Combined effect of polyethylene microplastics and Microcystis aeruginosa spent medium on Daphnia magna

The concomitant occurrence of microplastics (MPs) and toxic cyanobacteria in aquatic ecosystems is predicted to exacerbate toxicity to aquatic organisms, terrestrial animals and humans. We investigated the combined effects of naturally released extracellular metabolites from Microcystis aeruginosa, a harmful bloom-forming cyanobacterium and polyethylene (PE) MPs by first conducting a 48-h acute toxicity test, followed by an assessment of the recovery capacity of the test organism over 21 days. Immobilization in the combined MP + SM exposure reached 36.7% and 46.7% at 25 and 50 mg L⁻¹, respectively, which was approximately 2.2-fold and 1.7-fold higher than in the corresponding MP-alone treatments (16.7% and 26.7%), (p > 0.05). During the recovery period (21-d), a 50% higher mortality rate and reduced reproduction under combined treatment were observed compared to single treatment (p > 0.05). Subsequently, the oxidative stress response and the uptake of MPs in Daphnia magna were analyzed. The production of antioxidative materials was induced under single treatment, but it was not induced under combined treatment. The ingestion of MP particles by D. magna reached a six-fold increase in the combination treatment compared to the treatment with MPs alone (p < 0.05). These findings suggest that short-term (48-h) combined exposure can enhance MP ingestion, which may hinder recovery and ultimately lead to reduced reproduction and increased mortality of D. magna. The results of this study provide valuable insight into the management of aquatic systems regarding the ecotoxicological risk of MPs and harmful algal blooms.