Invertebrate responses to microplastic ingestion: Reviewing the role of the antioxidant system

Microplastic ingestion in invertebrates reduces somatic and reproductive growth. This could be caused by energy reserves being detracted from growth processes and redistributed to maintenance processes that preserve life. A potential sink for this diverted energy is the antioxidant system, which minimises oxidative damage and reinstates redox homeostasis following disturbances caused by exposure to pollution. Several microplastic studies have used genetic and molecular redox biomarkers to assess how microplastic ingestion affects the functioning of the antioxidant system. This systematic review synthesises the current understanding of redox biomarker responses in invertebrates that have ingested microplastics. We found that biomarker response information exists for only seven invertebrate taxa, and early life stages have received little scientific attention. The microplastics used by most studies were polystyrene (45% of studies), spherical (51% of studies), and were < 10 μm in diameter (31% of studies). We found multiple examples of microplastic ingestion posing an oxidative challenge to invertebrates, which required upregulation of antioxidant system components. However, the lack of systematic experiments prevented us from clearly identifying which characteristic of microplastics caused these responses. We identify several areas for consideration when investigating biomarker responses to microplastic ingestion and offer research priorities for future studies.