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 AgNO under pH 5 or 7 for 7 days. Notably, AgNO at 0.25 mg/L resulted in 100 % mortality in both pH conditions, while AgNO 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 Ca) 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/Ca 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.