How microplastics modulate the effects of bisphenol A during zebrafish development

In recent decades, plastic waste has become a serious environmental problem with implications for ecosystems and human health. When plastic is released, especially into water environments, it undergoes degradation, particularly by atmospheric factors such as ultraviolet (UV) radiation. This process breaks the plastic into smaller particles, known as microplastics (MPs). These small particles are found almost everywhere in the environment. Because of their small size and surface characteristics, microplastics can attract and carry other substances, such as chemical pollutants. Bisphenol A (BPA) is a synthetic compound commonly used in the production of polycarbonate plastics and epoxy resins, frequently detected in marine and fresh water. It is classified as an endocrine disrupting chemical, and its toxicological effects have been widely documented. In a previous study, we demonstrated that exposure to polystyrene microplastics (PS-MPs) of 1 µm and 3 µm in size negatively affects zebrafish embryonic development. Specifically, PS-MPs were shown to induce morphological malformations, promote apoptosis, increase heart rate, and disturb redox homeostasis. Based on the results previously obtained and on studies in the literature, this work aimed to investigate the interaction between PS-MPs and BPA during the early developmental stages of zebrafish. Embryos were exposed for 72 hours to PS-MPs (1 mgL-1) and BPA (25 µM) alone and in combination. We assessed some toxicity parameters, including hatching rate, spontaneous movement and heart rate, oxidative stress biomarkers, and the expression of development genes. The results showed that PS-MPs alone did not significantly alter the hatching rate, whereas BPA exposure delayed hatching. Both contaminants individually caused a reduction in spontaneous movements. Heart rate was increased following PS-MPs exposure but decreased with BPA. Interestingly, when embryos were co-exposed to PS-MPs and BPA, an antagonistic interaction was observed in several parameters: PS-MPs mitigated some of the toxic effects induced by BPA, particularly regarding hatching, spontaneous movement and heart rate. Regarding oxidative stress, both single BPA and combined exposure resulted in alteration of redox homeostasis, and the combination exhibited a synergistic effect. Gene expression analysis revealed that both pollutants individually downregulated developmental genes, but their combined exposure showed an antagonistic effect where PS-MPs mitigated the effect of BPA alone. These results show that microplastics and bisphenol A can interact in different ways. It is important to understand how these substances affect each other in order to better evaluate the risks they pose to the environment and to support future research on pollution in water ecosystems.