Environmental microplastics accumulate in gonads in a sex-dependent manner and alter reproductive success in zebrafish (Danio rerio)

Microplastics (MPs) are increasingly associated with physiological disruptions in aquatic organisms, yet the biological responses to environmentally sourced particles remain underexplored. This study investigated the reproductive toxicity of environmentally derived MPs collected from the Ikopa River (Antananarivo, Madagascar) in Danio rerio. Zebrafish were chronically exposed to cryomilled riverine MPs (1.2–50 µm) at concentrations of 100 and 1000 µg/L for 66 days, with daily reproductive assessments conducted over the final 21 days in accordance with OECD Test Guideline 229. Microplastic accumulation in gonadal tissue was assessed, along with subcellular responses via enzymatic assays in gonads and proteomic profiling in liver samples. Reproductive toxicity was evaluated through gonadal histology, fecundity, and fertility rates. MPs accumulated in gonads in a sex- and concentration dependent manner, with the highest levels in males exposed to 1000 µg/L (177.88 ± 102.65 particles/mg tissue, mean ± SD, n = 4). Despite MPs accumulation, no histopathological lesions were observed. However, significant oxidative stress and energy metabolism disruptions were identified in the liver, suggesting hepatic dysfunction as a potential driver of reproductive impairments. Furthermore, six polychlorinated biphenyl (PCB) congeners ranging from dozens to hundreds of ng/g MPs, and seven polybrominated diphenyl ether (PBDE) congeners in the range of a few ng/g MPs were detected on MPs surfaces, which may exacerbate toxicity via apoptosis inhibition. These findings provide novel mechanistic insights into how environmentally relevant MPs impair reproductive function in fish. The results underscore the necessity of incorporating environmental microplastics into toxicity testing frameworks to ensure accurate ecological risk assessment.