coming from captured fishes. This is one main global issue leading to overfishing of wild species (Cashion et al., 2017; Tacon & Metian, 2008). Several alternatives in lipid sources for fish diet have been tested and promising alternatives such as plants (e.g. rapeseed oil) or microalgae (e.g. Schizochytrium sp.) have been identified (Pérez-Pascual et al., 2020). Like other animals, fishes’ digestive tract is composed of a microbiota whose composition is linked to the host physiological state as well as its diet (Yukgehnaish et al., 2020). In reared fishes such as the European sea bass (Dicentrarchus labrax), replacing fish oil by other sources such as microalgae in their diet has been shown to modify the digestive microbiota composition (Pérez-Pascual et al., 2020).

Here, the aim of Katsoulis-Dimitriou et al. (2024), was to test the effect of three dietary microalgae blends on the midgut microbiota composition of the reared fishes. The authors compared the effect of a control diet (i.e. with only fish oil as lipid source, namely, FO) with that of three experimental diets with two thirds of the fish oil replaced by either a mixture of the microalgae Microchloropsis gaditana and Isochrysis sp. (now known as Tisochrysis lutea, MI), Phaeodactylum tricornutum and Isochrysis sp. (PI) or Schizochytrium sp. and P. tricornutum (SP). For each diet, 25 fishes were reared in each of the triplicated tanks and, after 80 days of experiment, a total of 10 fishes per diet were sampled. DNA was extracted from the midgut part of the intestine and a 16S rDNA-based metabarcoding approach was conducted to survey the associated bacterial community. Each diet type, FO, MI, PI and SP, was mostly characterized by a composition of specific abundant OTUs, indicating the clear influence of the oil composition on the digestive microbiota. When feeding with the MI diet, the authors also highlighted the presence of some candidate genera (e.g. Pseudoalteromonas, Pseudomonas, Bacillus and Rhodopseudomonas) as potential probiotics for fish aquaculture. Finally, in comparison to the fish oil diet, a predictive metabolic analysis of the bacterial community could suggest a differential expression of some polysaccharide metabolisms with the microalgae-based diets, highlighting a probable diet-based effect on the microbiota functioning.

The work from Katsoulis-Dimitriou et al. (2024) completes the current knowledge on using sustainable alternatives to traditional fish feed and its effect on the digestive microbiota composition of fishes. This work also opens new ways to be explored considering the enrichment of potential probiotics using microalgae-base diets. Further analyses testing specific functional approaches (e.g. transcriptomics, metabolomics) may allow completing the understanding of the gut microbiota functioning linked to diet composition. Finally, measurements on fish biometrics in a similar experiment should help understanding the contribution of a microalgal-diet to fish fitness.

References

Cashion, T., Le Manach, F., Zeller, D., & Pauly, D. (2017). Most fish destined for fishmeal production are food‐grade fish. Fish and Fisheries, 18(5), 837–844. https://doi.org/10.1111/faf.12209

Katsoulis-Dimitriou, S., Nikouli, E., Gkalogianni, E., Karapanagiotidis, I., Kormas, K. (2024) The effect of dietary fish oil replacement by microalgae on the gilthead sea bream midgut bacterial microbiota. BioRxiv, ver.3 peer-reviewed and recommended by PCI Microbiol https://doi.org/10.1101/2024.01.24.576938

Pérez-Pascual, D., Estellé, J., Dutto, G., Rodde, C., Bernardet, J.-F., Marchand, Y., Duchaud, E., Przybyla, C., & Ghigo, J.-M. (2020). Growth Performance and Adaptability of European Sea Bass (Dicentrarchus labrax) Gut Microbiota to Alternative Diets Free of Fish Products. Microorganisms, 8(9), 1346. https://doi.org/10.3390/microorganisms8091346

Tacon, A. G. J., & Metian, M. (2008). Global overview on the use of fish meal and fish oil in industrially compounded aquafeeds: Trends and future prospects. Aquaculture, 285(1–4), 146–158. https://doi.org/10.1016/j.aquaculture.2008.08.015

Yukgehnaish, K., Kumar, P., Sivachandran, P., Marimuthu, K., Arshad, A., Paray, B. A., & Arockiaraj, J. (2020). Gut microbiota metagenomics in aquaculture: Factors influencing gut microbiome and its physiological role in fish. Reviews in Aquaculture, 12(3), 1903–1927. https://doi.org/10.1111/raq.12416