Biochemical impacts of PET microplastics and cadmium on Danio rerio under environmental conditions

The escalating presence of microplastics (MPs) in aquatic ecosystems has sparked significant concern due to their detrimental effects on biota. A current debate revolves about the potential influence of type, size, and co-exposure with other contaminants on the toxicity of MPs. Here, two 21-day experiments with adult Danio rerio were carried out to (1) Verify the effects of different particle sizes of Polyethylene Terephthalate (PET-MPs) (300-425 µm and 53-75 µm), at two different environmentally relevant concentrations: 983 particles. L ^-1 and 330 particles. L ^-1 , and (2) demonstrate the possible effects of Cd (10 μg. L ^-1 ), 53 µm PET-MP (330 particles. L ^-1 ) and their mixture (PET-MP + Cd), on a set biochemical indicator: total proteins, carbonylated protein (CP), glucose, triglycerides, lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) activity. During exposure, there was no mortality. For the largest size (300-425 µm) there was a significant increase in CP levels, and a decrease in glucose and LDH levels compared to the control in both concentrations. In the experiment with PET-MPs co-exposed to Cd, the CP levels showed a significant increase in the PET-MP + Cd group compared to the control and PET-MP alone, evidencing oxidative stress because of the addition of Cd. LDH levels decreased significantly when Cd was added to the PET-MP. MDH of the Cd group showed a significant increase when compared to the control group and PET-MP alone. Our study evidenced that the toxicity of PET-MPs varied depending on the size, concentration, and combination with Cd.