Effects of Environmental Salinity on the Regulation of Trace Metals and Associated Transporters in the Euryhaline Tilapia (Oreochromis mossambicus).

In fish, trace metals play a crucial role in a range of physiological processes such as cellular respiration, enzymatic activity, and lipid metabolism. These processes, together with the concerted extrusion and uptake of ions via osmoregulatory tissues such as gills, are highly dependent on environmental salinity. The effects of salinity on the regulation of trace metals, however, remain poorly understood. With an emphasis on zinc, iron and manganese, this study first assessed trace metal concentrations in muscle and gill in the euryhaline Mozambique tilapia, Oreochromis mossambicus, acclimated to either FW or SW. Concentrations of manganese were higher in gills of SW-acclimated fish. In muscle, manganese and iron contents were higher in FW-acclimated fish, whereas zinc was higher in SW-acclimated fish. We then employed RNA-Seq analyses (n = 3 biological replicates per treatment) and DESeq2 statistical modeling with Benjamini-Hochberg FDR correction to reveal differential responses in specific trace metal transporters of the solute carrier superfamily (SLC). Branchial transcripts from fish acclimated to steady-state salinities were also compared with those from fish transferred from SW to a tidal regime (TR) for 15 days. A TR experimental paradigm is characterized by alternating between FW and SW every 6 h to simulate salinity fluctuations of estuaries, a common habitat of euryhaline fish such as the Mozambique tilapia. Most differentially expressed slc genes were observed between SW and a TR. From those, there were seven transcripts of the slc30 and slc39 families, primarily associated with the transport of zinc, differentially expressed between fish reared in SW and TR. Generally, the expression of slc genes varied to a greater extent between either FW or SW phases of TR and steady-state FW or SW than between the two phases of the TR or between steady-state FW and SW. Collectively, these data show salinity-dependent changes in tissue zinc, iron, and manganese and transcriptional activation of associated trace metal transporters in gill. Notably, most transcripts became differentially expressed when fish were transferred to a TR, suggesting that the regulation of trace metal transport is sensitive to frequent changes in salinity.