Many marine invertebrate taxa eat microplastics, and consequently suffer diminished growth and reproductive output. These sublethal effects can occur when energy is diverted away from growth and reproduction to fuel higher priority, homeostatic systems – like the antioxidant system, which maintains redox balance.
We tested how microplastics dosed at low concentrations (1 x 103 microplastics L-1) and high concentrations (1 x 106 microplastics L-1) affect antioxidant enzymes and molecules in the marine bivalve, Mytilus galloprovincialis.
Both microplastics treatments significantly altered the concentration of the antioxidant enzyme, catalase. Additionally, concentrations of the enzyme glutathione-S-transferase – and its substrate, glutathione – were drastically reduced by both microplastics treatments. These changes indicate that microplastics disrupted redox homeostasis, forcing mussels to deploy antioxidant defences to counteract the redox challenge. Concentrations of malondialdehyde and carbonyls – biomarkers of lipid and protein oxidation, respectively – were unaltered by microplastics, suggesting that mussels restored redox homeostasis before excess biomolecule damage occurred.
This study demonstrates that ingested microplastics challenge redox homeostasis in M. galloprovincialis. Whilst mussels neutralise this threat through antioxidant enzymes and molecules, such responses are energetically expensive and can divert energy away from growth and reproduction. The antioxidant system could, therefore, be an underlying mechanism of the microplastics-induced reductions in growth and reproduction reported in the literature.