Integrated histological and molecular assessment of avobenzone gill toxicity in adult zebrafish (Danio rerio)

Organic UV filters are widely used in personal care products and are increasingly being detected in aquatic environments, where they may pose a risk to aquatic organisms. Avobenzone (AVO) is one of the most widely used of these compounds worldwide. The European Commission has included it among substances to be systematically monitored across EU Member States due to concerns regarding its environmental fate and potential ecological risks. The available literature indicates that avobenzone can cause adverse biological effects in a wide range of aquatic species. However, research into the effects of avobenzone on fish is extremely limited, and no data on gill toxicity are currently available. In this study, we investigated the effects of two environmentally relevant concentrations of AVO (3 and 9 μg/L) on the gills of zebrafish ( Danio rerio ) under short-term (4 days) and long-term (21 days) exposure conditions. Morphological, ultrastructural, morphometric and molecular endpoints were evaluated to provide a comprehensive assessment of AVO-induced toxicity. Our results demonstrate that exposure to AVO induces pronounced, time- and dose-dependent alterations in gill tissue. These include chloride cell hypertrophy, epithelial lifting, macrophage proliferation and increased filament thickness and lamellar width, alongside reduced lamellar length. Furthermore, AVO exposure was found to significantly modulate the expression of genes involved in osmoregulation (atp1a1a.1 and a qp3a ), the oxidative stress response ( sod and cat ) and apoptosis ( casp3, casp8 and casp9 ). Overall, our findings provide novel evidence of avobenzone-induced gill toxicity in adult zebrafish and contribute to improving the environmental risk assessment of UV filters in aquatic ecosystems. Additionally, the information presented here underscores the importance of integrating morphological and molecular approaches when evaluating the toxic potential of emerging contaminants.