ALBERT

Description: 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.

Description: Introduction Various probiotics and immunostimulants have been shown to enhance the immune response and alter the disease resistance of aquaculture organisms. The bacterial energy storage compound poly-ß-hydroxybutyrate (PHB) for example improves resistance against pathogenic infections in shrimp (Laranja et al., 2014) and exhibits a controlling effect on the gut microbiota of juvenile sea bass which may result in the stimulation of immune functions (De Schryver et al., 2011). These properties might be of special importance for culturing early life stages since their immune system is not yet fully developed. Therefore, we assessed the potential immunostimulating effect of PHB in European sea bass (Dicentrarchus labrax) larvae in our study. Materials and methods We used rotifers as live carriers to feed PHB-accumulating bacteria (Alcaligenes eutrophus) to first-feeding larvae over a period of 14 days. Bacteria with a low (2.5%) and a high (75%) PHB content were used, respectively. Apart from the dose effect, we wanted to determine to which extend the point in time of the PHB administration matters. Therefore, PHB was added (in some experimental groups) directly to the water from the moment of mouth opening onwards. This led to the following experimental groups: 1) early and 2) later stimulation with a low PHB level, 3) early and 4) later stimulation with a high PHB level and 5) a control without any PHB. To estimate the immediate impact of PHB, larval mortality rates were monitored daily over the course of the experiment. Furthermore, larvae were sampled the first time after being fed for 3 days with PHB encapsulated in rotifers (and 5 days of PHB water treatment) and the second time after 14 days PHB via rotifers (and 16 days of PHB water treatment), respectively. Results and discussion Larval mortality rates were found to be the highest in the control group. During both sampling points larvae were slightly smaller and weighed less in the control group. The analysis of the gene expression profiles revealed that only certain immune genes such as cytokines (Interleukin-1ß, Interleukin 8 and TNFα) were affected by the PHB treatment. The data indicate that the application of PHB can provide a beneficial effect to sea bass larviculture in terms of higher survival rates. But further studies are required to verify the impact on the developing immune system of the larvae.