Acidified water promotes silver-induced toxicity in zebrafish embryos

Freshwater acidification is a global environmental challenge, yet the effects of acidic water on fish resistance to toxic Ag⁺ remain an unexplored area. To address this knowledge gap, zebrafish embryos were exposed to different concentrations (0 (control), 0.1, and 0.25 mg/L) of AgNO₃ under pH 5 or 7 for 7 days. Notably, AgNO₃ at 0.25 mg/L resulted in 100 % mortality in both pH conditions, while AgNO₃ at 0.1 mg/L resulted in higher mortality at pH 5 (85 %) compared to pH 7 (20 %), indicating that acidic water enhanced Ag⁺ toxicity. Several parameters, including body length, inner ear (otic vesicle and otolith) and yolk sac areas, lateral line hair cell number and morphology, the number of ionocytes (H⁺-ATP-rich cells and Na⁺/K⁺-ATP-rich cells), and ion contents (Ag⁺, Na⁺, and Ca²⁺) were assessed at 96 h (day 4) to investigate individual and combined effects of Ag⁺ and acid on embryos. Acid alone did not significantly alter most parameters, but it decreased the yolk sac area and increased the ionocyte number. Conversely, Ag⁺ alone caused reductions in most parameters, including body length, the inner ear area, hair cell number, and ionocyte number. Combining acid and Ag⁺ resulted in greater suppression of the otolith area, hair cell number, and Na⁺/Ca²⁺ contents. In conclusion, acidification of freshwater poses a potential risk to fish embryo viability by increasing their susceptibility to silver toxicity, specifically affecting sensory function and ion regulation.