Salinity as a shield: how elevated salinity mitigates zinc oxide nanoparticle toxicity in Javanese medaka (Oryzias...

The increasing environmental prevalence of zinc oxide (ZnO) nanoparticles (NPs) raises significant concerns about their ecotoxicological effects in aquatic ecosystems. This study investigates the sublethal toxicity of ZnO NPs (0-100 µg/L) to Javanese medaka (Oryzias javanicus) embryos across two ecologically relevant salinities (5 and 18 ppt) through chronic exposure. Dynamic light scattering revealed rapid NP aggregation (4,636-5,422 nm) with low stability (zeta potential 2.50-3.12 mV). Mortality exhibited a concentration-dependent pattern, with 18 ppt salinity providing significant protection (0.06% vs. 0.24% mortality increase per µg/L at 5 ppt). Cardiac responses showed time-dependent salinity interactions: initial cardiotoxicity at 18 ppt (53%-62% heart rate reduction at 5 days postexposure (dpe) reversed by 11 dpe, whereas 5 ppt exposures showed progressive impairment (32%-48% reduction). Hatching delays were concentration-dependent but diminished at 18 ppt, with 25 µg/L groups showing recovery by Day 20. Developmental abnormalities (pericardial edema, spinal deformities) were less severe at higher salinity. These findings demonstrate that elevated salinity mitigates ZnO NP toxicity. The study provides critical insights for environmental risk assessment of NPs in estuarine ecosystems, highlighting the need to consider salinity gradients in regulatory frameworks.