098 | Gill damage in zebrafish exposed to environmentally relevant concentrations of the UV filter avobenzone

Antonio Paolo Maria Graziani, Abdalmoiz I.m. Ahmed, Federica Talarico, Marcello Mezzasalma, Rachele Macirella, Elvira Brunelli | Department of Biology, Ecology and Earth Sciences DiBEST, University of Calabria, Rende [CS], Italy.

Avobenzone (AVO) is the most widely used organic UV filter worldwide, authorised for skin use in various regulatory contexts. Due to its widespread use, there are significant concerns about the environmental fate and impacts of AVO. Furthermore, its presence in surface waters poses a potential ecotoxicological risk, but its effects on aquatic organisms remain poorly understood. The aim of our study is to investigate the effects of AVO at two environmentally relevant concentrations (3 and 9 µg/L) on the Danio rerio model under acute (4 days) and chronic (21 days) exposure, using the gills as the target organ. Histological and ultrastructural analyses revealed a progressive pattern of gill injury that depended strongly on exposure concentration and duration. Histological examination in particular demonstrated severe gill damage, including epithelial lifting, hypertrophy and ectopia of chloride cells, and cytoplasmic vacuolisation. Furthermore, ultrastructural analysis revealed significant chloride cell damage, including mitochondrial swelling, disrupted cristae and apoptosis, primarily at higher concentrations and with prolonged exposure. A relative expression analysis was also conducted on the genes involved in oxidative stress (sod1 and cat) and those involved in the apoptotic pathway (casp3, casp8, casp9, apaf1). Our results reveal that low-dose exposure initially downregulated sod1 and cat, followed by a marked upregulation after 21 days, whereas high-dose exposure resulted in persistent suppression or no induction of antioxidant genes. Regarding the expression of apoptotic genes, after 4 days of exposure to the low concentration, casp3, casp8, and casp9 were downregulated. By contrast, after 21 days, casp8 and casp9 were upregulated, whereas casp3 expression was comparable to the control. Exposure to the high dose induced the upregulation of casp8 and casp9 after 4 and 21 days, whereas casp3 was consistently downregulated. Apaf1 expression was always downregulated except after prolonged exposure to the high concentration, indicating a dose- and time-dependent activation of the intrinsic apoptotic machinery. The results presented herein demonstrate that AVO can severely affect gill structure and ultrastructure, also altering the expression of genes associated with oxidative stress and apoptosis signalling. These effects depend on the concentration and duration of exposure. In conclusion, by integrating histological, ultrastructural, and molecular analyses, the present study bridges the gap between early molecular responses and organ-level dysfunction, providing mechanistic insight into how AVO exposure compromises gill integrity and function.