Poly-β-hydroxybutyrate administration during early life: effects on performance, immunity and microbial community of European sea bass yolk-sac larvae

Abstract

The reliable production of marine fish larvae is one of the major bottlenecks in aquaculture due to high mortalities mainly caused by infectious diseases. To evaluate if the compound poly-β-hydroxybutyrate (PHB) might be a suitable immunoprophylactic measure in fish larviculture, its capacity to improve immunity and performance in European sea bass (Dicentrarchus labrax) yolk-sac larvae was explored. PHB was applied from mouth opening onwards to stimulate the developing larval immune system at the earliest possible point in time. Larval survival, growth, microbiota composition, gene expression profiles and disease resistance were assessed. PHB administration improved larval survival and, furthermore, altered the larva-associated microbiota composition. The bacterial challenge test using pathogenic Vibrio anguillarum revealed that the larval disease resistance was not influenced by PHB. The expression profiles of 26 genes involved e.g. in the immune response showed that PHB affected the expression of the antimicrobial peptides ferritin (fer) and dicentracin (dic), however, the response to PHB was inconsistent and weaker than previously demonstrated for sea bass post-larvae. Hence, the present study highlights the need for more research focusing on the immunostimulation of different early developmental stages for gaining a more comprehensive picture and advancing a sustainable production of high quality fry.

Figure 1

PHB administration enhances survival of sea bass larvae. Depicted are Kaplan-Meier survival curves of sea bass larvae (10 to 22 dph) administered a low PHB dose from mouth opening onwards (LMO), a low PHB dose from first feeding onwards (LFF), a high PHB dose from mouth opening onwards (HMO), a high PHB dose from first feeding onwards (HFF) or no PHB (control), respectively. The dashed lines represent the 95% confidence intervals.

Figure 2

PHB affects the expression of (A) ferritin (fer) at 11 dph and (B) dicentracin (dic) at 22 dph in sea bass larvae. A low PHB dose from mouth opening onwards (LMO), a low PHB dose from first feeding onwards (LFF), a high PHB dose from mouth opening onwards (HMO), a high PHB dose from first feeding onwards (HFF) or no PHB (control) was administered, respectively. The figure displays the x-fold gene expression to the control. Data are presented as mean ± SEM. Treatments with different letters are significantly different at p < 0.05.

Figure 3

PHB controls the bacterial phyla composition in European sea bass larvae. (A) PCA plot visualizing the bacterial phyla composition in sea bass larvae (22 dph) administered a low PHB dose from mouth opening onwards (LMO), a low PHB dose from first feeding onwards (LFF), a high PHB dose from mouth opening onwards (HMO), a high PHB dose from first feeding onwards (HFF) or no PHB (control), respectively. (B) The corresponding scatterplot represents the contribution of each variable (bacterial phyla) to the total variability. Principle component 1 retains 15.5% and principle component 2 retains 12.9% of variance. (C) Percentage of different bacterial phyla in European sea bass larvae.

Figure 4

Larval survival differs between PHB treatments upon bacterial challenge test. Kaplan-Meier survival curves of sea bass larvae challenged with V. anguillarum at 22 dph after being administered a low PHB dose from mouth opening onwards (LMO), a low PHB dose from first feeding onwards (LFF), a high PHB dose from mouth opening onwards (HMO), a high PHB dose from first feeding onwards (HFF) or no PHB (control), respectively. The dashed lines represent the 95% confidence intervals.