Effect of oregano essential oil on growth and composition of gut prokaryote microbiota on striped bass (Morone saxatilis)

New sustainable strategies such as oregano essential oil (OEO) are being assessed to improve farmed fish's health and performance. Several studies have described the beneficial effects of OEO when used as a dietary supplement in freshwater species. However, information about its use in marine fish is scarce. The bioassay consisted of four experimental groups: a control and three levels of OEO dietary inclusion: OEO75 (0.75 ml kg-1), OEO100 (1.0 ml kg-1), and OEO125 (1.25 ml kg-1) in a seawater flow-through system. An initial sample was collected for microbiota analysis. After 70 days of the bioassay, data for growth parameters and samples for gut microbiota were taken. The final weight of OEO75 was higher, and its feed conversion rate was reduced. Bioinformatic analysis of the OEO experimental groups revealed a notable reduction in Proteobacteria (Vibrio, Flavobacteria, and Vibrionales order) abundance and an increase in Euryarchaeota phyla in a dose-dependent manner. The predicted functions of OEO-treated groups shifted from a nutritional role (OEO75) to replication, proliferation, and bacterial defense mechanisms (OEO100 and OEO125). The addition of OEO didn't improve the growth performance, but it modulated the gut microbiota by reducing the abundance of dominant bacteria and modifying their metabolic pathways.