Realistic mixture of illicit drugs impaired the oxidative status of the zebra mussel (Dreissena polymorpha)

• The toxicity of illicit drug mixtures to aquatic organism is utterly unknown.
• We measured oxidative alterations induced by an illicit drug mixture to the zebra mussel.
• The exposure to the realistic mixture imbalanced the activity of defense enzymes.
• Remarkable increases of oxidative damage were noticed.

Illicit drugs are considered to be emerging aquatic pollutants since they are commonly found in freshwater ecosystems in the high ng L−1 to low μg L−1 range concentrations. Although the environmental occurrence of the most common psychoactive compounds is well known, recently some investigations showed their potential toxicity toward non-target aquatic organisms. However, to date, these studies completely neglected that organisms in the real environment are exposed to a complex mixture, which could lead to dissimilar adverse effects. The present study investigated the oxidative alterations of the freshwater bivalve Dreissena polymorpha induced by a 14-d exposure to an environmentally relevant mixture of the most common illicit drugs found in the aquatic environment, namely cocaine (50 ng L−1), benzoylecgonine (300 ng L−1), amphetamine (300 ng L−1), morphine (100 ng L−1) and 3,4-methylenedioxymethamphetamine (50 ng L−1). The total oxidant status (TOS) was measured to investigate the increase in the reactive oxygen species’ levels, while the activity of antioxidant enzymes and glutathione S-transferase were measured to note the eventual imbalances between pro-oxidant and antioxidant molecules. In addition, oxidative damage was assessed by measuring the levels of lipid peroxidation and protein carbonylation. Significant time-dependent increases of all the antioxidant activities were induced by the mixture. Moreover, the illicit drug mixture significantly increased the levels of carbonylated proteins and caused a slight variation in lipid peroxidation. Our results showed that a mixture of illicit drugs at realistic environmental concentrations can impair the oxidative status of the zebra mussel, posing a serious hazard to the health status of this bivalve species.