The combined toxicity of polystyrene microplastics and glyphosate on Daphnia magna: Physiological and behavioral responses

Although microplastics (MPs) and glyphosate (Gly) commonly co-occur in aquatic environments, their combined toxic effects and underlying mechanisms remain poorly understood. In this study, Daphnia magna was exposed to polystyrene MPs (12 mg/L) and Gly (40 mg/L) individually and in combination for 96 h to evaluate their ecotoxicity. MPs accumulation, heart rate, swimming behavior, antioxidant enzyme activities (SOD, CAT, GST), malondialdehyde (MDA) content, apoptosis, intestinal histopathology, and expression of immunity- and reproduction-related genes were examined. MPs were rapidly ingested and persistently accumulated in the gut, and Gly co-exposure did not significantly alter accumulation levels. Combined exposure transiently increased the heart rate, significantly inhibited swimming behavior, induced oxidative stress, exacerbated apoptosis, and caused severe intestinal damage. A two-way ANOVA showed that the type of the MPs × Gly interaction differed among the measured responses: the interaction was synergistic for the stress- and detoxification-related indicators (GST activity, HSP70 and GST expression) and antagonistic for the oxidative-damage and reproduction-related indicators (SOD, MDA, CYP360 and Vtg), whereas no significant interaction was detected for CAT, DHB, EST, CYP314, FKBP39 or the cell death-associated fluorescence index. These results indicate that co-exposure to MPs and Gly produces non-additive effects in D. magna, with the type of interaction depending on the endpoint assessed. This study provides a scientific basis for assessing the ecological risks of combined MPs and pesticide pollution in aquatic environments.