There is an ongoing debate regarding the role of microplastics (MPs) in enhancing the effects of various chemical compounds, highlighting the need for more detailed analyses. In this study, neonates of the water flea (Daphnia magna) were exposed to polystyrene MPs (PS-MPs
3 µm
exposure concentration, 1.25 mg/L), a mixture of seven organic UV-filters (avobenzone, ethylhexyl triazone, homosalate, iscotrizinol, octinoxate, octisalate, and octocrylene
each at a low environmental concentration of 200 ng/L), or the combination of both pollutants for 3, 7, or 21 days. Results showed that PS-MPs alone decreased the body size of daphnids, while all treatments increased heart rate by the end of the 21-day exposure. On days 3 and 21, both PS-MPs and PS-MPs + UV-filters reduced swimming speed and total distance travelled. Additionally, PS-MPs increased the time of the first egg production, but decreased the egg number in the first production, total egg number, maximum egg number, and total neonate number during the 21-day treatment. Similarly, UV-filters or the combined pollutants increased the time of the first egg production and decreased the total neonate number. All treatments increased multixenobiotic resistance activity on days 3 and 7, while only UV-filters elevated CYP450 activity on day 3. PS-MPs or combined pollutants increased GST activity during early exposure but showed no effect on day 21. CAT activity was also affected by treatments in a time-dependent manner. These findings demonstrate that chronic exposure to PS-MPs and UV-filters, applied individually or in combination at a low environmental concentration, moderately impacts development, heart rate, and swimming activity in D. magna, while significantly altering reproduction and key cellular functions such as membrane transport activity, metabolism, and antioxidant defense. Co-exposure did not reveal a clear pattern of synergism or antagonism, suggesting that joint toxicity risks of these xenobiotics typically emerge at concentrations higher than low environmental levels. Future studies should explore potential interactions more thoroughly and assess transgenerational effects on reproduction and cellular defense pathways.