Microalgae and phytase dietary supplementation improved growth and gut microbiota in juvenile European seabass (Dicentrarchus labrax)

Abstract

Fishmeal and fish oil have been the main sources of protein and fatty acid for aquaculture fish. However, their increasing price and low sustainability have led the aquafeed industry to seek sustainable alternative feedstuffs to meet the nutritional requirements of fish and improve their health and performance. Plant proteins have been successfully used to replace fishery derivatives in aquafeeds, but the presence of anti-nutritional substances is a potential drawback of this approach. Thus, it has been reported that phytate breakdown can be caused by feed supplementation with exogenous phytase. The inclusion of microalgae has been proposed to improve gut functionality in fish fed diets with a high vegetable protein content. The aim of this study was to evaluate the effect on the growth and gut microbiota of European seabass (Dicentrarchus labrax) juveniles of a diet containing a blend of microalgae (Arthrospira platensis and Nannochloropsis gaditana) and different concentrations of phytase. An 83-day feeding trial was conducted, comprising four experimental diets with 2.5% microalgae and 500, 1,000, 2,000, or 10,000 phytase units (FTU)/kg feed and a microalgae- and phytase-free control diet. At the end of the trial, a significantly increased body weight was observed in fish fed the diet with the highest phytase concentration (10,000 FTU/kg) versus controls, although the gut bacterial composition did not differ from controls in alpha or beta diversity with either majority (Weighted UniFrac) or minority bacterial strains (Unweighted UniFrac). In comparison to the control group, the groups fed diets with 1,000 or 2,000 FTU/kg diets had a lower alpha diversity (Shannon's diversity index), while those fed diets with 500 FTU/kg or 1,000 FTU/kg showed distinct clusters in beta diversity (involving minority ASVs). According to these findings, the diet containing the 2.5% microalgae blend with 10,000 FTU/kg may be useful to increase the aquafeed quality and sustain the growth performance of juvenile European seabass.

Keywords: Arthrospira platensis; Dicentrarchus labrax; Nannochloropsis Gaditana; Fish nutrition; Gut microbiota; Microalgae; Phytase.

Fig. 1

Microbial composition at class level of juvenile European seabass (Dicentrarchus labrax) gut microbiota grouped by experimental diet (2.5% microalgae [A. platensis and N. gaditana] plus 500, 1,000, 2,000, or 10,000 FTU/kg phytase). Each bar represents the mean relative abundance of the different bacterial classes in samples from each group. Classes in the legend are ordered from the most to least abundant

Fig. 2

Microbial composition at genus level of juvenile European seabass (Dicentrarchus labrax) gut microbiota group by experimental diet (2.5% microalgae [A. platensis and N. gaditana] plus 500, 1,000, 2,000, or 10,000 FTU/kg of phytase). Each bar represents the mean relative abundance of the different bacterial genera in samples from each group. Genera in the legend are ordered from the most to least abundant

Fig. 3

Shannon’s diversity index of the gut microbiota of juvenile European seabass (Dicentrarchus labrax) fed with control diet or a diet supplemented with 2.5% microalgae (A. platensis and N. gaditana) and different concentrations of phytase (500, 1,000, 2,000, or 10,000 FTU/kg). Bars indicate the standard error of the mean

Fig. 4

Dimensional figures showing the first two axes of the Principal Coordinate Analysis and representing bacterial communities in the gut of juvenile European seabass (Dicentrarchus labrax) fed a control diet or a diet supplemented with 2.5% microalgae (A. platensis and N. gaditana) and different phytase concentrations (500, 1,000, 2,000, or 10,000 FTU/kg) using Weighted (A) and Unweighted UniFrac (B) distance matrices. Samples are color-marked by treatment (Control - red; 500 FTU/kg - blue; 1,000 FTU/kg – orange; 2,000 FTU/kg – green; 10,000 FTU/kg – purple). The proportion of variance explained by each PCo axis is indicated