Comparing the toxicity of polystyrene microplastic and diclofenac individually and mixed on neonate and adult Daphnia magna: Immobilisation and oxidative stress status

Research has underscored the global ubiquity of microplastics (MPs) in various environments1. However, the presence of multiple stressors in these environments, such as co-occurrence with pharmaceuticals and personal care products (PPCPs), adds complexity to the assessment of MP toxicity2 and could potentially alter the overall adverse effects upon exposure. The combined effect of MPs and PPCPs can potentially be additive, synergistic or antagonistic3. This study aimed to investigate the effects of polystyrene (PS) MP (fragments: 45-63 µm diameter) and diclofenac (DCF), two commonly occurring aquatic pollutants, on the model bioindicator organism Daphnia magna. In addition to mobility, oxidative stress status was used as an indicator of toxicity in response to exposure to these toxicants individually and combined at various environmentally representative concentrations (0.01, 0.1, 1.0, 10, 50, and 100 mg/L), for 24 and 48 h periods. The exposures were conducted in the dark to avoid photodegradation. Immobilisation was assessed according to the OECD guidelines4, and live neonates were collected to measure reactive oxygen species (ROS) to assess oxidative stress. The lethal dose 50 values (LD50) and the no and lowest observed effect levels (NOEAL and LOEAL) were calculated to determine whether combined exposure altered toxicity, i.e., additive, synergistic, or antagonistic effects.
PS and DCF individually at concentrations of 50 mg/L and higher caused significant immobilisation in neonates, but not adults, only after 48 h of exposure. In combination, adverse effects were observed after 24 h of exposure to 50 mg/L each. In neonates, ROS increased with all PS exposure concentrations, up to 2.4 times with the highest exposure concentration, and with adults on average 1.6 times. On the other hand, DCF did not have a significant effect. However, when combined, the LD50 of the individual compounds (21.4 mg/L for PS and 19.5 mg/L for DCF) decreased 1.6-fold for neonates. Similarly, the NOEAL decreased by 5-fold and the LOAEL by 2.5, indicating a synergistic effect. This reduction in dose descriptors highlights the risk of enhanced toxicity due to pollutant interactions, an issue of concern in environments where multiple contaminants coexist.
This research underscores the importance of evaluating the combined effects of different pollutants to better appreciate the potential environmental risks. These findings highlight the need for regulatory frameworks to consider complex mixtures of pollutants, as these can pose greater risks than the sum of their parts. Further studies should focus on elucidating the interaction mechanisms between these and other contaminants to inform more effective environmental protection and pollution mitigation strategies.