Sulfate (SO2−4) regulation is challenging for euryhaline species as they deal with large fluctuations of SO2−4 during migratory transitions between freshwater (FW) and seawater (SW), while maintaining a stable plasma SO2−4 concentration. Here, we investigated the regulation and potential role of sulfate transporters in Atlantic salmon during the preparative switch from SO2−4 uptake to secretion. A prepara-tory increase in kidney and gill sodium/potassium ATPase (Nka) enzyme activity during smolt development indicate preparative osmoregulatory changes. In con-trast to gill Nka activity a transient decrease in kidney Nka after direct SW expo-sure was observed and may be a result of reduced glomerular filtration rates and tubular flow through the kidney. In silico analyses revealed that Atlantic salmon genome comprises a single slc13a1gene and additional salmonid- specific duplica-tions of slc26a1 and slc26a6a, leading to new paralogs, namely the slc26a1a and - b, and slc26a6a1 and - a2. A kidney- specific increase in slc26a6a1 and slc26a1a during smoltification and SW transfer, suggests an important role of these sul-fate transporters in the regulatory shift from absorption to secretion in the kid-ney. Plasma SO2−4 in FW smolts was 0.70mM, followed by a transient increase to 1.14±0.33mM 2days post- SW transfer, further decreasing to 0.69±0.041mM after 1month in SW. Our findings support the vital role of the kidney in SO2−4 ex-cretion through the upregulated slc26a6a1, the most likely secretory transport can-didate in fish, which together with the slc26a1a transporter likely removes excess SO2−4, and ultimately enable the regulation of normal plasma SO2−4 levels in SW
