Seawater adaptation of freshwater cultured rainbow trout (Oncorhynchus mykiss): Insights into the gut microbiota and tissue gene expression profiles

Abstract

Mariculture of rainbow trout (Oncorhynchus mykiss) represents a significant development in the field of freshwater aquaculture, due to the fact that, as a large anadromous fish, it can overcome the geographical restrictions on the culturing of this species, whilst also providing a higher quality food source for human consumption. In order to explore the seawater adaptation of rainbow trout, a salinity accumulation trial was performed with nine salinity point from freshwater (‰) to seawater (32 ‰). A total of 81 rainbow trouts were obtained with nine of each salinity point, and the changes in its gut microbiota and gene expression in diverse tissues (gill, intestine, kidney, and liver) were uncovered. The 16S-Seq revealed significant changes in the diversity and composition of the gut microbiota among rainbow trout under different salinity conditions. These changes were initially acute and subsequently stabilised over time. Following transfer to an seawater environment, the gut microbiota of rainbow trout established a stable state that was distinct from that of individuals inhabiting freshwater. The gut microbiota of rainbow trout inhabiting seawater exhibited a simple yet stable co-occurrence network, robust fermentation and cell structure biosynthesis, and drift-dominant assembly, characteristics that differ significantly from those observed in freshwater populations. The application of RNA-Seq to tissue cells has revealed higher sensitivity of liver and intestine compared of kidney and gill to salinity stress. Based on the differentially expressed genes in all four tissues, DNA damage, cancer risks, and rhythm disorders were observed in rainbow trout cultured in seawater. It is hypothesised that the heightened cancer risks observed in rainbow trout under conditions of elevated salt stress may be attributable to fluctuations in the composition of certain rare gut bacteria, such as Psychrobacter celer, Lactobacillus, and Escherichia. The results of the present study have revealed the response of the gut microbiota and host physiology of rainbow trout to salinity stress. This provides a solid foundation for the development of mariculture technology for this species.