Ecotoxicology of Polystyrene Microplastic Fragments: Oxidative Stress Effects in Neonate Versus Adult Daphnia magna

Abstract Polystyrene is an inert polymer; however, sublethal effects have been observed in aquatic organisms exposed to microparticles of this plastic. Information on microplastic-related oxidative stress and related antioxidative responses as adverse effects and the underlying mechanisms of toxicity are limited. Daphnids are key contributors in aquatic ecosystems, linking primary producers to consumers and predators, facilitating energy transfer and thus being considered an ideal bioindicator organism to study the adverse effects of polystyrene fragments on the oxidative stress status and subsequent enzymatic antioxidant response. In the present study, neonates (≤ 24 h) and adult daphnids were acutely exposed to polystyrene fragments sieved to three size factions (< 25 µm, 45—63 µm, and 100—500 µm), and adverse effects were evaluated after 24 and 48 h. In adults, larger particles elicited reactive oxygen species by 61%, which was met with elevated superoxide dismutase (17%) and catalase activities (98%), restoring redox homeostasis within 48 h. However, in neonates, the reactive oxygen species increased with exposure to all polystyrene sizes within 24 h, and homeostasis was regained within 48 h without eliciting the enzymatic antioxidant defense. The adverse effects were associated with size fractions that were too large to be consumed, suggesting that leached additives may be involved. Further studies are needed to determine whether nonenzymatic antioxidants were responsible for neutralizing excess reactive oxygen species in neonates.