Is microplastic an oxidative stressor? Evidence from a meta-analysis on bivalves.

Microplastic pollution is a major threat to the marine environment attracting attention from scientific and public communities. Although we have sufficient evidence that microplastic is ubiquitous in all ecosystems, the question of the harmfulness of microplastic exposure is still under debate. Filter feeders like bivalves are commonly exposed to microplastics in water and sediments and thus can serve as excellent biological indicators for microplastic pollution. A relatively rich toxicological literature has been focusing on microplastic effects on bivalves but we have yet to reach an agreement on the toxic effects and mechanisms of microplastics. Here, we conducted a meta-analysis and bibliometrics analysis of the microplastic studies in bivalves. The bibliometric analysis (used to evaluate the general research trends) showed that the investigation of microplastic distribution in the marine environment and the molecular mechanisms of microplastic toxicity are the two major hot spots of research. Based on analyses of ecologically and environmentally relevant microplastics concentrations, particle sizes and polymer types, we discuss the physiological effects of microplastics on bivalves, and the severity and direction of the effects at the cellular, tissue, organ and organismal levels. The meta-analysis results show that microplastics can induce time-dependent oxidative stress in bivalves. Generally, the activities of antioxidant enzymes, such as glutathione peroxidase (GPx), glutathione-S-transferase (GST) and superoxide dismutase (SOD) increased during short-term exposure but declined after long-term exposure to microplastics. Non-linear response of GPx, GST and SOD enzymes to MP exposure over time indicate that these enzymes are not good biomarkers of MPs effects in marine bivalves. The tissue glutathione levels and catalase (CAT activity) showed an increase during both short- and long term MP exposures and thus can be used as oxidative stress biomarkers of sublethal MPs effects in marine bivalves.

Physiological effects of plastic particles on mussels are mediated by food presence.

Plastic particles cause toxic effects on marine organisms but whether food particles can affect the toxic effects of plastic particles on filter feeding animals remains unknown. To evaluate the intake and physiological effects of different size particles and their exposure ways, the thick shell mussels Mytilus coruscus were exposed to polystyrene (PS) nanoplastics (NPs, 70 nm) and microplastics (MPs, 10 µm) respectively for two weeks by mixing NPs/MPs with microalgae or exposed to MNPs individually after feeding. Intake of particles and their physiological effects including energy budget, digestive enzymes and oxidative responses were assessed after exposure. Results indicated food presence mediate the effects while MPs decrease the energy budget and increase the catalase activity and malondialdehyde levels. Moreover, exposure way significantly affected energy budget and size of particle had a significant impact on enzyme activities. Our results showed MPs induce more significant effects than NPs on mussels, emphasized the importance of particle exposure way and suggested that mixture exposure with microalgae alleviate the influences on mussels caused by plastic particles alone. This study emphasized that we need to take the food particles into account for evaluating the toxic effects of plastic particles on filter feeding animals in the natural environment.