Microbiota modification of Mytilus edulis larvae in response to the use of a new probiotic, the marennine, in aquaculture

In bivalve hatcheries, opportunistic pathogens have been associated with important mass mortality events of larvae and important economic lost for producers. New alternatives to the use of antibiotics, such as probiotics, have been proposed to limit the occurrence of such events in bivalve hatcheries and to stabilize bivalve production. Probiotics are microorganisms, or natural molecules, that are associated with beneficial effects for the host at different levels, especially to enhance their resistance to external stressors such as bacterial pathogens. It is know recognized that the composition of the host microbiota influences host health status and could be a target of probiotics. The aim of this study is to highlight the protective effect of a new probiotic, the marennine, on Mytilus edulis larvae during bacterial challenges in relation with a potential modification of the microbiota of the marennine-treated larvae. The main hypothesis is that the addition of marennine during larvae rearing processes could modify the conditions prevailing in the rearing medium and, as a consequence, the composition of the larvae microbiota, leading to a better resistance to bacterial infections. The marennine is a blue pigment originating from the diatom Haslea ostrearia, that have demonstrated a positive effect on larvae survival at a final concentration of 500 µg L-1. In this study, D-larvae (9 days old) and post-larvae (29 days old) were exposed for 96h to Vibrio splendidus with and without mareninne at a final concentration of 500 µg L-1. Our results demonstrated that at this concentration, the marennine have no direct antimicrobial effect on Vibrio splendidus growth kinetics. In addition, the presence of marennine did not modify the abundance of bacteria in the rearing medium, suggesting no direct antimicrobial effect of marennine on the bacterial load to which larvae were exposed during the experiments. Nevertheless, the presence of marennine increased the survival of D-larvae exposed to the pathogen but have no effect on post-larvae survival. The undergoing molecular analyses and the future metagenomics analyses of the larvae microbiota diversity will allow us to demonstrate if a modification in the larval microbiota’s richness might explain the increase of the survival rate during the production of blue mussels in aquaculture. Ultimately, our work will enable us to shed light on the importance of the larval microbiota in pathogen-resistance during bivalves rearing process