Response of gut microbiota to salinity change in two euryhaline aquatic animals with reverse salinity preference

While a digestive tract is known to harbor complex assemblages of microbiota, maintaining a functional, stable microbial community is important to the host's health but can be constantly influenced by various environmental factors. Salinity can lead to stress, including hyposaline or hypersaline stress to aquatic animals, which influence the growth condition of the animals. However, the relationship among the salinity, intestinal microbiota and growth condition of the aquatic animals remains unclear. In this study, two euryhaline species, Oreochromis niloticus (Nile tilapia) and Litopenaeus vannamei (Pacific white shrimp) were reared in water with different salinity and the growth condition verified that Nile tilapia and Pacific white shrimp had reverse salinity preference. The gut microbiota of Nile tilapia and Pacific white shrimp were compared by using 16S rRNA gene Illumina sequencing. Analysis of more than 280,000 quality-filtered sequences obtained from guts of Nile tilapia and Pacific white shrimp reared in different salinities showed that both host phylogeny and water salinity influenced the intestinal microbiota composition. Redundancy analysis combined with the heat map analysis indicated that among the shared operational taxonomy units (OTUs) in both Nile tilapia and Pacific white shrimp, 14 OTUs increased while 10 OTUs decreased in abundance toward to the increase of salt concentration. The proportion of bacteria regarded as opportunists increased while those regarded as commensal or beneficial bacteria decreased when the host facing hyposaline or hypersaline stress. The alteration of intestinal microbiota was likely attributed to the environmental selection for microbes that could grow better under high or low salinity, the host response to the salinity stress and subsequent stress exerting on the gut microbiota, or both. Statement of relevance: This manuscript studied the correlation between the intestinal microbiota and two euryhaline aquatic animals, Oreochromis niloticus (Nile tilapia) and Litopenaeus vannamei (Pacific white shrimp).